1 Remote Method Invocation (RMI) and Distributed Observers in Java Theodore Norvell.
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Transcript of 1 Remote Method Invocation (RMI) and Distributed Observers in Java Theodore Norvell.
1
Remote Method Invocation
(RMI) and Distributed Observers in JavaTheodore Norvell
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 2
The Proxy Pattern
The Client object uses its subject via an interface Thus it may be used with a real subject or with a
proxy object which represents the subject.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 3
The Proxy Pattern
The client calls the proxy, which
forwards the call (somehow) to the actual subject.
someObject : Client p : ProxyClass : Subject
operation( )
operation( )
setSubject(p)
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 4
RMI and the Proxy pattern
RMI uses the Proxy pattern to distribute objects across a network.
Recall that in the Proxy pattern a proxy and a subject share a common interface.
In RMI, objects call methods in a proxy (aka stub) on its own machine The proxy sends messages across the network to
a “skeleton” object The skeleton calls the subject object.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 5
One Remote Method Call.
(0) Client calls stub
(1) Stub messages skeleton
(2) Skeleton calls server (subject)
(3) Call returns
(4) Skeleton messages proxy
(5) Call returns
Client
Stub(Proxy)
Server
Skeleton
(1)
(0)(2) (3)
(4)
(5)
Call
Return
Message
Network
Object
Process(JVM)
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 6
Full disclosure The term “skeleton object” is out of date. The point
is that some code (that you don’t have to write) running on the server’s jvm will turn network traffic into method calls to the server object and method returns (or exceptions) into network traffic.
I’ll continue to use the term “skeleton object” to refer to the object(s) that perform these duties.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 7
Issues Concurrency
If there are multiple clients, the server may field multiple calls at the same time. So use synchronization as appropriate.
Argument passing Arguments are passed by value or “by proxy” not by
reference. Proxy generation
Proxy classes are automatically derived from the server class’s interface.
Lookup Objects are usually found via a registry (program
rmiregistry)
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 8
Nitty-Gritty The proxy and the server share an interface.
This interface must extend java.rmi.Remote. Every method in the interface should be declared to throw
java.rmi.RemoteException RemoteExceptions are thrown when network problems are
encountered, or when server objects no longer exist.
The server typically extends class java.rmi.server.UnicastRemoteObject The constructor of this class throws a RemoteException Therefore, so should the constructor of any specialization.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 9
Argument Passing Revisited Most arguments and results
are converted to a sequence of bytes; the bytes are sent over the net therefore the class should implement the
java.io.Serializable interface a clone of the argument/result is constructed on the other
side. The effect is pass by object value, rather than by object
reference. But
objects that extend java.rmi.server.RemoteObject instead have a proxy constructed for them on the other side I call this “pass by proxy”. Essentially pass by reference
So each argument, result, exception type should be a primitive type, Serializable, or extend RemoteObject
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 10
An Example – Distributed Othello Othello is a two person board game My implementation uses the observer pattern
so that, when the (game state) model changes, all observers are informed of the change.
I wanted to put the model on one machine and the observers on other machines.
Hence I implemented the observer pattern with RMI.
This is example othello-2 on the website.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 11
The Observer Pattern
Subject alerts Observers of changes of state.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 12
Observer Pattern
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 13
Object diagram for Othello 2
:BoardView
:Animator
: BoardView
:Animator
:RemoteGameModel
Client Host 0 Client Host 1
Server Host
Observes Observes
ObservesObserves
Notifies Notifies
Notifies Notifies
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 14
… with proxies and skeletons: BoardView
:Animator
:RemoteGameModel
Observes
Skeleton
Animator Proxy
Skeleton
Remote Game Model’s Proxy
Observes
Communicateswith
Communicates with
Notifies
Server Host
Notifies
Animator Proxy
: BoardView
:Animator
Observes Skeleton
Remote Game Model’s Proxy
Observes
Communicateswith
Communicateswith
Notifies
Notifies
Notifies
ClientHost
Another Client Host
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 15
The Observer Pattern for Othello
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 16
Proxy for the Remote Game Model
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 17
Proxies for the Animators
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 18
Animator / RemoteGameModel Relationship
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 19
Typical sequence —slightly simplified A remote client calls a synchronized mutator on the
RemoteGameModel via its stub & skeleton The RemoteGameModel updates its state and notifies each
Animator via its stubs & skeletons. The Observers (Animator objects) call the
RemoteGameModel accessor “getPieceAt” via its stubs and skeleton.
(Therefore this accessor must not be synchronized!) getPieceAt returns
The update routines return. The original mutator call returns and the
RemoteGameModel becomes unlocked.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 20
Typical sequence —slightly simplifiedsomeObj : RemoteGameModel : RemoteGameModel_Stub : Animator_Stub : Animator
move(int, int, int)
notifyObservers(...)
update(...)
update(...)
Across net with help of skeleton
getPieceAt(int, int)
getPieceAt(int, int)
Across net with help of skeleton
Calls to other accessors are similar
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 21
Concurrent Notification The previous sequence is slightly simplified.
In fact the Animators return from update immediately (so that all can be informed essentially at the same time). The Animation threads will inform the
RemoteGameModel of when they have completed their animation.
The RemoteGameModel waits until it has been informed that all animations are complete. The effect is that the animations can happen
concurrently, yet the RemoteGameModel does not unlock until all animations are complete and all models agree on the board state.
See RemotelyConcurrentlyObservable for details.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 22
Concurrent Notification
I simplifed this diagram a bit by omitting the callbacks from the animators to the game model and by omitting the stubs and skeletons.
During this time the animators animate (using new threads created in update). The last thing these threads do is call decrement (via RMI). Once all the animation threads have called decrement, the original server thread returns from waitForZero
someObj : RemoteGameModel : RemoteCountMonitor : Animator : Animator
move(...)
notifyObservers(...)
update(...)
update(...)
waitForZero( )
decrement( )
decrement( )
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 23
Naming the Server Normally an object’s address serves as a unique
identifier But this only makes sense in the context of a given JVM
process. We would like objects to have unique identifiers that are
unique in the world. The rmiregistry allows you to give a URI to an object And to obtain a proxy for an object that has a URI. URIs are: rmi://host/name The host must be running a rmiregistry process and that
process should have the appropriate class files on its CLASSPATH
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 24
Binding the server to a name.
The main routine for the server The static method bind in Naming gives a URI to
gameModelpublic static void main( String[] args ) { try { RemoteGameModel gameModel = new RemoteGameModel(); String name = args[0] ; Naming.bind( name, gameModel ) ; System.err.println("Game model bound to "+name);} catch( java.net.MalformedURLException e) { … } catch( AlreadyBoundException e) { … } catch( RemoteException e ) { … } }
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 25
Bind
i.jar
Webserver
java -Djava.rmi.server.codebase=U -cp s.jar ServerMainClass
bind
get
U is the URL of i.jar
gm
GameModel object
Proxy objects
Skeleton object
read
i.jar only needs the class filesfor the server object’s interface.
RMIRegistry
JVMs
Animator object
1. A call Naming.bind(uri,gm) is made.
1.1 A proxy object is created. 1.1.1 A serialization of the proxy is sent to the registry.
1.1.1.1 A copy is created in the registry’s vm.
The uri specifies the host (and port) of theRMI registry process.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 26
Looking up the server object
The clients obtain a proxy for the game model using Naming.lookup( URI )
From ClientMain.javapublic static void main( String[] args) {
RemoteGameModelInterface proxy = null ;
try {
String name = args[0] ;
proxy = (RemoteGameModelInterface)
Naming.lookup( name ) ; }
catch( java.net.MalformedURLException e) { … }
catch( NotBoundException e) { … }
catch( RemoteException e) { … }
… continued on the slide after next …
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 27
Looking up the server object
java -Djava.rmi.server.codebase=U -cp s.jar ServerMainClass
java -cp c.jar ClientMainClass
lookup(uri)
GameModel object
Proxy objects
Skeleton object
RMIRegistry
JVMs
Animator object
2. Naming.lookup(uri) is called on the client vm.
2.1 A lookup message is sent to the RMI registry.
2.1.1. A serialized copy of the proxy is sent to the client. And a proxy is built in the client vm.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 28
Closing the loop The client then can use the proxy. E.g.
Continuing the ClientMain main routine…Animator animator0 = null ;try { animator0 = new Animator( proxy ) ; }catch( RemoteException e ) { … }
The constructor for Animatorpublic Animator( RemoteGameModelInterface gameModel )throws RemoteException { super() ; … gameModel.addObserver(this); }
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 29
Adding a remote Observer.
Each Animator calls addObserver(this) on the RemoteGameModel’s proxy. Since Animator implements RemoteObject, it is
passed by proxy, meaning a proxy for the Animator is constructed in the JVM
of the RemoteGameModel. (see next slide.)
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 30
Adding a remote observer
gm
GameModel object
Proxy objects
Skeleton objects
JVMs
Animator objectani
gmp
addObserver(a)
addObserver( )
addObserver(anip)
anip
3 the animator (ani) calls addObserver(this) on the game model’s proxy (gmp)
3.1 the client vm sends a serialized proxy for ani to the server vm.
3.1.1 The animator’s proxy (anip) is constructed in the server vm.
3.1.1.1 An addObserver(anip) message is sent to the game model, which saves anip’s address to its observer list.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 31
The final hookup (again)
:Animator
:RemoteGameModel
Client Host 0
Observes
Skeleton Animator Proxy
Skeleton
Remote Game Model’s Proxy
Communicates with
Communicates with
Notifies
Server Host
Calls
Calls
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 32
Creating the stubs and skeletons First we write a server class: E.g.
public class RemoteGameModel
extends RemotelyConcurrentlyObservable
implements RemoteGameModelInterface
{
public RemoteGameModel() throws RemoteException {
super() ; } … }
We compile it with “javac” The stub objects will be created automatically at
run time.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 33
Starting an RMI registry In order to build a proxy object, the RMIRegistry
process needs access to the .class file(s) for the interface.
Option 0. Run the RMIRegistry with the appropriate .class files on its classpath.
Option 1. Put the class files on a webserver and, when the server is run, use the following option -Djava.rmi.server.codebase=URL
(Option 1 was illustrated earlier in the animation of the bind process.)
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 34
A Few Words of Warning
RMI makes it seductively easy to treat remote objects as if they were local.
Keep in mind Partial Failure
Part of the system of objects may fail Partial failures may be intermittent Network delays On a large network, delays are indistinguishable from
failures In the Othello example, failure was not considered. The
system is not designed to be resilient to partial failures.
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 35
A Few Words of Warning (cont.) Keep in mind (cont.)
Performance Remote calls are several orders of magnitude more expensive
than local calls (100,000 or more to 1) E.g. in the Othello example, this motivated splitting the model into
local (Animator) and remote (RemoteGameModel) copies. Concurrency
Remote calls introduce concurrency that may not be in a nondistributed system. E.g. in Othello, I had to be careful not to use synchronized for
accessors called by remote Observers and to consider the data integrity consequences of not doing so. (The reason was that the call to the mutator and the call to the accessor are in different threads each spun off the skeleton. Therefore the call to the mutator would lock out the call to the accessor, if both were synchronized.)
© 2004-10 T. S. NorvellEngineering 5895 Memorial
University RMI. Slide 36
A Few Words of Warning (cont.) Keep in mind (cont.)
Semantics changes In Java, local calls pass objects by pointer value. Remote calls pass objects either by copy or by copying
a proxy. E.g. in the Othello game as I converted from the
nondistributed to a distributed version, the semantics of some calls changed, even though I did not change the source code for those calls.