Seminarium on Component-based Software Engineering Callbacks and object re-entrance Marcello Bonsangue LIACS – Leiden University Fall 2004

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 31/12/2018 Seminarium CBSE

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!! 31/12/2018 Seminarium CBSE

Callback: message sequence chart User Client Library Client installs callback User calls library Library invokes callback Callback queries library Callback returns Calls Results Library returns 31/12/2018 Seminarium CBSE

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. 31/12/2018 Seminarium CBSE

The Directory service details LIBRARY 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); (* 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; Directory.RegisterNotifier(Notifier); END 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

A new Directory display client LIBRARY Directory PROCEDURE addEntry(n:Name,f:Files.File); BEGIN Locate entry named n If not found append a new entry to the files list in this directory Else replace old binding with new file Call all registered notifiers END 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); (* entry has been added under name n include in display *) END END Notifier; Directory.Register(Notifier); 31/12/2018 Seminarium CBSE

ThisFile(“Untitled”)=f Broken contract Request of a user to create file “untitled” Notifier() is called by the directory File is removed by Notifier() The AddEntry() post condition ThisFile(“Untitled”)=f is broken User Directory DirectoryDisplay RegisterNotifier(Notifier) AddEntry(“Untitled”,f) Notifier(“Untitled”) ThisFile(“Untitled”) RemoveEntry(“Untitled”) Notifier(“Untitled”) ThisFile(“Untitled”) Notifier returns RemoveEntry returns Notifier returns AddEntry returns 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

Test functions 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 *) Example: Java security manager use test function to protect critical services from being called by untrusted services. 31/12/2018 Seminarium CBSE

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. 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

The TextModel INTERFACE TextModel method max(): int; //pre true 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 //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); 31/12/2018 Seminarium CBSE

The TextObserver and TextView INTERFACE 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 //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 31/12/2018 Seminarium CBSE

Inserting a character Any callback? Text Model Text View Client Display Write insertNotification (Remove caret mark) (Update text display) (Redisplay caret mark) 31/12/2018 Seminarium CBSE

Inserting a character (revised) Text View Text Model It reflects better the situation: TextView depends on the state of TextModel See method Type Client Display Write insertNotification (Remove caret mark) (Update text display) (Redisplay caret mark) 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

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? 31/12/2018 Seminarium CBSE

Object re-entrance Class Store var size:Int := 0 rep :Repository method insert(elm int); size:= size+1 rep.add(elm) method check (max) if size < max return true method notify(repository:Repository):Store rep:= repository rep.register(self) Class Repository mystore :Store max = 100 method register(store) mystore:= store method add (elm) if store.check(max) then // code for adding elm to repository User myStore Display Notify(myrepository) Insert(elm) (add(elm) check(max) (Redisplay caret mark) 31/12/2018 Seminarium CBSE

Re-entrance (continued) MyStore in in inconsistent state when the message check(max) is received Obvious solution: Switch the command lines size:= size+1 rep.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 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

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 31/12/2018 Seminarium CBSE

Without method invocation Explicit dependencies between state and events Remember UML state machine obj!recall(book) obj?borrow(book) book on the shelf book on loan by obj obj?return(book) 31/12/2018 Seminarium CBSE