Note that as of 2.8, shunning has been removed (replaced by merging).

Let's say we have 4 members in a group: {A,B,C,D}. When a member (say D) is expelled from the group, e.g. because it didn't respond to are-you-alive messages, and later comes back, then it is shunned.

Shunning is essentially a majority vote to expel a slow or unresponsive member from the group. Such a member could cause degradation of performance for the entire group, so it needs to be excluded. Also, a slow member could slow or even prevent purging of stable messages (http://wiki.jboss.org/wiki/Wiki.jsp?page=JGroupsPbcastSTABLE), so in the worst case, all members could run out of memory because they would never purge stable messages. Exclusion of such a member resumes progress in the stability protocol.

Shunning causes a member to leave the group and re-join, if this is enabled on the Channel. To enable automatic re-connects, the AUTO_RECONNECT option has to be set on the Channel:

channel.setOpt(Channel.AUTO_RECONNECT, Boolean.TRUE);

To enable shunning, set FD.shun and GMS.shun to true.

Let's look at a more detailed example. Say member D is overloaded, and doesn't respond to are-you-alive messages (done by the failure detection (FD) protocol). It is therefore suspected and excluded. The new view for A, B and C will be {A,B,C}, however for D the view is still {A,B,C,D}. So when D comes back and sends messages to the group, or any individiual member, those messages will be discarded, because A,B and C don't see D in their view.

D is shunned when A,B or C receive an are-you-alive message from D, or D shuns itself when it receives a view which doesn't include D.

So shunning is always a unilateral decision. However, things may be different if all members exclude each other from the group. For example, say we have a switch connecting A, B, C and D.

If someone pulls all plugs on the switch, or powers the switch down, then A, B, C and D will all form singleton groups, that is, each member thinks it's the only member in the group. When the switch goes back to normal, then each member will shun everybody else (a real shun fest ). This is clearly not desirable, so in this case shunning should be turned off:

<FD timeout="2000" max_tries="3" shun="false"/> ... <pbcast.GMS join_timeout="3000" join_retry_timeout="2000" shun="false"/>