Redis 2.8 => 2 nodes configuration

[Gaetan]

Hello,

I'm curently trying to installed 2 node of Redis 2.8, with Master Slave configuration, and sentinel...

Server RE01 : Sentinel + Master server
Server RE02 : Sentinel + Slave server

When server is down, Sentinel automtiquely failover redis server, but, if OS crash (Sentinel + server down), the second sentinel don't want to put RE02 server as master (Quorum = 1)...

Is it normal ? A 2 sentinels  architecture is supported ?

If this architecture is possible, I will posted  our configuration...

Thank you !

[Jan-Erik Rediger]

It's not possible. For a proper quorum you need N/2 + 1 nodes to vote
for a new master. With N = 2, you still need 2 votes.
Therefore you need atleast 3 Sentinels to avoid the mentioned problem.

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[Carlos Abalde]

Hi,

Maybe I'm missing something here, but I think that technically speaking the architecture is supported simply setting the quorum value to 1 in sentinel.conf. Isn't it? A different discussion is if that's a reasonable idea. I think the answer to that question depends a lot on the specific scenario.

Cheers,

--
Carlos Abalde

[Salvatore Sanfilippo]

Hello, I'm in the process of improving the Sentinel configuration to
make certain things simpler, but if you check the current config, it
is already well-stated that you need the majority of Sentinels to
authorize a failover. Quorum is used for the failure detection, so
this is how it works:

Step 1) Failure detection: is quorum reached to consider the master
failing? If so, go to step 2.
Step 2) Is there the majority to authorize a failover? If so,
failover, otherwise do nothing.

Otherwise in your proposed setup, if the two servers where you have,
in each, a Sentinel+Redis server combo would no longer able to talk
with the other side, what you get is two masters since the minority
side is able to elect the slave to master. Moreover the majority is
needed in order to create an unique configuration number that is later
used in order to propagate the latest configuration for a given master
(who is the active master).

Salvatore

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Salvatore 'antirez' Sanfilippo
open source developer - Pivotal http://pivotal.io

"If a system is to have conceptual integrity, someone must control the
concepts."
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