[tarantool-patches] Re: [PATCH] replication: add is_orphan field to ballot

Kirill Yukhin kyukhin at tarantool.org
Wed Oct 9 14:33:12 MSK 2019


On 30 сен 20:54, Serge Petrenko wrote:
> A successfully fetched remote instance ballot isn't updated during
> bootstrap procedure. This leads to a case when different instances
> choose different masters as their bootstrap leaders.
> Imagine such a situation.
> You start instance A without replication set up. Instance A successfully
> bootstraps.
> You also have instances B and C both with replication set up to {A, B,
> C} and replication_connect_quorum set to 3
> You first start instance B. It doesn't proceed to choosing a leader
> until one of the events happens: either the replication_connect_timeout
> runs out, or instance C is up and starts listening on its port.
> B has established connection to A and fetched its ballot, with some
> vclock, say, {1: 1}.
> B retries connection to C every replication_timeout seconds.
> Then you start instance C. Instance C succeeds in connecting to A and B
> right away and bootstraps from instance A. Instance A registers C in its
> _cluster table. This registration is replicated to instance C.
> Meanwhile, instance C is trying to sync with quorum instances (which is
> 3), and stays in orphan mode.
> Now replication_timeout on instance B finally runs out. It retries a
> previously unsuccessful connection to C and succeeds. C sends its ballot
> to B with vclock = {1: 2, 2:0} (in our example), since it has already
> incremented it after _cluster registration.
> B sees that C has a greater vclock than A, and chooses to bootstrap from
> C instead of A. C is orphan and rejects B's attempt to join. B dies.
> To fix such ungentlemanlike behaviour of C, we should at least include
> orphan status in ballot and prefer fully bootstrapped instances to the
> ones still syncing with other replicas.
> We also need to use a separate flag instead of ballot's already existent
> is_ro, since we still want to prefer orphan instances over the ones
> explicitly configured to be read-only.
> Closes #4527

I've checked your patch into 1.10, 2.1, 2.2 and master.

Regards, Kirill Yukhin

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