[jboss-jira] [JBoss JIRA] Commented: (JGRP-549) ConnectionTable: spurious Connection instances on concurrent connects

[Bela Ban (JIRA)]

    [ http://jira.jboss.com/jira/browse/JGRP-549?page=comments#action_12368007 ] 
            
Bela Ban commented on JGRP-549:
-------------------------------

A simple mechanism for agreement on concurrent send is (on A):
- Before actively establishing a connection (new Socket()) to B, we place an entry for B into the conn table, the key is B, value is null
- On accept(B) (a passive connection establishment from B):
  - If there is no key for B, we create a new Connection for B and add it to the conn table
  - Else:
      - If B's address > A's address --> remove the existing entry for B and add the newly created Connection for B 
         to the conn table
      - Else close the incoming connection from B

We may still have to run getConnection() in a loop, random sleep after an unsuccessful attempt to create a connection (but terminate if the host cannot be reached !).

This determinism ensures that only one of the 2 connection end points will get closed, thus only 1 Connection will be created on concurrent send

> ConnectionTable: spurious Connection instances on concurrent connects
> ---------------------------------------------------------------------
>
>                 Key: JGRP-549
>                 URL: http://jira.jboss.com/jira/browse/JGRP-549
>             Project: JGroups
>          Issue Type: Bug
>            Reporter: Bela Ban
>         Assigned To: Bela Ban
>             Fix For: 2.6
>
>
> If we have members A and B, and they don't yet have connections to each other, and start sending messages to each other at the exact same time, then we run into the following scenario:
> - A attempts to connect to B (new Socket())
> - B attempts to connect to A
> - A accepts B's connection and adds an entry for B into its table
> - B adds an entry for A into its table
> - B gets the accept() from A, but doesn't add the new Connection into its table because there is already a Connection. This leads to a spurious Connection object, which will only be closed and garbage collected once A or B leaves the cluster
> - We used to close Connections which were already in the table, but this lead to ping-pong effects where concurrent initial senders always closed each others connections
> - Even worse: a new Socket() always creates a new Connection object and places it into the table, *regardless* of whether a Connection for a given destination already exists or not !
> GOAL:
> #1 Have only 1 TCP connection between any 2 members
> #2 Establish a handshaking mechanism, to avoid spurious connections
> SOLUTION:
> - When A establishes a connection to B, A needs to get agreement from B first (and vice versa)

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