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From Srikanth Veeramachaneni <sveer...@gmail.com>
Subject [MINA] Using MINA to write a proxy server like application
Date Fri, 02 Dec 2005 00:42:44 GMT
Hello,

I have an existing proxy-like server which currently uses blocking IO.
The role of the proxy server is to communicate with the client using SSL
and act as a proxy to the application server communicating with it using
plain TCP.

             TCP/SSL                           TCP
  Client -----------------> Proxy Server -----------> Application Server


The protocol being supported is a stateful synchronous command/response
protocol where the client initiates all the communication by sending
commands and the server responds by sending responses. The only exception
is that at the time of connection establishment the server sends a
greeting back to the client.

The normal process flow for the proxy would be something like below

  1. Client initiates SSL connection
  2. Proxy accepts connection and starts SSL handshake
  3. After SSL handshake is successfully completed, proxy establishes
     connection to the application server
  4. Read greeting from application server
  5. Send greeting to client
  6. Read command from client
  7. Send command to application server
  8. Read response from application server
  9. Send response to client
 10. Repeat steps 6 through 9

I am trying to see if I can use MINA to support the proxy server
and am in the process of building a prototype.

I basically took the EchoServer example and started modifying it to serve
my needs. I also made modifications to the SSLHandler class so that
sessionOpened() will be invoked a second time once the SSL handshake is
complete (as suggested by Maarten Bosteels).

At this point, in the sessionOpened() method, I am opening a session to
the application server using SocketConnector. I then set the source client
session as an attachment in the destination app server session and the
destination app server session as an attachment in the source client
session, so that the source client session and the destination app server
session can pipe data to each other in the dataRead() methods.

This approach works but the problem I am facing is that sometimes the
greeting from the APP server is being read even before the source client
session is set as an attachment to the destination app server session. The
source client session is absolutely needed here as I need to write the
greeting to it.

In order to get around this problem I made the dataRead() method of the
destination session handler wait in a loop until the source client session
becomes visible and then only it proceeds to write the greeting to the Client.

Finally my questions

 - I consider my above solution to the greeting problem a very bad hack.
   What is the best approach to handle such a need?
 - Is my approach overall reasonable? i.e. opening a session to
   the application server once the SSL handshake is completed and then
   piping the read event to each other in the dataRead() methods of each
   of these sessions and using the attachments to enable the data piping.
 - Are there better approaches or alternatives?

I am quite new to MINA and NIO in general. So, please feel free to point out
if I am totally being inefficient here.

Attached are the relevant classes from my prototype.

thanks,
Srikanth

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