Dead-Simple Async Control Flow with Coroutines
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This document discusses the challenges of asynchronous control flow in Node.js, particularly the issues with callback functions leading to unmanageable code structures known as callback hell. It proposes the use of coroutines, which integrate generator functions and promises to provide better readability and efficiency for asynchronous operations. The text also highlights various coroutine libraries and additional resources for implementation and further learning.
Dead-Simple Async Control Flow with Coroutines
- 1. Dead-Simple Async Control Flow with Coroutines by Travis Kaufman (@traviskaufman) github.com/traviskaufman/co-talk-examples
- 2. Outline Define the problem Async control flow in NodeJS Present the solution Coroutines (surprise!) Explore the implementation How coroutines work under the hood Coroutine libraries Start using them in your apps today! (time permitting) Advanced usage Sample code showing where coroutines really shine
- 4. Callbacks are UNINTUITIVE source: http://i.imgur.com/DEg3cPZ.png
- 5. Callbacks are UNINTUITIVE Simple Program: Get GH Username, print out 10 most recently pushed repos https://github.com/traviskaufman/co-talk-examples/blob/master/01-raw-callbacks.js
- 6. Callbacks are UNINTUITIVE Simple Program: Get GH Username, print out 10 most recently pushed repos https://github.com/traviskaufman/co-talk-examples/blob/master/01-raw-callbacks.js Code duplication
- 7. Callbacks are UNINTUITIVE Simple Program: Get GH Username, print out 10 most recently pushed repos https://github.com/traviskaufman/co-talk-examples/blob/master/01-raw-callbacks.js Code duplication “Diagonal Development” / Pyramid of Doom / Callback Hell / etc
- 8. Callbacks are UNINTUITIVE Simple Program: Get GH Username, print out 10 most recently pushed repos https://github.com/traviskaufman/co-talk-examples/blob/master/01-raw-callbacks.js Code duplication Refactoring callbacks means jumping around code to see logical flow of execution :(( “Diagonal Development” / Pyramid of Doom / Callback Hell / etc
- 9. https://github.com/traviskaufman/co-talk-examples/blob/master/02-async-lib-callbacks.js Callbacks are UNINTUITIVE The “solution”: use a library (e.g. caolan/async, a.k.a. the jQuery of NodeJS)
- 10. Callbacks are UNINTUITIVE The “solution”: use a library (e.g. caolan/async, a.k.a. the jQuery of NodeJS) Only have to handle this in one place! :D https://github.com/traviskaufman/co-talk-examples/blob/master/02-async-lib-callbacks.js
- 11. Callbacks are UNINTUITIVE The “solution”: use a library (e.g. caolan/async, a.k.a. the jQuery of NodeJS) Need to be familiar with “async.waterfall” API Change to accommodate lib usage? Only have to handle this in one place! :D https://github.com/traviskaufman/co-talk-examples/blob/master/02-async-lib-callbacks.js
- 12. Promises to the Rescue! https://github.com/traviskaufman/co-talk-examples/blob/master/03-raw-promises.js No triangles/pyramids/hells/doom/etc. Error handling separated from general logic
- 13. Promises to the Rescue! but... https://github.com/traviskaufman/co-talk-examples/blob/master/03-raw-promises.js No triangles/pyramids/hells/doom/etc. Error handling separated from general logic API Weirdness still... Still requires handler functions for promise settling
- 14. Can We Do Better? Readability of synchronous code I/O Efficiency of Asynchronous Code
- 16. YES! Introducing Coroutines ● coroutine(gen: GeneratorFunction) => Promise ● generator function yields Promises ● values from resolved promises are assigned ● errors from rejected promises are thrown within generator ○ note: could use try/catch inside generator to call handleError() ● Returns Promise for easy interop with other async code ● Best of both worlds! Readability/Intuitiveness + Efficiency/Non-blocking I/O https://github.com/traviskaufman/co-talk-examples/blob/master/04-co-promises.js
- 18. Coroutines = “Cooperative Routines” Cooperative threads are responsible for explicitly telling executor when they should be suspended Differs from preemptive threads which rely on executor to suspend/resume them Reference (Java-based): http://www.cafeaulait.org/course/ week11/32.html https://www.cs.mtu.edu/~shene/NSF-3/e-Book/FUNDAMENTALS/thread-yield.jpg
- 19. Coroutines = “Cooperative Routines” Generators provide cooperation through the yield keyword Explicitly tells IO-Loop (uv, etc.) to suspend execution Promises provide the routines Scheduled and run asynchronously by IO-Loop Generators + Promises = Cooperation + Routines = Coroutines! https://www.cs.mtu.edu/~shene/NSF-3/e-Book/FUNDAMENTALS/thread-yield.jpg yield new Promise(...)
- 20. A Simple Implementation < 25 SLOC!
- 21. Coroutine Libraries: tj/co My personal favorite Supports thunks as well as promises Supports yielding arrays/objects (Promise.race), as well as generators (delegation) https://github.com/tj/co
- 22. Coroutine Libraries: mozilla/task.js Similar to co Very powerful built-in scheduler Adds cancellation to promises Makes coroutines look a lot like thread-based scheduling http://taskjs.org/
- 23. “But wait, I need ES2015 for this!” Use babelJS (https://babeljs.io/) to transpile your code! Use the --harmony-generators flag with v0.11+ to enable generators (or just -- harmony) Check out petkaantonov/bluebird for an awesome promise polyfill/superset.
- 24. Advanced Example: “Retry” with exponential backoff
- 25. Advanced Example: DB Cursoring
- 26. Additional Resources MDN reference for generator functions https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function* Awesome Promise tutorial from HTML5Rocks http://www.html5rocks.com/en/tutorials/es6/promises/ “Promises and Generators: Control Flow Utopia” - Great presentation by Forbes Lindesay http://pag.forbeslindesay.co.uk/#/ “No Promises: Asynchronous Javascript with only generators” http://www.2ality.com/2015/03/no-promises.html Axel Rauschmayer takes it one step further and shows how you can use generators to write async code without the need for promises. Super interesting stuff!
- 27. THANK YOU SO MUCH! :D
Editor's Notes
- #5: one of the things we software engineers take most seriously is maintainability and readability. You want your code to represent the logical flow of steps to solve the problem at hand.
- #10: Mention how I <3 Async and, like jQuery, it is an amazing library
- #11: Mention how I <3 Async and, like jQuery, it is an amazing library
- #12: Mention how I <3 Async and, like jQuery, it is an amazing library