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by TheHydroImpulse 3565 days ago
FYI, Kafka doesn't need to fetch from disk every time as it caches the logs pretty aggressively, as long as you have enough memory.

Running Zk and Kafka on the same nodes is likely not the best thing.
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im_down_w_otp 3565 days ago
Why? I would think that, as long as there wasn't massive I/O contention between the two, that co-locating Kafka and Zookeeper on the same machines would mitigate a whole massive class of weird edge cases by removing one of the failure modes; the network boundary between the two critical components.

Though for my part I still don't understand why Zookeeper wasn't built as a library to add distributed strongly consistent coordination to software that needs/benefits from it rather than being an external service that needs to be connected to, and thus introduces a gnarly mess of new failure modes that make Zookeeper client behavior extremely critical and often fragile. Something that's more like a "libpaxos/libraft" (e.g. serf for Go-lang or riak_ensemble for Erlang) seems a lot more valuable. /shrug

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TheHydroImpulse 3565 days ago
But co-locating them won't actually remove a class of errors because Zk is not HA. The Kafka brokers need to communicate with the leader in the Zk cluster.

If we have K1,Z1 -- K2,Z2 -- K3,Z3 -- and one node goes down, you've now taken down both a broker and a Zk node. Remember, the brokers don't care about connecting to any Zk node, they want the leader. So you aren't gaining any more fault tolerant by co-locating them.

If there's a network partition between the leader Zk node and other nodes, the local Kafka broker won't actually be able to do much because the Zk cluster will elect a new leader, on another node, so again, you aren't gaining anything.

Moreover, you're now tying the scalability of Kafka with Zk. Zk doesn't scale linearly, so there's only so many nodes you may have in a cluster. Kafka, on the other hand, scales linearly. So if you're colocating them and you have to bump up Kafka, do you still start up Zk for those nodes (but they don't actually join the cluster)? You're now special casing and adding more edge cases.

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