Writing workers

[Nate Finch]

Totally belongs on the wiki.

On Mon, Jun 1, 2015 at 8:45 AM, John Meinel <john at arbash-meinel.com> wrote:

> This is one of those things that should probably end up on the Wiki.
> Thanks for writing it up.
>
> John
> =:->
>
>
> On Thu, May 28, 2015 at 5:34 PM, William Reade <
> william.reade at canonical.com> wrote:
>
>> Hi all
>>
>> I've noticed that there's a lot of confusion over how to write a useful
>> worker. Here follow some guidelines that you should be *very* certain of
>> yourself before breaking (and probably talk to me about anyway). If there's
>> any uncertainty about these, I'm more than happy to expand.
>>
>>   * If you really just want to run a dumb function on its own goroutine,
>> use worker.NewSimpleWorker.
>>
>>   * If you just want to do something every <period>, use
>> worker.NewPeriodicWorker.
>>
>>   * If you want to react to watcher events, you should probably use
>> worker.NewNotifyWorker or worker.NewStringsWorker.
>>
>>   * If your worker has any methods outside the Worker interface, DO NOT
>> use any of the above callback-style workers. Those methods, that need to
>> communicate with the main goroutine, *need* to know that goroutine's state,
>> so that they don't just hang forever.
>>
>>   * To restate the previous point: basically *never* do a naked channel
>> send/receive. If you're building a structure that makes you think you need
>> them, you're most likely building the wrong structure.
>>
>>   * If you're writing a custom worker, and not using a tomb.Tomb, you are
>> almost certainly doing it wrong. Read the blog post [0] or, hell, just read
>> the code [1] -- it's less than 200 lines and it's about 50% comments.
>>
>>   * If you're letting tomb.ErrDying leak out of your workers to any
>> clients, you are definitely doing it wrong -- you risk stopping another
>> worker with that same error, which will quite rightly panic (because that
>> tomb is *not* yet dying).
>>
>>   * If it's possible for your worker to call .tomb.Done() more than once,
>> or less than once, you are *definitely* doing it very very wrong indeed.
>>
>>   * If you're using .tomb.Dead(), you are very probably doing it wrong --
>> the only reason (that I'm aware of) to select on that .Dead() rather than
>> on .Dying() is to leak inappropriate information to your clients. They
>> don't care if you're dying or dead; they care only that the component is no
>> longer functioning reliably and cannot fulfil their requests. Full stop.
>> Whatever started the component needs to know why it failed, but that parent
>> is usually not the same entity as the client that's calling methods.
>>
>>   * If you're using worker/singular, you are quite likely to be doing it
>> wrong, because you've written a worker that breaks when distributed. Things
>> like provisioner and firewaller only work that way because we weren't smart
>> enough to write them better; but you should generally be writing workers
>> that collaborate correctly with themselves, and eschewing the temptation to
>> depend on the funky layer-breaking of singular.
>>
>>   * If you're passing a *state.State into your worker, you are almost
>> certainly doing it wrong. The layers go worker->apiserver->state, and any
>> attempt to skip past the apiserver layer should be viewed with *extreme*
>> suspicion.
>>
>>   * Don't try to make a worker into a singleton (this isn't particularly
>> related to workers, really, singleton is enough of an antipattern on its
>> own [2] [3] [4]). Singletons are basically the same as global variables,
>> except even worse, and if you try to make them responsible for goroutines
>> they become more horrible still.
>>
>> Did I miss anything major? Probably. If so, please remind me.
>>
>> Cheers
>> William
>>
>>
>> [0] http://blog.labix.org/2011/10/09/death-of-goroutines-under-control
>> [1] launchpad.net/tomb (apparently... we really ought to be using v2,
>> though)
>> [2] https://sites.google.com/site/steveyegge2/singleton-considered-stupid
>>
>> [3]
>> http://jalf.dk/blog/2010/03/singletons-solving-problems-you-didnt-know-you-never-had-since-1995/
>> [4]
>> http://programmers.stackexchange.com/questions/40373/so-singletons-are-bad-then-what/
>>
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