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From Apache Wiki <wikidi...@apache.org>
Subject [Lucene-hadoop Wiki] Update of "ipc" by MikeCafarella
Date Sun, 23 Apr 2006 06:39:15 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Lucene-hadoop Wiki" for change notification.

The following page has been changed by MikeCafarella:
http://wiki.apache.org/lucene-hadoop/ipc

New page:
= IPC = 

IPC, for InterProcess Communication, is a fast and easy RPC mechanism.
Unlike Sun's standard RPC package, it does not use standard Java
serialization and instead requires the author of every class to write
the relevant serialization method.  That extra work might seem like a drawback,
but if you've ever tried to debug Sun's class versioning system, you
realize that the extra work is in fact welcome.

IPC does not require a special compiler of any kind to create network
stubs and skeletons.  Rather, it uses introspection to examine a declared
"publicly-available interface" and determine how to marshall/unmarshall
arguments.

IPC is used as the internal procedure call mechanism for all of Hadoop
and Nutch.

== Use Model ==

IPC is a client/server system.  A server process offers service to others
by opening a socket and exposing one or more Java interfaces that remote callers
can invoke.  User server code must indicate the port number and an instance of
an object that will receive remote calls.  (see RPC.getServer())

A client contacts a server at a specified host and port, and invokes methods
exposed by the server.  User client code must indicate the target hostname
and port, and also the name of the Java interface that the client would like
to invoke.  While a single IPC server object can expose several interfaces
simultaneously, a client can invoke only one of them at a time.  (see RPC.getClient())

There is no way for an IPC server to invoke methods of the client.  There are places
in Hadoop where bidirectional communication is helpful (e.g., in ["DFS"], where the
Name and Data nodes must report status to each other).  In these cases, one side
acts as a client, making the same call over and over again.  The server always 
returns a special "status" object, which the client may then interpret as a 
request to perform work.

== Under the covers ==

The IPC mechanism automatically inspects the client's requested interface, plus
the server's exposed interfaces, and figures out how to marshall/unmarshall arguments
for the remote call.  This system works fine as long as all arguments in methods
consist of either Java's builtin types, or String, or an implementation of the Writable
interface.  (Or an array of one of those types)

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