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0 pointsKirinDave14y ago0 comments

Okay well if you don't want external dependencies, that's A Design Decision and it's fine.

What this doesn't answer is my first question: why do there need to external special-mode sentinel processes? Why not have the slaves negotiate themselves? You're already doing a bit of this work, wouldn't it be simpler?

You say the sentinels are "for free" and they may be extremely inexpensive to run; but they're not "for free" from an operational context. They're another process to provision, start, monitor, and consider.

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5 comments · 2 top-level

antirez14y ago· 3 in thread

Something is "available" or not depending on the observer and point of view. Users should be able to position Sentinels accordingly to the kind of failure detection they want, and usually where the slave are is not an objective point of view (that is instead where clients are usually).

Position, number of sentinels, level of agreement. Change this three elements and you can create many kind of setups.

About operations: Sentinels are designed to monitor other sentinels automatically and are as simple as possible to operate.

mrkurt14y ago

I don't disagree with allowing that flexibility, but letting Redis instances do dual duty would greatly simplify things for some really common types of deployments. It's awfully nice with Mongo to just run three members and get a pretty decent failover feature for "free", versus having to worry about maintaining an entirely different set of processes.

kenn14y ago

As to MongoDB, in case you only have two members in a replica set, you add an arbiter, not strictly three members.

An arbiter is indeed a lightweight mongod process that doesn't have storage, and should be added to break the tie when you have even number of members. I like the MongoDB's design, and it is probably optimal for 99% of use cases.

There are three key concepts: Split-brain, Quorum, and Fencing. I find this article fascinating.

http://techthoughts.typepad.com/managing_computers/2007/10/s...

antirez14y ago

I think that flexibility is absolutely needed for reliable failure detection.

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pedromelo14y ago

At least one small reason to have this as a small memory footprint process is that on certain situation, the Sentinel will fork()/exec() user scripts.

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