FVM: Federated Virtual Machines

FVM's SourceForge Home
  1. Intro
  2. LocalVm
  3. Starting a Server
  4. Examples
  5. ClassLoader
  6. Serialization
  7. Design Considerations
  8. Exceptions
  9. Security
  10. Reliability
  11. Configuration
  12. System Requirements
  13. License
  14. Javadoc

Intro

FVM is a small framework for writing Java applications that run across multiple Java Virtual Machines (JVMs). An application initiated on host #1 may execute portions of itself on host #2 if host #2 is running an FVM server. It does this using network ClassLoaders, java.io.ObjectIn/OutputStream and remote object references proxied by CGLIB.

LocalVm

Every JVM has a singleton instance of fvm.LocalVm, the entrypoint to the FVM framework. To get a reference to the local Vm use:
Vm localVm = LocalVm.vm();
To get a reference to another Vm use:
Vm otherVm = LocalVm.vm().connect("otherhost");
The other host must be running an FVM server.

Starting a Server

To start an FVM server, install a Java runtime environment and execute:
java -jar fvm.jar

Examples

TypicalFVM
System.out.println("Hello world");
Vm otherVm = LocalVm.vm().connect("otherhost"); otherVm.system().out().println("Hello world");
File f = new File("myfile.txt"); InputStream in = new FileInputStream( f );
Vm otherVm = LocalVm.vm().connect("otherhost"); IFile f = otherVm.fileSystem().getFile("myfile.txt"); InputStream in = f.openInputStream();
File f = new File("myfile.txt"); OutputStream out = new FileOutputStream( f, true );
Vm otherVm = LocalVm.vm().connect("otherhost"); IFile f = otherVm.fileSystem().getFile("myfile.txt"); OutputStream out = f.openOutputStream(true);
Runnable task = new com.myhost.MyTask(); new Thread( task ).start();
Vm otherVm = LocalVm.vm().connect("otherhost"); Runnable task = otherVm.newInstance( "com.myhost.MyTask", null ); otherVm.newThread(task).start();
com.myhost.MyTask and its dependencies need not be in the classpath on otherhost.
FVM will load them from the originating host.
Runtime rt = Runtime.getRuntime(); Process proc = rt.exec( new String[]{"myscript.sh"} ); proc.waitFor();
Vm otherVm = LocalVm.vm().connect("otherhost"); IRuntime rt = otherVm.runtime(); IProcess proc = rt.exec( new String[]{"myscript.sh"} ); proc.waitFor();
Class.forName("org.postgresql.Driver"); Connection conn = DriverManager.getConnection( "jdbc:postgresql://myhost/mydb",username,password);
Vm otherVm = LocalVm.vm().connect("otherhost"); otherVm.driverManager().loadDriver("org.postgresql.Driver"); Connection conn = otherVm.driverManager().getConnection( "jdbc:postgresql://myhost/mydb",username,password);
org.postgresql.Driver and its dependencies need not be in the classpath on otherhost.
FVM will load them from the originating host.

ClassLoader

The FVM ClassLoader attempts to find classes in the following locations and order:
  1. local JVM classpath
  2. FVM ClassLoader of other JVM associated with the current Thread
  3. FVM ClassLoaders of the JVMs listening on the hosts specified in the host.path configuration variable

Serialization

The following object types are passed by value. All other types are passed by reference. This includes objects inside of arrays, Collections and Maps.

Design Considerations

Remote method calls are much slower than local method calls. To minimize remote method calls, save references to remote objects called multiple times. For example:
SlowLess Slow
Vm otherVm = LocalVm.vm().connect( "otherhost" ); for( int i = 0; i < 10; i++ ) { otherVm.system().out().println( "Test " + i ); }
Vm otherVm = LocalVm.vm().connect( "otherhost" ); PrintStream out = otherVm.system().out(); for( int i = 0; i < 10; i++ ) { out.println( "Test " + i ); }
Classes tagged with the fvm.ValueObject interface are passed by value between JVMs. This means that method calls on instances of ValueObjects are always fast, local calls. Your classes should implement fvm.ValueObject if they 1.) represent business entities (like Java Beans) and 2.) don't care in which JVM their methods execute.

Exceptions

If a Throwable is thrown when calling a method on another JVM, FVM wraps that Throwable with a new fvm.FvmException and serializes it to the caller. FvmException extends RuntimeException and identifies the host in which the Throwable originated.

Security

FVM's purpose is to make it easy to run arbitrary code on FVM-enabled hosts. It's a security nightmare. FVM is intended for use behind a firewall on a trusted network.
Do not run an FVM server on a public network.
To mitigate the risk, FVM supports SSL, an "allowed hosts" list, and simple authentication.

Reliability

FVM has no built-in failover or recovery mechanism. A typical FVM application creates objects on multiple JVMs and passes object references between JVMs. The objects and references all live in memory so if one JVM fails, the application fails. Used in this way, FVM is not suitable for mission-critical applications.

If your application just uses FVM to perform procedural calls to stateless remote objects (similar to calling stateless EJBs or SOAP web services) then FVM should be no less reliable. Probably. Try it out.

Configuration

Normally, you just start FVM servers on all your firewalled hosts with no configuration. However, there are some config params you may control. FVM is configured with a .fvm.conf file located in your home directory. If FVM can't find this file, it creates one with insecure default values you may override. You can also override values on a per-JVM basis by passing -D arguments when starting the JVM.
You should change the default password in your .fvm.conf files.
KeyDescription
server.portthe port an fvm.impl.FvmServer will listen on
server.usernamethe username required for connections
server.passwordthe password required for connections
beacon.broadcast.enabledturns on/off broadcasts of multicast "I am here" messages [true/false]
beacon.broadcast.multicast.groupip for multicast beacon broadcasts
beacon.broadcast.interval.secondsfrequency of beacon broadcasts (in seconds)
hosts.allowedcomma-separated list of hosts allowed to connect to this FVM
host.patha classpath composed of hostnames instead of directories/jars.
ssl.enabledturns on/off use of SSL for connections to/from this host [true/false]
keystore.fileJava keystore file to use if SSL is enabled
keystore.passwordpassword for Java keystore file

System Requirements

FVM runs on JDK 1.4 and above.

License

FVM has the same license as CGLIB:
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