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by HenryR 5532 days ago
It's not clear to me how, or if, failure detection is to be integrated with Doozer. Keith has said on the thread accompanying the announcement blog post that Doozer doesn't have the 'baggage' of sessions (which are used in ZooKeeper to manage timeout failure and the removal of certain kinds of data which can be used to model lock revocation).

Without some way of knowing if a process fails, it's hard to do leader election, locks, other synchronisation patterns. It would be a reasonable design choice to do failure detection completely out of band with the sequentially consistent store, but I'd like to understand their architecture better.
2 comments
kaib 5531 days ago
Just curious, how familiar are you with Paxos? I'm asking because failure detection is pretty much isomorphic with distributed consensus and generally considered hard. Ie. how do you differentiate between down and slow?
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HenryR 5531 days ago
Extremely familiar - see my articles at

http://the-paper-trail.org/blog/?p=173

and http://the-paper-trail.org/blog/?p=190

for some tutorials I wrote on the subject.

You're correct that failure detection and consensus are very deeply related, in that a strong failure detector is 'sufficient' for consensus.

But my point is about client failure detection, not failure detection between servers (which must have some kind of timeout system; that's ok - you just sacrifice liveness in a few pathological cases rather than sacrificing correctness). If I am to implement leader election with Doozer, does Doozer provide any tools to help us with deciding when to elect a new leader? There's no reason it should, but ZooKeeper, for example, does have that in its arsenal.

Doozer doesn't, AFAIK, expose consensus as a primitive; that's not its model. So the fact that it uses Paxos, or ZAB, or 2PC or whatever doesn't make a difference to its clients.

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kaib 5530 days ago
Replied to you via email.
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bmizerany 5531 days ago
Please see: http://xph.us/2011/04/13/introducing-doozer.html
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HenryR 5531 days ago
That was the introductory blog post to which I referred - note the similar discussion in the comments.
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