Wednesday, January 03, 2018

Injecting a split brain into a JGroups cluster

Thanks to a contribution by Andrea Tarocchi, we can now inject arbitrary views into a JGroups cluster, generating split brain scenarios.

This is done by a new protocol INJECT_VIEW, which can be placed anywhere in the stack, e.g. like this:

  <MERGE3 max_interval="30000"

  <VERIFY_SUSPECT timeout="500"  />


The protocol exposes a method injectView(), which can be called by Let's say we have a cluster {A,B,C}: we can now inject view {A} into A, and {B,C} into B and C, as follows: op=INJECT_VIEW.injectView["A=A;B=B/C;C=B/C"]

(Note that '/' is used to separate node names; this is needed, because ',' is used to seperate arguments by default in probe) .

The result is that node A is now its own singleton cluster (not seeing B and C), and B and C only see each other (but not A).

Note that the member names are the logical names given to individual nodes.

Once the split has been injected, MERGE3 should remedy it by merging A, B and C back into a single cluster, after between 10-30 seconds (in the above configuration).

Thanks to Andrea for creating this nifty tool to experiment with cluster splits!

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