Writing workers

[William Reade]

https://github.com/juju/juju/wiki/Guidelines-for-writing-workers

On Mon, Jun 1, 2015 at 3:32 PM, Nate Finch <nate.finch at canonical.com> wrote:

> 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/
>>>
>>> --
>>> Juju-dev mailing list
>>> Juju-dev at lists.ubuntu.com
>>> Modify settings or unsubscribe at:
>>> https://lists.ubuntu.com/mailman/listinfo/juju-dev
>>>
>>>
>>
>> --
>> Juju-dev mailing list
>> Juju-dev at lists.ubuntu.com
>> Modify settings or unsubscribe at:
>> https://lists.ubuntu.com/mailman/listinfo/juju-dev
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.ubuntu.com/archives/juju-dev/attachments/20150601/aeb60184/attachment-0001.html>