Wednesday, December 23, 2009

How WLS Serves Request

1. A client contacts the ListenThread, the entry point into WebLogic Server, which accepts the connection. It then registers the socket with a WLS component known as the SocketMuxer for further processing.

2. The SocketMuxer is responsible for reading and dispatching all client requests to the proper WLS container. It then adds this socket to an internal data structure for processing and makes a request of an ExecuteThreadManager to create a new SocketReaderRequest. This request is then dispatched by the manager to an ExecuteThread

3. As a result, the ExecuteThread becomes a SocketReader thread - it will continually run the SocketMuxer’s processSockets and checks the muxer’s queue to determine if there is work to be done. If an entry exists, it pulls it off the queue and processes it.

4. The SocketReader thread reads the client request and determines the protocol type of this client request, to create a new protocol specific MuxableSocket.

5. The MuxableSocketDiscrminator stores a new MuxableSocket with the implementation matching the protocol of the client. It also returns true to the SocketReader to notify it that the message is complete and it can be dispatched.

6. MuxableSocketDiscriminator re-registers the protocol specific version of the MuxableSocket that was created earlier. The net result is that “Step 2” is repeated, and a new protocol specific MuxableSocket is placed in the SocketMuxer’s queue for processing.

7. A socket reader will get the new protocol specific MuxableSocket off the queue and read it. It will then checks to see if the message is complete, based on the protocol. If it is, it will invoke the protocol specific MuxableSocketDiscriminator

8. Before the work requested by the client can be performed, there may be many iterations of “step 7”. This is determined by the protocol – for example, t3 will read a portion of the message, dispatch it so it can act upon the portion of the protocol read thus far.

9. The subsystem will create an ExecuteRequest and send it to an ExecuteThreadManager for processing. The request is dispatched to an ExecuteThread, and the result is returned to the client.

From a high level overview’s perspective, the SocketMuxer can be explained as follows. Each and every socket connection that comes into WebLogic Server is “registered” with the SocketMuxer - which then maintains a list of these connections, each represented by a form of the MuxableSocket interface. It then becomes the responsibility of the SocketMuxer to read and dispatch each client request to the appropriate subsystem. This is a fairly elaborate process, which is illustrated by steps 2 through 8 above.

There are only a few key things to know about the SocketMuxer:

First, it has a data structure in which it stores a socket entry for each client connected to WebLogic Server.
Second, a “socket reader” is the main component of the SocketMuxer - which really is just an execute thread that is running the SocketMuxer’s processSockets() method.

Third, the SocketMuxer does most of its work through the only interface it knows how to operate on – the MuxableSocket interface.

Socket Reader:

A SocketReaderRequest is merely an implementation of an ExecuteRequest, which is sent to the ExecuteThreadManger by the invocation of the registerSocket(). When the ExecuteThread invokes the execute() method of the SocketReaderRequest, the SocketMuxer’s processSockets() method is invoked.
So, a socket reader thread is simply a normal execute thread which runs the main processing method of the SocketMuxer, processSockets().

The acceptBacklog parameter of Weblogic server is passed to ServerSocket. The value of acceptBacklog means "The maximum queue length for incoming connection indications (a request to connect) is set to the backlog parameter. If a connection indication arrives when the queue is full, the connection is refused. "

Thus if too many connects come on the server at the same time, the server would queue this connects and process them one at a time. The value does not mean that only that many clients can connect to the server.

It does not limit the number of connections made. It limits the number of potential connections that can lie in the backlog queue. So for e.g.: AcceptBacklog is 2. If hundreds of connections were made to the server and the server has one thread to accept new connections.

This thread accepts the new connection and dispatches it to a new thread and then goes back to listening to new connections. Sample code is

while (true) {

Socket sock = serversocket.accept(); // Line 1
new MyThread(sock).run(); // Line 2


Here the thread accepts a new connection at line1. Dispatches to new thread in line 2. Evaluates the while expression and goes back to line 1. So in between the time it takes for it to get back to line 1(say T1) many new connections requests are made by the clients. These new connections lie on the accept backlog queue and this queue length is controlled by the accept backlog parameter.

If the queue length is 2, and between this time T1 several hundred connections are made to the server only 2 would get accepted and rest of them rejected. For rejection there must be too many simultaneous requests to the server, if it’s not simultaneous then the chances of queue getting full is less.

Note: Thanks to Sreedevi for helping me to provide this valuable information.