JGroups Protocol MERGE2

External

• JGroups Manual http://www.jgroups.org/manual/html/protlist.html#MERGE2
• JGroups Wiki http://community.jboss.org/wiki/JGroupsMERGE2

Internal

• JGroups

Relevance

• JGroups 3.4.3 (in this version MERGE2 is broken, does not handle "re-incarnation" well)

Overview

MERGE2 merges split groups.

Merging Process

If a group splits, MERGE2 detects the split and re-joins the group.

Split situations:

• A member is inactive (long GC), it is excluded from group, then comes back to life and it's part of an "older" view - the new view multicasted the coordinator does not include it.
• Real network partitions that leads to two subgroups with two coordinators.

The merge protocol is run by coordinator.

On the coordinator, the FindSubgroupsTask of the MERGE2 protocol periodically sends a FIND_ALL_VIEWS down the stack, which is handled by the implementation of the Discovery protocol (PING, MPING, JDBC_PING, etc). Upon receiving a FIND_ALL_VIEWS, the discovery protocol sends a GET_MBRS_REQ message.

For more details on GET_MBRS_REQ requests handling by the Discovery protocols, see:

GET_MBRS_REQ requests handling by the Discovery protocols

The FIND_ALL_VIEWS events (and consequently GET_MBRS_REQ messages) are sent at random intervals between 'min_interval' and 'max_interval' milliseconds.

The result of GET_MBRS_REQ is a list of PingData. The example below shows a FIND_ALL_VIEWS result indicating that no partition happened, for a cluster of three nodes.

TODO h45gt3

TRACE org.jgroups.protocols.MERGE2 (Timer-5,shared=tcp) -- Discovery results:
[host03/tcp]: view_id=
[host01/tcp]: view_id=[host01/tcp|3] ([host01/tcp|3] (2) [host01/tcp, host03/tcp])

This example shows a FIND_ALL_VIEWS result indicating that a cluster member (host02) maintains an obsolete view:

TODO b347tg

DEBUG org.jgroups.protocols.MERGE2 (Timer-2,shared=tcp) -- host01/tcp found different views : [host01/tcp|2], [host01/tcp|3]; sending up MERGE event with merge participants [host01/tcp, host02/tcp].
Discovery results:
[host02/tcp]: coord=host01/tcp
[host01/tcp]: coord=host01/tcp

Note the coordinator is the same, but the views have different versions. This is a situation when the merge process will be initiated.

If another coordinator for the same group receives this message, it will also initiate a merge process.

The merge process does not merge state. The app has to handle the callback to merge state.

Configuration

JGroups

  <MERGE2 max_interval="100000" min_interval="20000"/>

WildFly

<subsystem xmlns="urn:jboss:domain:jgroups:2.0" default-stack="tcp">
   <stack name="tcp">
      ...
     <protocol type="MERGE2"/>
     ...
   </stack>
</subsystem>

Parameters

max_interval

If 'max_interval' is smaller or equal with 'min_interval', we get a configuration error:

21:14:16,436 ERROR [MERGE2] @main max_interval has to be greater than min_interval

and the stack won't start.

Also See

MERGE3