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 <[email protected]> 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 > [email protected] > Modify settings or unsubscribe at: > https://lists.ubuntu.com/mailman/listinfo/juju-dev > >
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