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added paragraph on locking and merges

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Bela Ban
Bela Ban committed Jan 21, 2011
1 parent 5d7c7e1 commit 7df52d7e49663c76dc9f87a18b42d71320a871cd
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  1. +38 −0 doc/manual/en/modules/blocks.xml
@@ -732,6 +732,44 @@
<xref linkend="PEER_LOCK">PEER_LOCK</xref> and <xref linkend="CENTRAL_LOCK">CENTRAL_LOCK</xref>. The locking
protocol has to be placed at or towards the top of the stack (close to the channel).
+ <section>
+ <title>Locking and merges</title>
+ <para>
+ The following scenario is susceptible to merging: we have a cluster view of {A,B,C,D} and then the cluster
+ splits into {A,B} and {C,D}. Assume that B and D now acquire a lock "mylock".
+ This is what happens (with the locking protocol being CENTRAL_LOCK):
+ <itemizedlist>
+ <listitem>There are 2 coordinators: A for {A,B} and C for {C,D}</listitem>
+ <listitem>B successfully acquires "mylock" from A</listitem>
+ <listitem>D successfully acquires "mylock" from C</listitem>
+ <listitem>The partitions merge back into {A,B,C,D}. Now, only A is the coordinator, but C ceases
+ to be a coordinator</listitem>
+ <listitem>Problem: D still holds a lock which should actually be invalid !</listitem>
+ </itemizedlist>
+ There is no easy way (via the Lock API) to 'remove' the lock from D. We could for example simply release
+ D's lock on "mylock", but then there's no way telling D that the lock it holds is actually stale !
+ </para>
+ <para>
+ Therefore the recommended solution here is for nodes to listen to MergeView changes if they expect
+ merging to occur, and re-acquire all of their locks after a merge, e.g.:
+ <screen>
+ Lock l1, l2, l3;
+ LockService lock_service;
+ ...
+ public void viewAccepted(View view) {
+ if(view instanceof MergeView) {
+ new Thread() {
+ public void run() {
+ lock_service.unlockAll();
+ // stop all access to resources protected by l1, l2 or l3
+ // every thread needs to re-acquire the locks it holds
+ }
+ }.start
+ }
+ }
+ </screen>
+ </para>
+ </section>

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