seminarium on component-based software engineering

Seminarium onComponent-based Software Engineering

Marcello BonsangueLIACS – Leiden UniversityFall 2004

Callbacks and object re-entrance

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Libraries A library is a collection of procedures It is always the client to call the library Library operations run to completion before

returning control

Only states before and after a call are relevant to clients, library intermediate states are not observable

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Callbacks A callback is a procedure that can be

registered to a library so that the library can call it back Example: registration of client code to be

called when library catch user events

They are a mechanism for extensibility

Callbacks operate on the intermediate state of a library!!

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Callback: message sequence chart

Client User LibraryClient installs callbackUser calls libraryLibrary invokes callbackCallback queries library

Callback returns

Library returnsCallsResults

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A Directory service example The directory service supports callbacks

to notify clients about changes in the managed directory

A client can use this mechanism to implement a visual display of the directory content.

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The Directory service detailsLIBRARY Directory

Name=ARRAY OF CHAR;Notifier=PROCEDURE(IN name:Name);

PROCEDURE ThisFile(n:Name): Files.File;(*pre n <>“” *)(*post result=file named n OR (result=NIL and

no such file *)PROCEDURE addEntry(n:Name,f:Files.File);(* pre n<>“” and f<>NIL *)(* post ThisFile(n)=f *)PROCEDURE RemoveEntry(n:Name);(* pre n<>”” *)(* post ThisFile(n)=NIL *)

PROCEDURE RegisterNotifier(n:Notifier);(* pre n<>NIL *)(* post n registered, will be called on AddEntry

and RemoveEntry *)PROCEDURE UnregisterNotifier(n:Notifier);(* pre n<>NIL *)(* post n unregistered, will not be called

anymore *);

DirectoryDisplay (V 1.0)

IMPORT Directory;PROCEDURE Notifer(IN n:DirectoryName);BEGIN IF Directory.ThisFile(n) = NIL THEN (*entry under name n has been removed – delete n in display *)ELSE (* entry has been added under name n – include

n in display *)END Notifier;

BEGIN Directory.RegisterNotifier(Notifier);END

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The Directory contract ambiguity What happens on redefinition of an entry?

Notifier is not called … Redefine uses thisfile (that does not call notifier)

…called once … Redefine uses addEntry (that calls notifiers)

…twice Redefine uses removeEntry and addEntry (and

both call notifier)

The contract should specify who call notifier

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The Directory contract ambiguity Is the notifier called before or after the

directory is updated?

It is only sensible to call it after the update.

It should be in the contract

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A new Directory display clientLIBRARY Directory

PROCEDURE addEntry(n:Name,f:Files.File);BEGINLocate entry named nIf not found append a new entry to the files

list in this directoryElse replace old binding with new file

Call all registered notifiersEND AddEntry;

DirectoryDisplay (V 1.1)

IMPORT Directory;PROCEDURE Notifier(IN n:Directory.Name);BEGIN IF Directory.ThisFile(n) = NIL THEN (* entry under name n has been removed – delete n in display *) ELSE IF n = “untitled” THEN Directory.RemoveEntry(n); ELSE (* entry has been added under name n

include in display *) END ENDEND Notifier;BEGIN Directory.Register(Notifier);END

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Broken contract Request of a user to create file

“untitled”

Notifier() is called by the directory

File is removed by Notifier()

The AddEntry() post conditionThisFile(“Untitled”)=f

is broken

User Directory DirectoryDisplayRegisterNotifier(Notifier)AddEntry(“Untitled”,f)Notifier(“Untitled”)ThisFile(“Untitled”)RemoveEntry(“Untitled”)

Notifier(“Untitled”)

ThisFile(“Untitled”)Notifier returnsRemoveEntry returns

Notifier returnsAddEntry returns

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What is the problem The contract of the library does not says

that notifiers do not have to change the file passed Difficult to express as it is a transitive problem

Solutions: Incorporates histories of interactions in

specifications Undecidable or very complex

Use state test functions in specifications Test whether a particular operations is available

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Test functions

Example: Java security manager use test function to protect critical services from being called by untrusted services.

LIBRARY Directory

PROCEDURE InNotifier():BOOLEAN; (* pre true *) (* post a notifier call is in progress *)PROCEDURE AddEntry(IN n:name:Name;f:Files.file(); (* pre (not InNotifier() and n<>”” and f<>NIL *) (* post ThisFile(n) = f *)PROCEDURE RemoveEntry(IN n:Name); (* pre not InNotifier() *) (* post ThisFile(n)=NIL *)

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Callbacks in objects We can structure an application in

layers. Object can references other objects on different layers: Above the referenced object

Method invocation = regular call Below the referenced object

Method invocation = up-call

Every method invoked as an up-call is a possible callback.

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A text processing example A text model is a sequence of

characters that supports random access to read,insert and delete characters

Text observers can register with the model and will be notified whenever a character is inserted or deleted

A text viewer extends an observer and provides visualization

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The TextModelINTERFACE TextModel

method write(int pos,char ch);//[len:int;txt:array of char;//pre len=this.length(); (all i:0<=i<len:txt[i]:=this.read(i)) and len< this.max() and 0<=pos<=len//post this.length() = len +1 and (all i:0<=i<=len:this.read(i):=txt[i]) and this.read(pos) = ch and (all i: pos<=i<=this.length():

this.read(i)=txt[i-1]

method delete(int pos);// delete char at pos pos and shift all successive

char back to one

method max(): int;//pre true//post result=maximum length this

text instance can have

method length():int//pre true//post 0<=result<=this.max() and

result=length of text

method read(int pos):char;//pre 0<=pos<=this.length()//post result=character at position pos

method register(TextObserver x);

method unregister(TextObserver x);

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The TextObserver and TextViewINTERFACE TextObserver

method insertNotification(int pos);//pre character at position pos has just

been inserted

method deleteNotification(int pos);//pre character that was at position pos has

been deleted

INTERFACE TextView EXTENDS TextObserver

method text():TextModel//pre true//post result <> null

method caretPos():int//pre true//post 0<=result<=this.text().length()

method type(char ch)//pre caret:=this.caretPos():this.text().length()

< this.text().max()//post this.caretPos()=caret+1 and

post(this.text().write(caret,ch))

method setCaret(int pos);//pre 0<=pos<=this.text().length()//post this.caretPos()=pos

method posFromCoord(int x,int y);//pre (x,y) is valid screen coordinate//post result=x,y-coordinates corresponding to

text position pos

method insertNotification(int pos);//post display updated and

this.caretPos()=pos+1

method deleteNotification(int pos);//post display updated and

this.caretPos()=pos

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Inserting a character

ClientText Model

Text View Display

WriteinsertNotification(Remove caret mark)(Update text display)

(Redisplay caret mark)

Any callback?

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Inserting a character (revised)

ClientText View

Text Model Display

WriteinsertNotification(Remove caret mark)(Update text display)

(Redisplay caret mark)

It reflects better the situation:TextView depends on the state of TextModel See method Type

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Inter-object consistency Control is transferred to other objects while

still updating. In the example: Display calls TextView’s map methods

But they can be called by other objects too! Multiple observers could be registered

Which one gains control of display?

Consistency can be achieved by a disciplined programming semaphores synchronized methods monitors

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Intra-object consistency The real problem: Re-entrance

Re-entrance = an object method is invoked while another is still executing.

Methods have the right to change the state of their object, but what if it is called on an object that is not in a consistent state?

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Object re-entranceClass Repository mystore :Store

max = 100method register(store)

mystore:= storemethod add (elm)

if store.check(max) then // code for adding elm to repository

Class Storevar size:Int := 0 rep :Repository method insert(elm int);

size:= size+1rep.add(elm)

method check (max)if size < max return true

method notify(repository:Repository):Storerep:= repositoryrep.register(self)

User myStore Display

Notify(myrepository)Insert(elm)(add(elm)check(max)

(Redisplay caret mark)

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Re-entrance (continued) MyStore in in inconsistent state when

the message check(max) is received Obvious solution:

Switch the command linessize:= size+1rep.add(elm)

in the code of method insert(elm int)

It requires inspection of code, but components must be independently verifiable based on their contracts at interfaces

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Self-interference & re-entrance Leads to inconsistent object state

Re-entrance is necessary Recursive calls (direct or indirect)

As for callbacks it can be addressed by: Introducing test functions for clients Adding time-stamps to the state Allowing histories of communication in specification Not using method invocation

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Multithreading and re-entrance Locking doesn’t solve the re-entrance

problem

When strict re-entrance is solved, but leads to self-inflicted deadlocks

When relaxed(as in Java), suffers by re-entrance as in single threaded systems

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Histories Alternative to specification contracts

Histories are sequences of messages

Restricting the permissible traces we can get valid state transitions

Permissible traces are hard to be checked dynamically

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Without method invocation Explicit dependencies between state

and events Remember UML state machine

obj?borrow(book)

obj?return(book)

book on the shelf

book on loan by obj

obj!recall(book)