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CLS & asyncListener: asynchronous observability for Node.js

Forrest Norvell
Forrest Norvell

際際滷s from my presentation at PDXNode in October 2013 before RealtimeConf. Thanks to Tracy Abrahms, Ben Acker, and the rest of the PDXNode community for accommodating and hosting me at the last minute!

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continuation-local storage https://github.com/othiym23/continuationlocal-storage
cls needed a way to pass around per-request state that wouldnt break (or change) user code stashing state on the side of req and res is yucky thread-local storage is nice but Node is single-threaded and were thinking about chains of continuations, so each set of values needs to be tied to a speci鍖c request chain
a dirt-simple example var cls = require('continuation-足local-足storage'); function magic() { console.log("value is %s", cls.getNamespace('test').get('value')); } var test = cls.createNamespace('test'); test.run(function () { test.set('value', 'hi there!'); setImmediate(magic); }); test.set('value', 'oh no!');
things to note magic does not have test in scope inside test.run, values have their own scope tests values persist across the call to setImmediate
whats going on? namespace.run clones the current context the contexts are entered and exited like domains the state is persisted across process.nextTick, timers, and all other core async functions
{create,add,remove}AsyncListener https://github.com/joyent/node/pull/6011 https://github.com/othiym23/async-listener
THERES NO WAY IM LETTING ANOTHER THING AS SLOW AS DOMAINS IN CORE TREVNORRIS
srs bzns very powerful but low-level API captures every async event in the Node process and makes it observable makes it simple to pass state to callbacks without changing their code also makes it easy to write global error handlers that have some state
the API a listener function that is 鍖red for every async event, returns a value that will be passed to the decorators on this async events callbacks a bundle of optional before, after, and error callbacks that will be wrapped around functions that this async event is responsible for an optional value that can be passed to before / after / error instead of the results of the listener
how it go??! in 0.11.8 and earlier, a JavaScript poly鍖ll that monkeypatches all the things (so, pretty slow) in 0.11.9+, a whole bunch of code in C++ and JavaScript (but still, not a performance king)
how it stable? poly鍖ll has pretty good coverage, doesnt appear to slow apps down too much in practice (just recently solidi鍖ed enough) poly鍖ll is in use in the New Relic transaction tracer the native version is stable enough that Trevor rewrote domains to use it (and got a nice performance boost when domains arent in use)
what for? CLS, obviously low-level logging modules pure JS pro鍖lers long stacktrace modules making the details of asynchronous execution more observable, basically

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CLS & asyncListener: asynchronous observability for Node.js

  • 2. cls needed a way to pass around per-request state that wouldnt break (or change) user code stashing state on the side of req and res is yucky thread-local storage is nice but Node is single-threaded and were thinking about chains of continuations, so each set of values needs to be tied to a speci鍖c request chain
  • 3. a dirt-simple example var cls = require('continuation-足local-足storage'); function magic() { console.log("value is %s", cls.getNamespace('test').get('value')); } var test = cls.createNamespace('test'); test.run(function () { test.set('value', 'hi there!'); setImmediate(magic); }); test.set('value', 'oh no!');
  • 4. things to note magic does not have test in scope inside test.run, values have their own scope tests values persist across the call to setImmediate
  • 5. whats going on? namespace.run clones the current context the contexts are entered and exited like domains the state is persisted across process.nextTick, timers, and all other core async functions
  • 7. THERES NO WAY IM LETTING ANOTHER THING AS SLOW AS DOMAINS IN CORE TREVNORRIS
  • 8. srs bzns very powerful but low-level API captures every async event in the Node process and makes it observable makes it simple to pass state to callbacks without changing their code also makes it easy to write global error handlers that have some state
  • 9. the API a listener function that is 鍖red for every async event, returns a value that will be passed to the decorators on this async events callbacks a bundle of optional before, after, and error callbacks that will be wrapped around functions that this async event is responsible for an optional value that can be passed to before / after / error instead of the results of the listener
  • 10. how it go??! in 0.11.8 and earlier, a JavaScript poly鍖ll that monkeypatches all the things (so, pretty slow) in 0.11.9+, a whole bunch of code in C++ and JavaScript (but still, not a performance king)
  • 11. how it stable? poly鍖ll has pretty good coverage, doesnt appear to slow apps down too much in practice (just recently solidi鍖ed enough) poly鍖ll is in use in the New Relic transaction tracer the native version is stable enough that Trevor rewrote domains to use it (and got a nice performance boost when domains arent in use)
  • 12. what for? CLS, obviously low-level logging modules pure JS pro鍖lers long stacktrace modules making the details of asynchronous execution more observable, basically