Re: [jgroups-dev] Need information about Jgroup communication in jgroups 2.6.5

On 2/13/11 3:07 PM, Ashish Giri wrote:
> Hi Team,
>
>
>
> We are stuck to a scenario when we have three servers[A, B and C].All three
> servers are interconnected.
>
>
>
> What will happen if route to B from A is disconnected. However B is still
> reachable from C and C is reachable from A.
>
>
>
> How view will be represented in this case in each server?

It depends on the failure detection protocols and on who is coordinator. 
Assuming A is coordinator and VERIFY_SUSPECT is in the stack, the 
following will happen:
- In FD, A pings B, B pings C and C pings A
- A won't get any heartbeat acks from B
- A suspects B and broadcasts a SUSPECT event
- A's VERIFY_SUSPECT will try to reach B again, but doesn't get any response
- A excludes B and installs the new view {A,C} in  A and C
- B still has view {A,B,C}
- When B tries to ping C, C shuns B
- B then leave the cluster and tries to reconnect. Because it cannot 
talk to A, this will go on forever


You can actually simulate the above case easily by adding <DISCARD 
use_gui="true"/> just above UDP. This will pop up a dialog box, in which 
you can interactively drop communication  to/from any member, even 
asymmetric links are possible (A can talk to B. but B cannot talk to A).

The above behavior was changed in 2.8: shunning [2] doesn't exist 
anymore, it was replaced by merging, and merging was also improved to 
handle asymmetric cases [1].


[1] http://belaban.blogspot.com/2009/04/those-damn-edge-cases.html
[2] http://belaban.blogspot.com/2009/06/shunning-has-been-shunned.html


-- 
Bela Ban
Lead JGroups / Clustering Team
JBoss

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