Strong Code Mobility Leuven 14 oktober 2004 Why

Strong Code Mobility Leuven, 14 oktober 2004

Why Code Mobility? Relocation of services is necessary in environments where the context frequently changes Users moving about geographically Collaborating service components need to migrate independently Migration must be seamless Leuven, 14 oktober 2004 2

Work Package on Code Mobility Strong Mobility Progressive Mobility Smart Mobility Proof-of-concept high-level virtual machine supporting strong code mobility High-level programs Chit. Chat VM Java Virtual Machine Leuven, 14 oktober 2004 3

Strong Mobility: Approach Proof-of-concept high-level virtual machine supporting strong code mobility High-level programs interpreted by Chit. Chat VM resume execution Chit. Chat VM make run-time state explicit in Java objects Java Virtual Machine restore state Java Virtual Machine serialize state to bytestream send stream through Java I/O sockets Leuven, 14 oktober 2004 4

Types of Mobility Data Context Control Context Resources Context Weak Mobility Semi Strong Mobility Full Mobility Java Applets Most Middleware Solutions Threads ‘become’ java. io. Serializable Process Migration (for e. g. load-balancing) Leuven, 14 oktober 2004 5

Why Strong Code Mobility? public void m() { public void n(Object obj) { Semi strong mobility Object x =. . . ; //. . . is far less expressive… o. n(this); move(obj, some. Location); In an Am. I environment, you // perform some computation with x } cannot anticipate every move! } public void m() { should be executed at the remote location… public void n(Object obj, Object x) { Object x =. . . ; o. n(this, x); //. . . // code hereafter never executed! move(obj, some. Location); obj. after. Move(x); } public void after. Move(Object x) { } // perform some computation with x } Leuven, 14 oktober 2004 6

Why not just use ‘Java’? No provisions for mobility Middleware/language extensions interfere with standard Java semantics often give up JVM compatibility Technical problems with recursive transmission of classes A Virtual Machine can abstract from the underlying host system Leuven, 14 oktober 2004 7

Move Considered Harmful Imagine combinations of… regular control flow (if, while, …) late binding polymorphism meta-programming, reflection, aspects move Which objects will be residing where? Conjecture: move is the ‘goto’ of mobility move(obj, 134. 184. 43. 120) goto 0 xff 408 a 7 e move(doc, the. Printer) Wanted: goto label. Mobility Structured We need abstractions to control the “loci of objects” Leuven, 14 oktober 2004 8

Chit. Chat: Structured Mobility Model based on active objects New kind of method ‘modifier’: move Move methods ‘pull’ objects from one VM to another: o. m() move Leuven, 14 oktober 2004 move 9

Structured Mobility: Example active object mobile. Service { public move void migrate() { // computation happens at site of caller // adapt to new context mobile. Service. migrate. To(. migrate() ) } public void migrate. To(host) { host. accept(this); } } active object host { public void accept(object) { if (object meets requirements) { object. migrate(); . accept( host ) mobile. Service } } } Leuven, 14 oktober 2004 10

Demo: Chat Client Application Simple client-server architecture Server automatically relocated to host of ‘most popular’ client server send. Msg(“Hello World”) client Leuven, 14 oktober 2004 client 11

Demo: Chat Client Application active object chat. Server { Object[] clients; int occupancy, max. Clients, max; a chat. Server is an active object… public chat. Server(channel, max. Clients) { clients = new Object[max. Clients]; occupancy = max = 0; …with a reference to this. max. Clients = max. Clients; its connected clients this. register(channel); } public move void come(nam) { System. out. println(“arrived at client ”+nam); } public Object register. Client(nam) { move method used by clients to pull the server towards them download new chat. Client(nam); } } Leuven, 14 oktober 2004 weak mobility: client code can be downloaded by a remote machine 12

Demo: Chat Client Application active object chat. Client extends chat. Server { String nam; int count; clients have a dynamic relation with the server } public chat. Client(nam) { this. nam = nam; count = 0; super { if (occupancy == max. Clients) error("Sorry, channel is full"); else clients[++occupancy] = this; } public void send. Msg(msg) { } if (++count > super. max) { super. come(nam); public void receive. Msg(from, msg) { super. max = count; System. out. println(from+”: ”+msg); if I am the most active client, } } pull the server towards me super { for (int i=0, i < occupancy, i++). . . clients[i]. receive. Msg(nam, msg); } broadcast the message to } all other clients Leuven, 14 oktober 2004 13