training idms
TRANSCRIPT
IDMS
Integrated Database Management System
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Topics Covered Introduction to DBMS Logical and Physical Database Structure Record Characteristics Set Characteristics IDD and DDDL Data Description Language (DDL) Data Manipulation Language (DML) Recovery & Restart of Database Locking Facilities
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Introduction to DBMS
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TYPES of DBMS
Inverted List (e.g. Datacom DB)
Hierarchical (e.g. IMS)
Data records are typically connected with embedded pointers to form a tree structure. Each node (except root) can have one and only one parent
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• Network (e.g. IDMS)The database forms a mesh structureEntity-Relationship is implemented using Record type and Set.
• Relational (e.g. Oracle, Sybase, etc)Entity-Relationship is implemented in the normalized form. Data represented in the form of rows and columns (two dimensional table)
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Logical & Physical Database Structure
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A database can be viewed as consisting of Entities. An entity is an object about which we are storing data. Every entity has attributes. e.g. Employee is an entity which has attributes like, emp-code, emp-name, emp-address, emp-sex, emp-dept etc. There exists relationship between different entities. We can represent the relationship between our entities using an entity-relationship (E-R) diagram as below.
LOGICAL DATABASE STRUCTURE
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SALARY
BONUS
TEAM
PLAYER GAME
E-R Diagram
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The example discussed here:
Taken from a typical American baseball league.
A team has many players but at a given time, a player is attached to only one team.
A team plays many games. Games can be further identified by Game-Home and Game-Away.
THE EXAMPLE
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A player plays many games and same way a game is played by many players.
A player has many salary records since his salary (contracts) may have undergone changes during a season. Similarly, a player might have many bonuses awarded to him.
Team, Player, Game, Salary and Bonus represent Entities.
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Attributes of Team could be Team-Name, Team-City, Team-Address etc.
The relationship between entities is shown by line diagram. We use cardinality symbols to represent type of relationship. The crow-foot end of the line diagram represents Many type and single line represents One type of the relationship.
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For example, consider relationship between Team and Player. Team end has a single line whereas Player end has a crow-foot. This represents a One-to-Many relationship. It means a team has many players. But a player is attached to only one team.
Note that relationship between Player and Game is Many-to-Many. i.e a game has many players and a player plays many games.
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In IDMS, entities are implemented using Record types and relationships are implemented using Set types.
IDMS does not allow direct implementation of many-to-many relationship.
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JUNCTION RECORD
It is a way of implementing many-to-many relationship in IDMS. It is a record type having attributes that are common (intersection) to both record types participating in many-to-many relation.
Thus, many-to-many relation between player and game can be resolved as below.
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NOTE:
Here, referring to our earlier example of American baseball league, we had many-to-many relationship between entities player and game.
PLAYER GAME
POSITION
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That is to say, a player has played in
many games and conversely, a game
has many players in it. To resolve this
M:M relationship, we introduce a
junction entity, Position record.
Position is called Junction record
because it represents junction or
intersection between Player and Game
record.
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A player has many positions assigned to him for different games. But for a game he has a fixed position. Main attribute of the record is “player-position” for a game. It thus resolves many-to-many relationship between player and game. In other words, we can easily move from a player record occurrence to a game record occurrence using the position record occurrence.
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BACHMAN DIAGRAM
Most installations use a line with an arrow at one end to show each one-to-many relationship instead of using crow’s foot notation. Such a diagram is called Bachman Diagram. Here, the end of the relationship without an arrow always represents “One” side of the relationship and the end with arrow always represents “Many” side of the relationship.
Our earlier E-R diagram is represented using Bachman diagram as:
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SALARYTEAM
PLAYER GAME
BONUS POSITION
BACHMAN DIAGRAM
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IDMS database is divided into one or more areas. An area is defined as the major named subdivision of addressable storage in database.
An area is further subdivided into pages. Page is a smallest unit of data transfer between main memory and hard disk.
PHYSICAL DATABASE STRUCTURE
Database Area, Page and File
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Each page stores control information and can store up to a maximum 255 database records.
Database areas can be of different page sizes. e.g AREA-A can be of page size 2048 bytes where as AREA-B of same database can be of page size 4096 bytes.
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Page size for an area is decided based on the record types that will be residing within that area.
ALL record occurances of a
particular record type are stored
within the SAME area. But, an area may store occurances of
multiple record types. Hence this holds true for a database page as well.
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Record types are assigned to areas by designer.
Areas are mapped to files.
Many or all areas can be mapped into one file if all areas have the same page size.
Each area can be mapped into a different file.
One area can be mapped into several files.
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AREAS and FILES MAPPING
Page
Area
Logical Database
Physical Database
File
Block
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Team Area
921 922 … 930
IDMS File 1
Position Area
1021 1022 … 1040
Salary Area
821 822 ... 830
IDMS File 2
Area and File Mapping…..
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SALARYTEAM
PLAYER GAME
BONUSPOSITION
ADVANTAGES OF MULTIPLE AREAS
SALARY-AREAPOSITION-AREA
TEAM-AREA
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As shown in the previous slide, our example of baseball database is divided into three physical areas namely Team-area, Position-area and Salary-area.
The advantages of dividing a database into separate physical areas are:
Processing efficiency
Records those are accessed together during most of the processing can be grouped together into same area for efficiency.
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Security
One can restrict access to certain record types. e.g. salary-area can be restricted to only finance department.
Database recovery and backup
Database can be initialized, reorganized and backed up on area-by-area basis. Backup of most updated areas can be made more frequent than other areas.
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Concurrent updating
A program can request exclusive use of an area and prevent other programs from accessing it concurrently.
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CONCEPT of DB-KEY
Each Record occurrence stored in the database is assigned an unique numeric identifier, called Database Key (db-key). A record’s db-key consists of a 32-bit field that contains a 23-bit page number and an 8-bit line number. The page number identifies the page in which the record is stored and the line number identifies location of the record occurrence within the page.
The format of db-key is as below:
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8-bits23-bits1-bit
(Not
Used)
Line NumberDatabase Page NumberSign Bit
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RECORD TYPE v/s RECORD OCCURRENCE
It is important to make distinction between Record Type and Record Occurrences.
Record type is like a template. It describes the format of all occurrences of a given record type stored in database.
Record occurrence represents the smallest directly addressable unit of data. It consists of fixed or variable number of characters that are subdivided into units called Data Elements.
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Example of Record Type versus Record Occurrences
Employee Record Type ……….
Emp-code
PIC 9(4)
Emp-name
PIC X(30)
Sex
PIC X
Emp-dept
PIC X(10)
Employee Record Occurrences …..…...
1000 ABC M TECH
1500 XYZ F SALES
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Database consists of collection of record occurrences.
The physical records stored in a database consist of more than the data elements used by the application program.
IDMS also maintains information about relationships that exists between records.
Relationships are implemented by linking record occurrences together with pointers.
STRUCTURE of DATABASE RECORDS
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Pointers contain addresses of related record occurrences and are stored along with the data portion of the record occurrences.
A record occurrence in database consists of two parts. Data portion and Prefix portion.
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Data elements values are stored in data portion. Pointers to related record occurrences are stored in prefix portion.
Application programs deal only with data portion of the record occurrence. Whereas, IDMS system maintains the pointers in the prefix portion.
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PREFIX Portion DATA Portion
Pointer 1
Pointer 2
Pointer 3
…..Data element 1
Data element 2
...
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Schema is the logical definition of a database. A schema is a complete database description (all records types & record elements, set types, files and areas).
There is ONE and ONLY ONE schema for a given database.
SCHEMA and SUBSCHEMA
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A subschema defines a subset of the schema. One or more subschemas can be associated with a schema. A load module is created for each subschema. At runtime, a program can access only one subschema. A program can not access IDMS database by referring to the schema.
A subschema is similar to a view in a relational database.
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Even if a program requires access to ALL record types, data elements and ALL set types defined in the schema, the designer must still define a subschema that includes all the entities defined in the schema.
A subschema can restrict access to a program. For example, it may give only read-only access to an area, or it may not allow deletion of some record types.
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Record Characteristics
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The characteristics that apply to record types are:
Record Name
Record Identifier
Storage Mode
Record Length
Location Mode
Duplicates Option
Area Name
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RECORD NAME
Each record type must be assigned a 1
to 16 character name that identifies the
record type. The name must begin with
an alphabetic character. The
application program must reference the
record’s name in DML (Data
Manipulation Language).
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It is a number that serves as in internal
identifier for the record type. It is in the
range 100 through 9999. Each record
type must be assigned an unique record
identifier within the installation. DBAs
assign this number to each record type.
Application programs do not refer to
record type using this number.
RECORD IDENTIFIER
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STORAGE MODE
It indicates whether record occurances of the record type are fixed or variable length and whether they are stored in compressed format.
Allowable codes are:
F (fixed length)
V (variable length)
C (compressed)
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NOTE:
Compressed mode can be used along with fixed or variable length storage modes. For example, one can mention storage mode as ‘FC’ or ‘VC’. DBMS takes care of compression and decompression of data.
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RECORD LENGTH
It is expressed in BYTES. It is the
actual data length for fixed-length
record or the maximum data length
for variable-length record.
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LOCATION MODE
It defines the way record occurances are stored in the database.
Allowable location modes are:
CALC
VIA
DIRECT
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In CALC mode, a particular data
element within the record is declared as
the CALC-key. At the time of storing
the record in the database, IDMS
system uses value of the data element
to calculate the page number for
storing a record. Records stored with a
CALC mode can be retrieved from disk
in a single access.
CALC Mode
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IDMS system uses randomizing routine
to distribute records evenly over its area,
minimizing overflow conditions and
leaving space for adding new records.
For retrieving the record (stored with
CALC location mode), appropriate
value for the data element is moved to
the storage area in the application
program and then DML retrieval
function is executed.
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VIA Mode
Records stored with VIA location mode
are stored near another database record. This mode is generally used for storing
member records on the same page
containing owner record or on a page
near their owner record. This mode tends to reduce disk accesses
needed to retrieve all the records of a set
occurrence.
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CALC retrieval is not possible in
case of records stored with VIA
location mode. Generally the owner
record is assigned CALC location
mode for easy access of member
records.
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DIRECT Mode
In DIRECT location mode, application
program explicitly specifies the page
into which the record should be stored.
To retrieve this record, programmer
must specify its database address, i.e
db-key of the record. This mode is less often used than
CALC and VIA mode.
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DUPLICATES OPTION
It is specified only for record types that
are stored using CALC location mode. It
specifies whether records with duplicate
CALC-key values are allowed and if so,
how would they be stored in the
database.
Possible codes are as on next slides:
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DN (Duplicates Not Allowed)
Record occurances with
duplicate CALC-key value will
not be accepted. IDMS will
give an error if application
program tries to store a record
with duplicate CALC-key.
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DF (Duplicates First)
IDMS will store the record with
duplicate CALC-key value
Before any record in the database
that has matching CALC-key
value. When CALC retrieval is
made using CALC-key value,
newly stored record will be
retrieved first.
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DL (Duplicates Last)
IDMS will store the record with
duplicate CALC-key value After
any record in the database that has
matching CALC-key value. When
CALC retrieval is made using
CALC-key value, newly stored
record will be retrieved last.
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AREA NAME
It is the name of the area into which all record occurances of the record type are to be stored. e.g. CUST-AREA, SALARY-AREA, SALES-AREA etc.
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Data Structure Diagram (DSD) for Documenting Record Characteristics
Record name
Record-id Storagemode
Recordlength
Location mode
Calc key or VIA set name Duplicates option
Area name
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Example : DSD for Employee Record Type
EMPLOYEE
1000 F 150 CALC
EMP-CODE DN
HR-AREA
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Set Characteristics
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A Set Occurrence consists of one occurrence of owner record type and any number of member record occurrences.
WHAT IS A SET ?
A Set consists of an OWNER record type and one or more MEMBER record types.
SET OCCURRENCE ?
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A set occurrence that consists only of an owner record occurrence and no member record occurrences is called Empty Set occurrence.
Walking a Set
Records in each set occurrence are physically linked together by pointers.
Accessing members by following the pointers from one record occurrence to the next is called Walking the Set.
Empty Set Occurrence
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• Any record type can be member in one or more sets.
• Any record type can be owner of one or more sets.
• Any record type can be member in one set and owner in another set.
• An owner record can own the same member record in more than one set.
• A record cannot be both owner and member in the same set.
RULES & FEATURES FOR SET RELATIONSHIP
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Office
Employee
Department
Office-EmployeeDept-Employee
Any record type can be member in one or more sets
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Any record type can be owner of one or more sets
Order
Cust-Order
Customer
Parts
Cust-Parts
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Any record type can be member in one set and owner in another set
Order
Cust-Order
Customer
Items
Ord-Items
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An owner record can own the same member record in more than one set
Class
Teacher
Teach-Class Examiner-Class
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DBA typically assigns characteristics to each set when defining the IDMS database in the schema DDL.
The characteristics are:
Set Name
Linkage Options
Membership Options
Order Option
Duplicates Option
Set Characteristics
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SET NAME
A unique name must be given to each set
type in the database. The name can be
maximum of 16 characters. The set name
must be referenced whenever an application
program accesses records using that set
relationship. Usually it will be owner record
name followed by member record name e.g.
CUST-ORDER.
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LINKAGE OPTIONS
It indicates the types of pointers that are used to implement the set. Pointers provide flexibility in accessing records in a set occurrence. Pointers are stored in the prefix part of the database record occurrences.
Available options are:
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N (NEXT pointer)Each record in the set contains a pointer to the next record occurrence. This option allows to access member records only in the forward direction. Next pointer is mandatory for all sets. All other pointers are optional.
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NP (NEXT and PRIOR pointer)
With this, in addition to Next
pointer, each record contains
pointer to prior record occurrence
in the set. This allows us to access
member records in both forward
and backward direction.
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NO (NEXT and OWNER pointer)
With this, in addition to Next
pointer, each record contains pointer
to owner record occurrence in the
set. This allows us to access the
owner record directly from any
member record occurrence.
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NPO (NEXT, PRIOR and OWNER pointer)
Each record in the set contains all three pointers. This option allows to access member records in both forward and backward direction and also allows to access the owner record directly from any member record occurrence.
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Xansa Mumbai DATAPN
EMP-A DATA PN
EMP-B DATAPN
EMP-C DATAPN
Record Occurrences with Next, Prior & Owner Pointers
Office
Employee
Office-Employee
O
O
O
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The Membership options specify how a member record may be connected to or disconnected from a set occurrence.
The option is defined in two parts.
MEMBERSHIP OPTIONS
First part is a Disconnect Option, indicating the way a record is disconnected from a set.
Second part is the Connect Option, indicating how a record is connected to a set.
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It specifies whether a member record can
be later disconnected from a set once its
membership has been established.
Possible values are:
M (Mandatory)
The record cannot be disconnected
from a set unless that record is
deleted (erased) from the database
using the ERASE command.
Disconnect Option
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O (Optional)
A record occurrence can be disconnected from a set. The record remains in the database. It can be connected to some other set occurrence. It is optional to use ERASE for such records.
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It specifies whether or not a member record is automatically connected to a set occurrence when it is added to the database.Possible values are: A (Automatic)
Automatic means when a member record is inserted in database, IDMS will automatically connect it to all its owner records (provided currencies have been established for owners).
Connect Option
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M (Manual)
Manual option specifies that after
inserting a record, programmer
must explicitly connect it to its
owner record by issuing
CONNECT statement.
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MA - Mandatory Automatic
MM - Mandatory Manual
OA - Optional Automatic
OM - Optional Manual
Combinations for Membership Options
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The order option specifies logical order in which member record occurrences are placed within a set occurrence. Options available are: FIRST
Each new member record occurrence is placed immediately after the owner record (in the next direction). This option achieves a member record in LIFO.
ORDER OPTION
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LAST
Each new member record occurrence is placed immediately before the owner record (in the prior direction). This option achieves a member record in FIFO. Prior pointer is a must to specify this option.
NEXT
Each new member record occurrence is placed immediately after the member record occurrence that was last accessed (in the next direction).
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PRIOR
Each new member record occurrence
is placed immediately before the
member record occurrence that was
last accessed within the set (in the
prior direction). Prior pointer is a
must to specify this option.
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SORTED
Each new member record occurrence is placed in ascending or descending sequence, based on the value of designated sort-control data element (sort-key) in each record occurrence. When the record is placed into a set, DBMS examines the sort-key value in each member to find the logical position of new member record in the set.
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This option is useful only in case where set is defined with order option as Sorted. It indicates the action to be taken when a duplicate sort-key value occurs.
DN (Duplicates Not Allowed)
Record occurances with duplicate sort-key value will not be stored in the set. IDMS returns an error code if program tries to store such a record.
DUPLICATES OPTION
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DF (Duplicates First)
Record with duplicate sort-key
value is stored Before any
existing record in the set that has
matching sort-key value. Most
recently stored duplicate record
will be retrieved first.
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DL (Duplicates Last)
The record occurrence with
duplicate sort-key value is stored
After existing record in the set
that has matching sort-key value.
Most recently stored duplicate
record will be retrieved last.
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IDMS allows retrieval of records using an index.
In conventional sets, member records are chained together by pointers. In an indexed set, DB-key values of member record occurrences are stored in a specified order in one or more index records.
Adds flexibility to data retrieval and retrieval is made faster in some cases.
INDEXED SETS
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Indexed sets are useful in cases where: Records need to be accessed using
alternate-key (apart from CALC key).
Walking very long sets and when only key values of a member record is required.
Member records need to be retrieved randomly using partial key value.
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Indexed sets can be implemented using two database record types or a single record type and a system defined record type.
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Player
Position
Player-Position-Index
(Position-Name)
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User-defined Owner Record
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Employee
System-defined owner record
(Employee-Name)
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SR7
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Index between Player and Position
records allows to access position
records using Position-name values.
System owned index on employee
implements alternate index on
employee-name.
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SALARY
PLAYER
BONUS
TEAM
POSITION
GAME
SALARY-AREA
SALARY-AREA
TEAM-AREA
TEAM-AREA
TEAM-AREA
POSITION-AREA
DN
DN
DN
VIA
VIA VIA
CALC CALC
CALC40
70
25 50
80
701000
1100
1200
2000
25003000
F
F
F
F
F
F
PLAYER-BONUS GAME-POSITION
PLAYER-NAME
PLAYER-SALARY
GAME-PLAY-DATE
TEAM-NAME
PLAYER-SALARY
NPO
OA
NEXT
TEAM-PLAYER
NPO
OM
LAST
PLAYER-BONUS
NPO
OA
NEXT
PLAYER-POSITION
NPO
OA
NEXT
GAME-POSITION
NPO
MA
NEXT
TEAM-GAME
NPO
MM
NEXT
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IDD & Data Dictionary Definition Language (DDDL)
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IDD stores meta-data about all the data items in the database (The data values are not stored in IDD).
Stores information about users, application programs, files, record, data element, module (date routine, error handling routine) and application systems.
IDD is integrated with every software component provided in IDMS like ADSO, OLQ and Report Generator. Each software component accesses IDD to get information about data items and programs.
INTEGRATED DATA DICTIONARY (IDD)
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The data dictionary definition language (DDDL) is used to create, update or delete entity occurrences in IDD.
DDDL provides five verbs to manipulate entities:
ADD to add a new entity.
MODIFY to change entity description.
DATA DICTIONARY DEFINITION LANGUAGE (DDDL)
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DELETE to remove an entity.
DISPLAY to display an entity description.
PUNCH to copy entity information from IDD to a file.
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ADD ELEMENT CITY-NAME ELEMENT DESCRIPTION ‘TEAM CITY’ PICTURE X(20) USAGE DISPLAY.
DISPLAY PROGRAM STATSEDIT.
DELETE PROGRAM NAME IS STATSEDIT.
Examples of DDDL
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MODIFY ELEMENT CITY-NAME PICTURE X(25). MODIFY PROGRAM NAME IS STATSEDIT
VERSION IS 1LANGUAGE IS COBOLESTIMATED LINES 5000.
SIGNOFF. ( to exit from IDD )
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Data Description Language (DDL)
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Data Description Language (DDL) is used to define:
SCHEMAS
DBA describes the logical structure of the database by coding set of schema DDL statements. There is a single schema for a given database.
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DEVICE-MEDIA CONTROL
LANGUAGE (DMCL) MODULES
Many aspects of the database’s
physical structure, such as disk
device assignments and page sizes
are described in DMCL module
definition.
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SUBSCHEMAS
The views that individual
applications programs have of the
database are defined in
subschemas and described with set
of subschema DDL statements.
Any number of subschemas can be
defined for a given database.
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The logical structure of a database is defined in a set of schema DDL statements.
Schema DDL statements define the following:
Schema name Database file names Database areas Data elements and records Sets
SCHEMA DDL STATEMENTS
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The schema DDL is processed by the
schema compiler. The schema
compiler reads the schema DDL
statements and stores a description of
the schema in the data dictionary.
The data dictionary is the central
repository of information that IDMS
maintains about the databases under its
control.
Process
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Schema
DDL Statements
Schema Compiler
Data Dictionary
The schema compiler can also copy from the data dictionary descriptions of records that have been previously defined and stored in the data dictionary.
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ADD SCHEMA NAME IS DDSCHEMA VERSION 1
ADD FILE NAME IS CUST-FILE ASSIGN TO CUST.ADD FILE NAME IS ORDER-FILE ASSIGN TO ORDER. ADD AREA NAME IS CUST-AREA PAGE RANGE IS 505000 FOR 100 WITHIN FILE CUST-FILE.ADD AREA NAME IS ORDER-AREA PAGE RANGE IS 809000 FOR 100 WITHIN FILE ORDER-FILE.
Example Of Schema DDL
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ADD RECORD NAME IS CUST-REC RECORD ID IS 3000 LOCATION MODE IS CALC USING (CUST-NO)
DUPLICATES ARE NOT ALLOWEDWITHIN AREA CUST-AREA.
02 CUST-NO PICTURE IS 9(6) USAGE IS DISPLAY.02 CUST-NAME PICTURE IS X(40) USAGE IS DISPLAY.
ADD RECORD NAME IS ORDER-REC RECORD ID IS 3100 LOCATION MODE IS CALC USING (ORDER-NO)
DUPLICATES ARE NOT ALLOWEDWITHIN AREA ORDER-AREA.
02 ORDER-NO PICTURE IS 9(7) USAGE IS DISPLAY.02 ORDER-STATUS PICTURE IS X(1) USAGE IS DISPLAY.02 ORDER-REQ-DT PICTURE IS 9(8) USAGE IS DISPLAY.
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ADD SET NAME IS CUST-ORDER
ORDER IS NEXT
MODE IS CHAIN LINKED TO
PRIOR
OWNER IS CUST-REC
MEMBER IS ORDER-REC LINKED
TO OWNER
MANDATORY
AUTOMATIC.
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It completes the mapping of logical areas into physical files.
It also identifies and describes the journal files that are used by IDMS for recording changes made to the database. Journal files are used to implement IDMS backup and recovery facilities.
DMCL DDL STATEMENTS
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DMCL DDL statements define the following:
DMCL module name Schema name associated with this
DMCL module Sizes of the I/O buffers Information about database areas Information about journal files used
for backup and recovery
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The DMCL definition is processed by the DMCL compiler. The DMCL compiler reads the DMCL source statements and stores a description of the DMCL module in the data dictionary.
As the DMCL compiler processes DMCL source, it must have access to the corresponding schema definition in the data dictionary. For this reason, schema must be compiled before its associated DMCL module.
Process
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The DMCL compiler also generates a set of assembler language source statements. These are in turn processed by assembler and the linkage editor to form an executable DMCL load module. The linkage editor places the load module into a load module library. The DMCL load module is used by IDMS at run-time when an application program accesses the database files described by the DMCL module.
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DMCL Compiler
Assembler
Linkage Editor
DMCL Source
StatementsData
Dictionary
Load Library
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DMCL DDL statements contain following four sections: Device-Media Description section
This sections gives name to the DMCL statements. It further provides other documentary details about the DMCL module.
Buffer section
It assigns page size and defines buffers that IDMS will use to handle database accesses.
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Area section
Information about each area is copied from the schema into this section of the DMCL. The section assigns each area to buffers that were defined in the buffer section.
Journal section
The section describes disk files to be used for journaling. Journal files are used to keep track of database activity.
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DEVICE-MEDIA DESCRIPTION.DEVICE-MEDIA NAME IS DMCLBASE OF SCHEMA NAME SCHBASE VERSION 1.DATE. DD/MM/YY.INSTALLATION. XANSA MUMBAI
INDIA.BUFFER SECTION.
BUFFER NAME IS XXBUFFPAGE CONTAINS 4096 CHARACTERSBUFFER CONTAINS 5 PAGES.
JOURNAL BUFFER NAME IS JJBUFFPAGE CONTAINS 3188 CHARACTERSBUFFER CONTAINS 3 PAGES.
Example Of DMCL DDL
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AREA SECTION.COPY SALARY-AREA AREA
BUFFER IS XXBUFFJOURNAL SECTION.
JOURNAL BUFFER IS JJBUFF.FILE CONTAINS 3000 BLOCKS.FILE NAME IS XANJRNL1
ASSIGN TO SYSJRNL1.FILE NAME IS XANJRNL2
ASSIGN TO SYSJRNL2.
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Subschema DDL statements specify the following:
Subschema, Schema and DMCL module name
Areas accessible in this subschema
Records and data elements accessible and
Sets included in the subschema
SUBSCHEMA DDL STATEMENTS
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The subschema compiler reads subschema source statements and stores a description of the subschema in the data dictionary.
The compiler also stores an executable load module in an area of the data dictionary, known as Load Area.
IDMS uses this load module whenever an application program using this subschema is under execution.
Process
123
Like DMCL compiler, subschema compiler must have access to the corresponding schema definition in the data dictionary. Thus, schema must have been compiled before any of its associated subschemas can be compiled.
After compiling subschema, data dictionary has all the required information about the logical and physical structures of the database.
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Data Dictionary
SubschemaDDL
SubschemaCompiler
Load Area
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ADD SUBSCHEMA NAME SUBSCHA OF SCHEMA NAME SCHDBA VERSION 1 DMCL NAME SUBADMCL OF SCHEMA NAME SCHDBA VERSION 1 COMMENTS ‘LIMITED VIEW OF SCHDBA’
ADD AREA NAME IS CUST-AREA.ADD AREA NAME IS ORDER-AREA.ADD RECORD NAME CUST-REC
ELEMENTS ARE CUST-ID CUST-NAME.ADD RECORD NAME ORDER-REC
ELEMENTS ARE ALL.ADD SET NAME ORDER-ACTION.ADD SET NAME CUST-ORDER.GENERATE.
Example Of Subschema DDL
126
Data Manipulation Language (DML)
127
In an IDMS application program, there
are no file description statements or Open
and Close statements found in a typical
batch Cobol file processing application
system.
To access data stored in the database,
DML verbs are used.
Two new terms Run Unit and Currency
are important to understand.
128
The BIND RUN-UNIT statement is used to specify the start of a run unit.
A run unit ends when program completes all database processing and issues a FINISH statement.
CONCEPT of RUN UNIT
The run unit is that portion of program’s
processing during which it has access to
one or more database areas and services.
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The program may perform some
processing before it begins a run
unit.
The program may perform some
processing even after it finishes a run
unit.
A program may begin and end more
than one run unit.
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IDMS internally stores pointers to
the most recently accessed records.
It uses these pointers to access
subsequent records. These pointers
are referred to as CURRENCY.
At the beginning of a program, all
currencies are null.
CONCEPT of CURRENCY
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Currency is established by DML verbs FIND, OBTAIN, STORE, CONNECT, DISCONNECT and ERASE.
There are four currencies maintained for a database: current of run unit current of record type current of set current of area
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Current of Run Unit is a record which was most recently accessed.
Current of Record type is the most recently retrieved or inserted record of that record type.
Current of Set is the most recently retrieved or stored record of that set. Record may be a member or owner of that set.
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Current of Area is the most recently retrieved or stored record of that area.
When a record is erased, currency of its record type and its sets is set to null. Run unit and area currencies are unchanged.
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DML Processor
Compiler
Linkage Editor
Source Statements Data
Dictionary
Load Library
IDMS APPLICATION PROGRAM PREPARATION
135
The previous slide shows how an IDMS application program written in Cobol host language is compiled and made ready for execution.
Application program with embedded DML statements is first read by DML processor. It validates DML statements and creates translated version of the source program where in DML statements are replaced by appropriate calls to IDMS.
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DML processors are available for a number of commonly used programming languages. DML processor may also generate a listing of errors. DML processor interacts with data dictionary for copying descriptions of various database elements like schema, subschema, record types etc.
Output from the DML processor acts as input to the host programming language compiler like a Cobol compiler.
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After the translated source program is
compiled by host language compiler, the
resulting object module is processed by
Linkage editor to create an executable
load module.
The Linkage editor also includes copy of
a routine called IDMS Interface Module
in final load module. IDMS service
requests from the program are converted
in to calls to IDMS Interface Module.
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Following are typical changes one
notices in a n IDMS Cobol application
program.
IDENTIFICATION DIVISION
remains exactly the same as before.
ENVIRONMENT DIVISION has
IDMS-CONTROL section.
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DATA DIVISION begins with
SCHEMA SECTION which gives
the subschema used by the
program.
PROCEDURE DIVISION has
DML verbs for data retrieval and
data updation.
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.….
ENVIRONMENT DIVISION.
IDMS-CONTROL SECTION.
PROTOCOL.
MODE IS BATCH DEBUG.
IDMS-RECORDS WITHIN WORKING-STORAGE SECTION.
CONFIGURATION SECTION.
…..
INPUT-OUTPUT SECTION.
FILE CONTROL.
…..
IDMS-CONTROL SECTION
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The PROTOCOL statement is a
compiler-directive statement that
specifies the manner in which DML
processor will generate CALL
statements.
Application program must specify one
of the following modes to access
database:
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BATCH (a batch program with
database processing)
IDMS-DC (a non-batch program i.e.
an online program called by a dialog
process)
DC-BATCH (a batch program with
access to DC queues)
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‘DEBUG’ instructs DML processor to
produce additional code in the resulting
source code which is helpful during
program debugging.
The IDMS-RECORDS clause specifies
that the data definitions for records and
data elements will be copied automatically
from the data dictionary and placed at the
end of working storage section.
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.….
DATA DIVISION.
SCHEMA SECTION.
DB SUBEMP WITHIN SCHEMP.
FILE SECTION.
…..
WORKING-STORAGE SECTION.
…..
DATA DIVISION
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As explained earlier, DATA DIVISION
begins with SCHEMA SECTION which
gives the subschema used by the
program.
As shown in previous slide, SUBEMP is
the name of the subschema which will be
used by the program.
Further, the SCHEMP is the schema
from which SUBEMP subschema is
derived.
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BIND RUN UNIT.
BIND EMPLOYEE.
BIND MANAGER.
BIND PROJECT.
BIND SALARY.
BIND STATEMENT
Above example displays the typical
code that might be found in the
Procedure Division of an IDMS
Cobol program.
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The first statement BIND RUN-UNIT
establishes addressability for the IDMS
communications block and causes
IDMS to load the subschema identified
in the Schema section. Next four BIND
statements establish addressability for
the working-storage areas that will be
used to contain record occurrences.
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Generally, following one statement is found in the source code.
COPY IDMS SUBSCHEMA-BINDS.
When the source is pre-compiled, the above single statement is replaced by previous set of five Bind statements.
BIND statement identifies the record types that the program will access.
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COPY IDMS SUBSCHEMA-BINDS.
COPY IDMS IDMS-STATUS.
COPY STATEMENT
Copy statement is a compiler directive
statement.
In first example above, the copy
statement generates appropriate Bind
statements at the time of pre-compile.
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As in second example, the copy
statement copies module called
IDMS-STATUS from data
dictionary into the source code
when it is pre-compiled. This
particular module (routine) checks
result of DML statement.
This helps in reducing coding effort.
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READY EMPLOYEE-AREA USAGE IS RETRIEVAL.
READY SALARY-AREA USAGE EXCLUSIVE UPDATE.
READY USAGE IS RETRIEVAL.
READY STATEMENT
Ready statement can be thought as an
equivalent to Open statement for a
conventional file. It gives the program
access to all or named areas defined in
the subschema and begins a run-unit.
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The usage-mode clause in the Ready
statement indicates whether the program
will only retrieve data or update it as
well.
If no areas are mentioned in the Ready
statement, then all areas mentioned in
the subschema will be made available to
the program by default in the mode
specified in the Ready statement.
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FINISH.
FINISH STATEMENT
Finish statement is used to end a run
unit by releasing database resources and
terminating database processing.
Non-database processing of the program
may continue after the Finish statement,
but database can not be accessed (unless
a fresh run unit is started).
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MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.
FIND CALC EMPLOYEE.
FIND STATEMENT
Find statement is used to locate a record
occurrence in the database. The record
occurrence is placed in the system
buffers but the data element values are
still not available in the variable storage
of the program.
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There are situations when we do not need
actual values of a record occurrence like:
We may need to verify that a particular record occurrence exists
We need to establish currencies for some subsequent record retrieval
In the example, the required employee information is moved to the CALC-key data element of Employee.
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Next, Find statement will locate the record occurrence with matching employee-code.
If the Find is successful, currencies are set
appropriately but the individual data
element values for the located record are not
yet made available to the program.
In case, data element values are required
then Find can be followed by Get statement.
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MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.
FIND CALC EMPLOYEE.
GET.
GET STATEMENT
Get statement is used to make available to
the program a record occurrence that was
previously located by the Find statement.
Get statement can qualify the record type
or it can be unqualified as in this case.
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To continue with our earlier Find example,
we have followed it with unqualified Get.
This will transfer record occurrence values
to the variable storage.
If Get is qualified then IDMS will verify
whether the previously located record is
Employee record or not. If it does not
match then IDMS will return error code.
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The Obtain statement is equivalent of a Find followed by a Get.
It is most widely used statement for
data retrieval in IDMS.
There are many syntax variations
available for Obtain statement, based
on location mode of records and mode
of retrieval.
OBTAIN STATEMENT
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CALC-key Retrieval
To perform Calc-key retrieval, the record
must have location mode specified as
CALC and we must know the Calc-key
value for the record to be retrieved.
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MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.
OBTAIN CALC EMPLOYEE.
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As shown in previous example, we move
the Calc-key value for the record to be
retrieved to the data element of Employee
record type that is defined as Calc-key
(Employee-Code) and issue Obtain
statement to get the record contents.
IDMS uses the Calc-key value supplied to
calculate the target page for the data
retrieval.
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Duplicate CALC-key Retrieval
Sometimes the record type is defined
with duplicate allowed option. We can
try and find out if duplicate record exists
for a given Calc-key value.
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MOVE INPUT-EMP-NAME TO EMPLOYEE-NAME.OBTAIN CALC EMPLOYEE.OBTAIN DUPLICATE EMPLOYEE.
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As shown in previous example, we
obtain the first record with matching
Calc-key value in normal way.
The second Obtain statement tries to
obtain duplicate record with same
Calc-key value. If it exists then IDMS
fetches the record, else an error code is
returned.
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Member Record Retrieval
Many times, we need to retrieve owner
record for a set occurrence and then
proceed to retrieve one or more of its
member records.
Following are some of the examples of
such typical member retrieval criteria.
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OBTAIN FIRST EMPLOYEE WITHIN DEPT-EMP.
OBTAIN LAST EMPLOYEE WITHIN DEPT-EMP.
OBTAIN NEXT EMPLOYEE WITHIN DEPT-EMP.
OBTAIN PRIOR EMPLOYEE WITHIN DEPT-EMP.
OBTAIN OWNER WITH DEPT-EMP.
OBTAIN 5 WITHIN DEPT-EMP.
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By first retrieving the owner record, we
establish the currency for the subsequent
member retrievals. Owner becomes
current of run unit, record type and set
type. Now to access members within the
set becomes relatively simple task.
“Obtain First” gets the first member
record within the set.
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“Obtain Last” gets the last member
record within the set. For this, the set
must have been defined with prior
linkage option.
Next option is known as Walking the Set.
“Obtain Next” gets each member in
sequence. This statement is executed in a
loop generally till end-of-set is reached.
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“Obtain Prior” gets the prior member
record within the set. For this, the set
must have been defined with prior
linkage option. This works in same
manner as Next option, except that it
is in the reverse direction.
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We can obtain owner of the set by
“Obtain Owner” statement.
If set is defined with Owner linkage
pointer then owner is obtained directly.
In case such pointer is absent, IDMS
walks through the entire set and
obtains the set owner.
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We can obtain specific member record
within the set by its relative position with
respect to owner in the ‘next’ direction.
“Obtain 5” will fetch the fifth member
record in the set.
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Current Record Retrieval
Many times, we need to retrieve records
based directly on various currency.
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OBTAIN CURRENT EMPLOYEE.
OBTAIN CURRENT WITHIN DEPT-EMP.
OBTAIN CURRENT WITHIN EMPLOYEE-AREA.
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In first case, IDMS obtains the record
that is current of the Employee record
type.
In second case, current of the Dept-Emp
set type is fetched. Depending on
currency established, either Dept record
or an employee record is obtained.
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In last case, current of employee-area
is obtained. e.g. if employee-area
contains employee, project, salary
record types then depending on
currency, the statement will fetch
record occurrence of any one of these
record types.
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Area Sweep Retrieval
Sometimes, we need to retrieve records by
doing an area sweep. IDMS scans through
the area in physical sequence. Area sweep
is used when we need to access all records
within an area, irrespective of the record
types and/or their sequence.
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OBTAIN FIRST WITHIN EMPLOYEE-AREA.
OBTAIN NEXT WITHIN EMPLOYEE-AREA.
OBTAIN LAST WITHIN EMPLOYEE-AREA.
In first case, the record having lowest db-key value within the employee-area will be fetched. In second case, record having next highest db-key value will be obtained. The sweep in reverse direction as in last case can also be made.
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OBTAIN FIRST EMPLOYEE WITHIN EMPLOYEE-AREA.
OBTAIN LAST EMPLOYEE WITHIN EMPLOYEE-AREA.
As shown above, the record type can
also be qualified during the area sweep.
Only record type mentioned in the
statement (employee) are obtained
during the sweep. Occurrences of other
record types are ignored.
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Sort-Key Retrieval
We can retrieve records based on sort-
key values, provided the set is defined as
Sorted order option.
In our earlier example of American
baseball league database, suppose that
set Player-Position is defined as Sorted.
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The sort-key data element is Position-
name of Position record.
Then we can retrieve Position record
directly based on sort-key value as below.
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….
OBTAIN CALC PLAYER-REC.
MOVE INPUT-POSITION-NAME TO POSITION-NAME.
OBTAIN POSITION-REC WITHIN PLAYER-POSITION
USING POSITION-NAME.
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In the example, a position-name value is
moved to sort-key data element and then
Obtain statement is issued using reference
to the sort-key element.
The retrieval appears similar to CALC, but
in this case the search is made only within
the current set occurrence and not within
entire record type.
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DB-Key Value Retrieval
If we know the dB-key value of a record
then we can directly obtain the record.
This is useful in cases where we know
that a particular record would be
processed again later on during the
execution of the program.
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When a record is successfully retrieved
or located, its dB-key value is available
in the IDMS Communication Block. We
can store this value in a variable storage
area for later use. Later on, we can use
the stored dB-key value to retrieve the
record and process it.
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…..
MOVE INPUT-PLAYER-NAME TO PLAYER-NAME.
OBTAIN CALC PLAYER-REC.
…..
MOVE DBKEY TO SAVE-DBKEY.
…..
OBTAIN DB-KEY IS SAVE-DBKEY.
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Note that the dB-key of a record may
undergo change whenever a database is
reorganized. Hence it is not always
advisable to save dB-key values for a
long time with an intention of using
them later on.
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Index Set Retrieval
Using index sets, following types of
retrieval is made possible which is
otherwise very difficult.
Generic key retrieval.
Random and sequential retrieval based
on a key other than Calc-key.
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Suppose in our baseball example, Player
record has a system indexed set IX-
PLAYER-NAME on Player-Name
element, then we can retrieve a player
record by supplying a partial key to
IDMS. Such retrieval is not possible
unless we have index set. In Calc, we
have to supply complete key value.
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01 WS-PLAYER-NAME.
05 FIRST-INITIAL PIC X.
05 FILLER PIC X(29).
…..
MOVE LOW-VALUES TO WS-PLAYER-NAME.
MOVE WS-INITIAL-LETTER TO FIRST-INITIAL.
MOVE WS-PLAYER-NAME TO PLAYER-NAME.
OBTAIN PLAYER-REC WITHIN IX-PLAYER-NAME USING PLAYER-NAME.
…..
OBTAIN NEXT PLAYER-REC WITHIN IX-PLAYER-NAME.
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Say we want to retrieve all players
starting with ‘C’.
We move ‘C’ to ws-initial-letter and rest
is filled with low values. This is called
Partial key value.
Then the program will obtain first player
starting with name ‘C’.
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First obtain statement is random retrieval
using index set. Whereas the second
obtain statement is the example of the
sequential retrieval using index set.
Program has to check name field after
every retrieval to determine if it has
reached the last record with specific name.
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The Modify statement is used to change data element values in an existing record occurrence.
Important considerations for a record to be successfully modified are:
Ready the concerned area(s)
with one of the update options.
MODIFY STATEMENT
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Making the record to be modified as current of run unit. This is done by retrieving the record before modifying it, by issuing Obtain.
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MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.
OBTAIN CALC EMPLOYEE.
IF DB-STATUS-OK
MOVE NEW-EMP-ADDRESS
TO EMPLOYEE-ADDRESS
MODIFY EMPLOYEE
PERFORM IDMS-STATUS.
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Above example shows how an existing
Employee record is modified.
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The Store statement is used to add
new record occurrence to the database.
Adding a new record involves:
Constructing new record
occurrence in the record’s area in
variable storage.
STORE STATEMENT
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Establish currency with correct set
occurrences in which the new
record participates as automatic
member. FIND is used to set
currencies.
Issue Store statement specifying
the name of the record type we are
adding.
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Syntax for Store is :
STORE EMPLOYEE.
While adding a new record, following
information is analyzed.
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Location where new record is added.
If location mode is CALC, then based
on Calc-key value, storage page is
calculated. For VIA, optimum
location is found based on the
location of owner record. If its
DIRECT, then program specifies the
page number through db-key.
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Where in the set occurrences the new
record should be inserted.
Before issuing Store, set order option
is also examined. Depending on
order option like first, last, next etc,
the new record is inserted in
appropriate position within the set.
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The Erase statement is used to delete
a record occurrence from the
database.
Erase statement often affects not only
the record that is being deleted but, in
some cases, also members of sets
owned by that record.
ERASE STATEMENT
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There are four options available for the
Erase statement.
ERASE
ERASE ALL MEMBERS
ERASE PERMANENT MEMBERS
ERASE SELECTIVE MEMBERS
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ERASE
We first begin by retrieving the record
we want to delete and thus the record is
made current of run unit. Then Erase is
issued naming the record type.
The record is first disconnected from all
sets in which it participates as a
member.
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Then the record occurrence is
removed (erased) from the database.
If the record is the owner of any sets,
then the record will be erased ONLY
IF those sets are empty. If we try to
erase an owner record of a non-empty
set, IDMS returns an error status code.
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MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.
OBTAIN CALC EMPLOYEE.
IF DB-STATUS-OK
ERASE EMPLOYEE
PERFORM IDMS-STATUS.
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Above example shows how an existing
Employee record is erased from the
database with unqualified Erase statement.
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ERASE ALL MEMBERS
First the record occurrence is removed
from the database.
Then all its members, both mandatory
and optional, are also removed from
the database.
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ERASE PERMANENT MEMBERS
First the record occurrence is erased from
the database.
Then all mandatory members in all sets
owned by the record are also removed.
Optional members are NOT removed but
they are disconnected from their respective
set occurrences.
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Lastly, if the erased member is the
owner of any sets then it is treated as if
an Erase Permanent is issued for it. Its
mandatory members are removed and
optional members are disconnected.
This process continues until all
members have been processed.
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ERASE SELECTIVE MEMBERS
First the record occurrence is erased from
the database.
Then all mandatory members in all sets
owned by the record are also removed.
Optional members are removed, provided
they do not participate in any other sets.
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Lastly, if the erased member is the
owner of any sets then it is treated as
if an Erase Selective is issued for it.
Its mandatory members are removed
and optional members are removed if
they do not participate in any other
sets.
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Connect statement is used to link a record
occurrence into a set.
Here, we name both the record type and
the set type.
The statement causes the current of
record type to be connected to the set
occurrence that is current of set type.
CONNECT STATEMENT
CONNECT EMPLOYEE TO DEPT-EMP.
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In case of set defined with Automatic
member option, the member record gets
connected automatically at the time of
addition to the database.
With set defined with Manual member
option, Connect statement is used to
manually set up link with a set
occurrence.
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Disconnect statement cancels the membership
of a record in a set occurrence in which it
currently participates as a member.
The record that is being disconnected, must be
defined as an Optional member of that set. If
not then IDMS returns an error code.
DISCONNECT STATEMENT
DISCONNECT EMPLOYEE FROM DEPT-EMP.
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QUEUE RECORDS
We write temporary data to a queue record in data dictionary and make this data available to other IDMS COBOL programs (DC-BATCH Mode).
Queue records are relatively permanent records in that they are not deleted when the system is shutdown or when the system crashes. They are deleted either explicitly by programmer or after the retention period specified for the queue is over.
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As the data remains permanent, it is global in a sense. Thus, queues are used to pass data information from one application to another. Specifically this feature is very widely used in online applications using ADSO.
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QUEUE MANAGEMENT COMMANDS
Commands to write a record to a queue, retrieve a queue record and delete a queue record are:
PUT QUEUE ID queue-id GET QUEUE ID queue-id DELETE QUEUE ID queue-id
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PUT QUEUE ID queue-id
This command is used to store a
queue record in the data dictionary in
the queue identified by the queue-id.
An added parameter of Last or First
can be mentioned to instruct whether
the new queue record will be placed
at the beginning or end of the queue.
Default is Last.
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GET QUEUE ID queue-id
This command is used to transfer the
contents of a queue record to a
specified location in the variable
storage. Added parameter is Delete or
Keep. Delete will remove the record
from the queue after retrieval. Keep
will retain the record in the queue.
Default is Delete.
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DELETE QUEUE ID queue-id
This command is used to delete one
or all queue records in a specified
queue. Added parameter is Current or
All. Current will delete only the
current record of the queue. All will
delete all queue records, including the
header record.
Default is Current.
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MAJOR CODES OF ERROR-STATUS
01 : FINISH
02 : ERASE
03 : FIND/OBTAIN
05 : GET
06 : KEEP
07 : CONNECT
08 : MODIFY
09 : READY
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11 : DISCONNECT
12 : STORE
14 : BIND
18 : COMMIT
19 : ROLLBACK
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Recovery and Restart of Database
219
IDMS provides facilities for recovering from problem situations and for restarting after failures occur.
Journal files are used for the recovery and restart facilities.
These facilities are dependant on the two operating environments in which an IDMS run unit executes.
220
IDMS application program can be run in two operating environments: Local Mode and Central Version.
All online applications that use IDMS/DC or some other telecommunication facilities must run under central version.
An application program that operates in batch mode can be run either under the control of central version or in local mode.
IDMS OPERATING ENVIRONMENTS
221
In central version, a single copy of the IDMS DBMS controls the operation of a particular set of IDMS application program run units.
No two central version are allowed to update the same database area. All run units that execute under the control of a particular central version are chosen such that they all access the same group of database areas.
Central Version
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Multiple copies of IDMS central version can execute in same region with each copy accessing different set of database areas.
Central version implements facilities for
recovering from system failures.
It is the preferred method of running an
IDMS application.
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A batch program that operates in local mode has its own copy of IDMS DBMS loaded into its partition, region or address space.
IDMS provides no automatic restart and
recovery facilities for applications running
in local mode.
Batch retrieval programs are good
candidates for running in local mode.
An application runs faster in local mode.
Local Mode
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IDMS maintains set of journal files that
are used to recover from failures and to
restart aborted run units.
IDMS provides automatic restart and
recovery facilities for applications
running under central version AND for
run units those use journal files.
Journal files can be put on disk or tapes.
JOURNAL FILES
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CHECKPOINTING
When a run unit begins by issuing BIND run-unit statement, IDMS system writes Begin checkpoint record on the journal file. This contains information about current status of the run-unit.
As run-unit executes, it may update records in database. As each update request is processed, IDMS writes Before Image and After Image of the updated record to the journal file.
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When applications ends the run-unit by issuing FINISH statement, IDMS writes End checkpoint record on to the journal file.
Thus, at the end of execution of a run-unit the journal files contains:
Begin checkpoint
Before and After images of all updated records and
End checkpoint
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RECOVERY and RESTART
Following are typical scenarios when run unit is terminated abnormally.
Program error may cause the run unit to terminate abnormally.
IDMS system may encounter situations that require it to terminate one or more run units abnormally. e.g. Wait time limit has been exceeded to avoid deadlock.
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In situations like these, when a run unit is
terminated abnormally after it has updated
one or more database records; IDMS
works backwards in the journal file until it
reaches the Begin checkpoint record and
uses the information in the Before images
to reverse effect of database updates
already made by the aborted run unit. The
run unit can then be restarted from the
beginning.
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RECOVERY UNIT
All run units that terminate normally, create at least two checkpoint records on the journal file: BEGIN CHECKPOINT when Ready/Bind statement is executed and END CHECKPOINT when FINISH statement is executed. Before and After images that are recorded between these two checkpoints constitute a record of all updating the run unit performed.
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The processing that a run unit performs between checkpoints is called Recovery Unit.
In the simplest form, the entire run unit constitutes a single recovery unit.
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INTERMEDIATE CHECKPOINTS
In real life scenario (for online applications), a separate run unit is started for each incoming transaction. Very few database updates are made during each run unit. Each run unit is a single recovery unit.
At times, it is required to divide the run unit into many smaller run units to reduce the time for recovery in case of run unit failure.
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This leads us to the concept of
Intermediate checkpoints.
To create intermediate checkpoints, run
unit periodically issues a COMMIT
statement to make permanent all
database updates that has already been
made.
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THE COMMIT VERB
Commit verb is used to write Intermediate checkpoints on the journal file.
There are two forms of Commit verb:
COMMIT and
COMMIT ALL
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In COMMIT, IDMS releases all locks except those placed on current records.
In COMMIT ALL, IDMS releases all locks including those on current records.
In case of abnormal termination of run unit, IDMS works backwards in the journal file and reads till the most recent COMMIT checkpoint is encountered.
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Now the journal file is not read till
Begin checkpoint. The run unit can
now be restarted from the point at
which it wrote the most recent Commit
checkpoint.
A simple thumb rule of 500 or 1000
database updates is used to issue a
Commit statement.
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THE ROLLBACK VERB
Sometimes an application program may
itself decide to back out all the updates it
made to the database since it wrote the last
intermediate (commit) checkpoint.
To back out the changes, we can issue a ROLLBACK statement.
It has two forms:
ROLLBACK and
ROLLBACK CONTINUE
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ROLLBACK
It writes a checkpoint record to the
journal file, nullifies all currencies,
automatically restore the database
using the before images from the
journal and terminate the run unit.
After issuing the unqualified Rollback,
to access the database again, we must
reissue the Bind.
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ROLLBACK CONTINUE
This form causes IDMS to restore the
database but does not terminate the run
unit.
One can access the database immediately
without reissuing the Bind.
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Locking Facilities
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Locking facilities are provided for
controlling access to areas and to individual
records when two or more run units require
access to the same database areas.
IDMS provides three levels of locking
facilities.
Area Locks
Area Usage Modes
Record Locks
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AREA LOCKS
Area locks are used to prevent areas from being updated by more than one IDMS central version or by more than one application running in local mode.
If an area is locked that is required by a
local mode application, that local mode
application ABENDS.
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If required area is NOT locked, area
lock is set by the local mode
application and no other local mode
application is given access to that area.
An area locked by a local mode
application is released when that
application executes its FINISH
statement (end of run-unit).
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If required area is locked that is required by an IDMS central version, execution of that central version continues, but no run-unit executing under the control of that central version will be given access to the already locked area. The area is said to be Varied Offline to that central version. Run-units that attempt to access an area that is varied offline are abnormally terminated.
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If required areas are NOT locked, then those areas are locked by the central version. Run-units executing under the control of that central version can access those areas. No other central version will be given access to locked areas.
An area that is locked by a central version is released only when the central version is shut down.
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AREA USAGE MODES
Area usage modes control how database areas under the control of a central version can be accessed.
The six possible usage modes for a database area are as follows:
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(Shared) Retrieval
Protected Retrieval
Exclusive Retrieval
(Shared) Update
Protected Update
Exclusive Update
The ‘Shared’ word is optional.
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Retrieval or Shared Retrieval
We are requesting only retrieval access
to the area. All other run units under
the same central version will be
allowed to RETRIEVE or UPDATE
records in the same area, provided they
do no specify usage mode Exclusive.
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Protected Retrieval
We are requesting retrieval access to the area and it also specifies that no other run unit should update records in the same area for the duration of our run unit. All other run units under the same central version will be allowed to retrieve records in the same area provided they did not specify usage mode Exclusive.
If another run unit is already updating records
in the area, our run unit will wait till that run
unit finishes.
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Exclusive Retrieval
We are requesting retrieval access to the area
and it also specifies that no other run unit
should retrieve or update records in the same
area till our run unit finishes execution. If
another run unit is already accessing the
area, our run unit will wait till that run unit
finishes.
This mode is not often used.
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Update or Shared Update
We are requesting updating access to the
area. All other run units under the same
central version will be allowed to retrieve
or update records in the same area as long
as they do not specify Protected or
Exclusive usage mode options in Ready
statements.
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Protected Update
We are requesting update access to the area and
specifying that other run units should not be able
to update records in the same area till our run unit
finishes. All other run units will be allowed to
retrieve records in the same area as long as they
do not specify Protected or Exclusive as usage
modes.
If another run unit is already accessing the area,
our run unit will wait till that run unit finishes.
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Exclusive Update
We are requesting update access to the area
and further specifying that no other run unit
should either retrieve or update records in the
same area while our run unit is executing.
This option gives our run unit exclusive use
of the area.
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Sha
red
Upd
ate
Sha
red
Ret
rieva
l
Pro
tect
ed U
pdat
e
Pro
tect
ed R
etrie
val
Exc
lusi
ve U
pdat
e
Exc
lusi
ve R
etrie
val
Shared Update Y Y N N N N
Shared Retrieval Y Y Y Y N N
Protected Update N Y N N N N
Protected Retrieval N Y N Y N N
Exclusive Update N N N N N N
Exclusive Retrieval N N N N N N
Run
Uni
t 1
Run Unit 2
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IMPLICIT RECORD LOCKS
It is maintained only for run-units operating under Central Version.
When we use usage mode options that allow more than one run-unit to access the same area while updating is taking place, IDMS sets Implicit record locks to prevent same record from being updated simultaneously by two or more run-units.
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Record locks are generally maintained for shared and protected update mode and shared retrieval mode.
Implicit locks can be either Shared or Exclusive.
Implicit Shared lock guarantees that only one run-unit is allowed to update a record while others can retrieve the same record.
Implicit Exclusive lock ensures that no other run-unit can either update or retrieve the record.
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Implicit shared locks is placed on a record when it is retrieved and Implicit Exclusive lock is placed on a record when it is accessed through a DML update verb.
Shared Lock remains in effect till currency changes.
Exclusive Lock remains in effect till run-unit ends (Finish) or until Commit is issued.
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Explicit locking is the most efficient way of placing record locks. It can be set by coding the KEEP statement or by the KEEP clause of the FIND/OBTAIN statement.
Explicit lock remain is effect until Finish is executed or Commit is issued.
e.g. Obtain Keep Calc Employee.
Obtain Keep Exclusive Calc Employee.
First is explicit shared lock and second is explicit exclusive lock.
EXPLICIT RECORD LOCKS
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Another way of setting explicit locks is to code separate Keep statement after a record is retrieved.
e.g.
Keep exclusive current.
Keep current within Dept-Employee.
Keep exclusive current within employee-area.
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