ProducerPool - easier remote publishing

[jehiah]

go-nsq does not provide good primitives to handle failures when publishing to a nsq instances; Handling publish errors is left as an exercise for the developer when that logic is important for reliable message creation. Common guidance has been to prefer publishing to a colocated nsqd instance, but that configuration is less desirable in many cloud environments.

I'm proposing we add a ProducerPool which provides a Publish interface that can be configured with multiple nsqd instances and which will retry publishing to another instance on error. This will provide a simple way for applications publishing messages to handle write failures in a fault tolerant way.

Based on outcome of nsqio/nsq#1300 this producer may also lazily organize nsqd instances into groupings of node local, zone local, region local or global and provide prioritization base on topology.

Functionally this is similar to the reliable publishing in nsq_to_nsq where publish happens in a round robin, or host pool to a pool of nsqd instances (but without the same backpressure).

cc: nsqio/nsq#1254 which tracks facilitating use of nsq in a cloud environment.

[mreiferson]

Sounds good 👍

[jehiah]

Thanks to @jharshman for contributing a PublishAsync implementation. I still want to work on making this transparently pick the best nsqd to write to.

[ploxiln]

another possibly related idea: "publish to N different nsqd" (e.g. 2) for users that want stronger guarantees that no messages are ever lost

[mreiferson]

Not sure if this is ready for review, but a few things as is:

• Looks like the target nsqd are static in a ProducerPool, meaning if the list needs to change, a user is expected to discard a ProducerPool instance and create a new one with a new list (and new connections). If so, then ProducerPool needs some sort of Close() func to cleanup.
• PublishAsync shouldn't be managing its own goroutine directly, perhaps a ProducerPool instance should manage the lifecycle of a goroutine that handles retries?

[jehiah]

Not sure if this is ready for review, but a few things as is:

No, not RFR (see below) - but I think it has some useful contents and figuring out the right way to expose initialization needs some thought so definitely RFC, so comments are good.

Based on outcome of nsqio/nsq#1300 this producer may also lazily organize nsqd instances into groupings of node local, zone local, region local or global and provide prioritization base on topology.

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PublishAsync shouldn't be managing its own goroutine directly, perhaps a ProducerPool instance should manage the lifecycle of a goroutine that handles retries?

I don't like how this implementation spawns goroutines, but i don't really like needing to mange a long lived goroutine either; This implementation should be "correct" so that's a start. I'd love this targeted an interface (I have a version in use that does) that would make it easier to land testing, but this package needs some re-work to get to a point of targeting an interface - sort of out of scope of this PR.