Writing workers

[John Meinel]

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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