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90 lines
3.5 KiB
Markdown
90 lines
3.5 KiB
Markdown
gls
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===
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Goroutine local storage
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### IMPORTANT NOTE ###
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It is my duty to point you to https://blog.golang.org/context, which is how
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Google solves all of the problems you'd perhaps consider using this package
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for at scale.
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One downside to Google's approach is that *all* of your functions must have
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a new first argument, but after clearing that hurdle everything else is much
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better.
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If you aren't interested in this warning, read on.
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### Huhwaht? Why? ###
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Every so often, a thread shows up on the
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[golang-nuts](https://groups.google.com/d/forum/golang-nuts) asking for some
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form of goroutine-local-storage, or some kind of goroutine id, or some kind of
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context. There are a few valid use cases for goroutine-local-storage, one of
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the most prominent being log line context. One poster was interested in being
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able to log an HTTP request context id in every log line in the same goroutine
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as the incoming HTTP request, without having to change every library and
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function call he was interested in logging.
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This would be pretty useful. Provided that you could get some kind of
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goroutine-local-storage, you could call
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[log.SetOutput](http://golang.org/pkg/log/#SetOutput) with your own logging
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writer that checks goroutine-local-storage for some context information and
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adds that context to your log lines.
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But alas, Andrew Gerrand's typically diplomatic answer to the question of
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goroutine-local variables was:
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> We wouldn't even be having this discussion if thread local storage wasn't
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> useful. But every feature comes at a cost, and in my opinion the cost of
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> threadlocals far outweighs their benefits. They're just not a good fit for
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> Go.
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So, yeah, that makes sense. That's a pretty good reason for why the language
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won't support a specific and (relatively) unuseful feature that requires some
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runtime changes, just for the sake of a little bit of log improvement.
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But does Go require runtime changes?
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### How it works ###
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Go has pretty fantastic introspective and reflective features, but one thing Go
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doesn't give you is any kind of access to the stack pointer, or frame pointer,
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or goroutine id, or anything contextual about your current stack. It gives you
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access to your list of callers, but only along with program counters, which are
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fixed at compile time.
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But it does give you the stack.
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So, we define 16 special functions and embed base-16 tags into the stack using
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the call order of those 16 functions. Then, we can read our tags back out of
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the stack looking at the callers list.
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We then use these tags as an index into a traditional map for implementing
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this library.
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### What are people saying? ###
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"Wow, that's horrifying."
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"This is the most terrible thing I have seen in a very long time."
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"Where is it getting a context from? Is this serializing all the requests?
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What the heck is the client being bound to? What are these tags? Why does he
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need callers? Oh god no. No no no."
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### Docs ###
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Please see the docs at http://godoc.org/github.com/jtolds/gls
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### Related ###
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If you're okay relying on the string format of the current runtime stacktrace
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including a unique goroutine id (not guaranteed by the spec or anything, but
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very unlikely to change within a Go release), you might be able to squeeze
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out a bit more performance by using this similar library, inspired by some
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code Brad Fitzpatrick wrote for debugging his HTTP/2 library:
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https://github.com/tylerb/gls (in contrast, jtolds/gls doesn't require
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any knowledge of the string format of the runtime stacktrace, which
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probably adds unnecessary overhead).
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