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Distributed Computing Environment
M. Habibullah Pagarkar
Aniket Kate
Prashant Ponde
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DCE Basics
n Vender Independent Distributed Computing Environment
n Not An Os Or An Application
n An Integrated Set Of Services And Tools That Can Be Installed As A Coherent Environment On The Top Of Existing Os
n Serve As A Platform For Building & Running Distributed Applications
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Layers
Comp Hardware
Operating System
Application
n Middleware Software Layer DCE
n Conceals Differences Between Machines & Provides Transparency
OS & Network
DCE Software
DCE Applications
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DCE: Blend Of Technologies
Components:
n Thread Package
n RPC Facility
n Distributed Time Service
n Name Services
n Distributed File Services
n Distributed Security Services
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DCE Cells
n Group Of Users, Machines Or Other Resources Having A Common Purpose & Share Common DCE Services
n Helps To Break Down A Large System Into Manageable Units
n Minimum Configuration :Ø Cell Directory ServerØ Security ServerØ Distributed Time ServerØ One Or More Client Machines
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Factors For Cell Boundaries
n Purpose
n Administration
n Security
n Overhead
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Threads in DCE
n Fundamental building blocks
n On top of OS+DCE components
n Run mainly in User Space
n Thread package – collection of user level library procedures that allow processes to manipulate threads
n 4 States – Running, Ready, Waiting, Terminated
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Problems with threads
n Problem 1 – Signalsq Synchronous – memory violation
handled by current thread
q Asynchronous – external
given to the threads (if any) waiting for it
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Problems in Threads
n Problem 2 – standard libraryq If allocating memory and thread switched :
inconsistent data
n Solutionq Jackets around some library procedures (read,
fork are jacketed)
q mutex
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Problems in Threads
n Problem 3 – Error No.q System calls returned with error no.
q If one makes call, but just after completion another scheduled, error no. gets over-written.
n Solutionq Thread specific version of err no. saved and
restored
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Scheduling
n Similar to scheduling processes
n Threads in DCE have priorities
n DCE has three scheduling algorithmsq FIFO
q Round Robin
q Default
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Synchronisation
n mutexq Used when it is essential to prevent multiple thread from
accessing the same resource at the same time. Eg. Moving through a linked list
q Fast mutex – analogous to a lock
q Recursive mutex – if subroutine also wants a lock, it is granted
q Attempt to lock a mutex that is already locked returns an error
n Condition Variablesq Conjunction with mutex
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RPC
n DCE based on client/server modeln RPC system makes it possible for a system
to access a remote system by simply calling a remote procedure
n RPC hides details from clientsn Handles message transports in both
directionsn As a result, client and servers are highly
independent
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RPC
n Interface definition defines the syntax of callsn Written in IDL – Outputs header, client stub,
server stubn Server location done in 2 stepsq Locate server machineq Locate correct process on that machine
n To communicate reliably, an end point is required which are dynamically assigned
n Performing RPC – At-most-once
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Security In DCE
A Security Infrastructure That Provide:
Ø Authorization
Ø Authentication
Ø Encryption At Application
Selectable Levels
Ø Central Security Database
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Main Components
Authenticationserver
Privilegeserver
Registry Server
Registry Database
Registryeditor
ACL editor
Login process
Clientprocess
Application server
ACLmanager
Resource ACL
Security server node
Administrator
Client node Application server node
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DCE Distributed File System
n Derived from Andrew File Systemn Uses threads to handle multiple file access
requests simultaneously and RPC for client/server and directory service
n File Model – unstructured. One file can contain 242 bytes
n File System Model –FilesàDirectoriesàFilesetsàAggregate.
n One disk partition has one aggregate
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DFS
n Distributed File Services uses data caching model
n DFS server machine has following componentsq Episodeq Token managerq File exporterq Fileset serverq Fileset location serverq Replication server
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File Sharing / File Caching
n Every read operation on the file sees the effect of all previous write operations performed on that file
n Local memory used for caching in diskless terminals
n As long as client possesses the token for specified operation, data is valid.
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Replication
n Filename mapped to all file-servers having a replica of the file.
n Unit of replication is a filesetn Replication server is responsible for
maintaining consistencyn Updates only on primary copy, replication
server periodically sends updated copy outwards
n Every update to a file results in a log
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THANK YOU……