Robust Error Handling in Node.js

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Who are you? • I'm Lewis, I like JavaScript (and other things) • Made Bee-Queue, Redis-backed job queue for Node.js • Like Celery, Resque, Kue, Bull • Worked on raven-node, Sentry's Node error reporting SDK • It captures and reports about 100 million errors per week

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Why should you want robust error handling in Node?

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This talk is for you if...

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You've ever written code like this: fs.readFile('myFile.txt', { encoding: 'utf8' }, function (err, data) { console.log(data); });

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Or this: http.get(myUrl, function (res) { doSomethingWith(res); });

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Or even this: try { data = JSON.parse(userInput); } catch (e) { // this should never happen } doSomethingWith(data);

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"this should never happen"

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Famous last words

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We can do better!

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This is a little better: fs.readFile('myFile.txt', { encoding: 'utf8' }, function (err, data) { if (err) return console.error(err); doSomethingWith(data); });

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But we can do a lot better

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Overview of topics • Background and how Node is special/different • Handle what you can, avoid what you can't • The robust game plan to follow • Exceptions, Callbacks, Promises, EventEmitters, and more • Catching, reporting, shutting down, restarting gracefully

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Error Mechanisms in Other Languages • Python: try/except and raise • Ruby: begin/rescue and raise • PHP: try/catch and throw • Lua: pcall() and error()

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So Node has some similar thing, right?

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Well, sure, this works: try { throw new Error('boom!'); } catch (e) { console.log('Aha! I caught the error!'); }

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Well, sure, this works: try { throw new Error('boom!'); } catch (e) { console.log('Aha! I caught the error!'); } $ node try-catch.js

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Well, sure, this works: try { throw new Error('boom!'); } catch (e) { console.log('Aha! I caught the error!'); } $ node try-catch.js > Aha! I caught the error!

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But where try-catch comes up short: try { setTimeout(function () { throw new Error('boom!'); }, 0); } catch (e) { console.log('Aha! I caught the error!'); }

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But where try-catch comes up short: try { setTimeout(function () { throw new Error('boom!'); }, 0); } catch (e) { console.log('Aha! I caught the error!'); } $ node try-settimeout.js

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But where try-catch comes up short: try { setTimeout(function () { throw new Error('boom!'); }, 0); } catch (e) { console.log('Aha! I caught the error!'); } $ node try-settimeout.js /Users/lewis/dev/node-error-talk/try-settimeout.js:3 throw new Error('boom!'); ^ Error: boom! at Timeout._onTimeout (/Users/lewis/dev/node-error-talk/try-settimeout.js:3:11) at ontimeout (timers.js:365:14) at tryOnTimeout (timers.js:237:5) at Timer.listOnTimeout (timers.js:207:5)

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Why? • Try-catch is synchronous, setTimeout is asynchronous • Callback is queued and will throw the error later • Catch block is no longer waiting to catch the exception • Run-to-completion semantics & event loop are behind this

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How Node is Different • Other languages: • Each process handles one request at a time • Everything is synchronous, try/catch works ﬁne • Easy to keep one request from blowing everything up • Node: single process, cooperative concurrency, asynchronous I/O • Handles multiple requests at the same time in the same thread • Try/catch doesn't work well in asynchronous world • Any one request blowing up could mess up the others

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What this means for us • We need other mechanisms to handle asynchronous errors • We can't try/catch and throw exceptions for everything • An error in one request can take down the entire server • We have to be extra careful to keep things online • Our program can end up in unknown states • Only correct thing to do might be shut down!

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Errors vs Exceptions What's the difference, anyway?

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Errors Error is just a special class in JavaScript - You can pass an Error object around like any other value - Runtime errors throw an Error object - Has a message and a stack property - Also RangeError, ReferenceError, SyntaxError, others var myError = new Error('my error message');

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Exceptions Exception: what happens when you throw something - Usually you throw an Error object - Call stack unwinds looking for a catch block - You can throw anything, not just Error objects...but don't - If an exception is unhandled, Node will shut down throw new Error('something bad happened!'); throw 'something bad happened' // avoid this

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Stack trace example function a() { // call stack here is [a] b(); } function b(x) { // call stack here is [b, a] try { c() } catch (e) { console.log(e.stack); } } function c() { // call stack here is [c, b, a] throw new Error('boom'); } a();

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Error: boom at c (/Users/lewis/dev/node-error-talk/try-catch.js:14:9) at b (/Users/lewis/dev/node-error-talk/try-catch.js:7:5) at a (/Users/lewis/dev/node-error-talk/try-catch.js:2:3) at Object. (/Users/lewis/dev/node-error-talk/try-catch.js:17:1) ... This is useful! We want to see it!

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Errors vs Exceptions in Async Node Land • We're generally not going to throw Exceptions • We're going to pass Error objects around a lot

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Three Guiding Principles

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Always know when your errors happen

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Avoid patterns like: function (err, result) { if (err) { /* drat, ignore */ } } try { ... } catch (e) { // this should never happen } Promise.catch(function (reason) { // surely this won't happen }); req.on('error', function (err) { // oh well, not gonna do anything });

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Handle what you can, avoid what you can't

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Operational Errors vs Programming Errors • Operational error: recoverable - expect and handle these • Typically an Error object being passed around • Programming error: nonrecoverable - try to avoid these • Typically an Exception thrown

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Operational Errors to Expect • Network timeouts • Database is down • Disk got full • 3rd party API returning errors: S3 goes down • Unexpected or missing user inputs (JSON.parse)

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Programming Errors to Avoid • Silently ignoring/swallowing errors instead of handling them • Classic JavaScript errors • TypeError: undefined is not a function • TypeError: Cannot read property 'x' of undefined • ReferenceError: x is not defined • Invoking a callback twice • Using the wrong error mechanism in the wrong place

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What to do with each: • Operational errors • Known; handle manually wherever they may occur • Recoverable if handled correctly: S3 being down doesn't kill us • Avoid assuming anything is reliable outside your own process • Programming errors • Unknown; catch with global error handler • Nonrecoverable: we're gonna have to abandon ship • No amount of additional code can ﬁx a typo • Use a linter to help avoid many common problems

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Don't keep running in an unknown state

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Don't keep running in an unknown state • State shared across multiple requests: less isolation • Unexpected error in one request can pollute state of others • Polluted state can lead to undeﬁned behavior • Memory leaks, inﬁnite loops, security issues • Only way to get back to a known good state: bail out, restart

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This leads us to the game plan

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The game plan 1. Follow the guiding principles 2. Know and use different mechanisms for effective handling 3. Have a global catch-all for the errors you couldn't handle 4. Use a process manager so shutting down is no big deal 5. Accept when it's time to pack up shop, clean up, shut down

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1. Know and use different mechanisms for effective handling

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Error Mechanisms in Node • Try/Catch - throw and try/catch • Callbacks - err ﬁrst argument and if (err) • Promises - reject(err) and .catch() • Async/Await - Sugar for promises + try/catch • EventEmitters - error events and .on('error') • Express - next(err) and error-handling middleware

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Callbacks & Try-catch function readAndParse(file, callback) { fs.readFile(file, { encoding: 'utf8' }, function (err, data) { if (err) return callback(err); var parsed; try { parsed = JSON.parse(data); } catch (e) { return callback(e); } callback(null, parsed); }); }

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Promises var p = new Promise(function (resolve, reject) { fs.readFile('data.txt', function (err, data) { if (err) return reject(err); resolve(data); }); }); p.then(parseJson) .then(doSomethingElse) .catch(function (reason) { // if readFile or JSON parsing or something else failed, // we can handle it here });

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Async/Await try { await somePromiseThatRejects() } catch (e) { // e is the rejection reason! }

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EventEmitters • Servers, sockets, requests, streams • Long-lived objects with asynchronous stuff going on • They can emit error events: listen for them! • If an error event is emitted without an error listener... • The Error object will be thrown instead! • How operational errors become programming errors

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EventEmitter Request Example var req = http.get(url, function (res) { doSomething(res); }); req.on('error', function (err) { // we caught the request error, let's recover }

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Express Error Middleware app.post('/login', function (req, res, next) { db.query('SELECT ...', function (err, user) { if (err) return next(err); }); }); app.use(function (req, res, next, err) { // spit out your own error page, log the error, etc next(); });

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2. Have a global catch-all for the errors you couldn't handle

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Basic Global Error Handler process.on('uncaughtException', function (err) { console.log('Uncaught exception! Oh no!'); console.error(err); process.exit(1); });

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3. Use a process manager so shutting down is no big deal

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Process managers and Node • Run multiple server processes • One of them dying won't take us ofﬂine • Node cluster module • Process managers: systemd, pm2, forever, naught • Will automatically restart processes when they die • Some provide further Node-speciﬁc functionality

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Example with naught var server = http.createServer(...); process.on('uncaughtException', function (err) { console.log('Uncaught exception! Oh no!'); console.error(err); // tell naught to stop sending us connections & start up a replacement process.send('offline'); process.exit(1); }); server.listen(80, function () { // tell naught we're ready for traffic if (process.send) process.send('online'); });

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4. Accept when it's time to pack up shop, clean up, shut down

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When we catch a "fatal" error, we want to: • Quit accepting new connections • Start reporting whatever we're gonna report • Tell proc manager we're gonna die so it starts replacement • Wait for any existing requests, sockets, etc to be dealt with • Close any open resources, connections, etc • Shut down

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We want to report that stack trace! function reportError(err, cb) { // send the stack trace somewhere, then call cb() } process.on('uncaughtException', function (err) { process.send('offline'); reportError(err, function (sendErr) { // once error has been reported, let's shut down process.exit(1); }); });

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Maybe get a text message: var myPhone = "..." function reportError(err, cb) { console.error(err); twilio.sendTextMessage(myPhone, err.message, cb); }

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Also useful to report operational errors db.query('SELECT ...', function (err, results) { if (err) { return reportError(err); } doSomething(results); });

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But then the database goes down...

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Alternatively, use Sentry The raven npm package is Sentry's Node error reporting SDK: var Raven = require('raven'); Raven.config(''); function reportError(err, cb) { console.error(err); Raven.captureException(err, cb); }

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Graceful shutdown server = http.createServer(...); function shutDownGracefully(err, cb) { // quit accepting connections, clean up any other resources server.close(function () { // could also wait for all connections: server._connections reportError(err, cb) }); } process.on('uncaughtException', function (err) { process.send('offline'); shutDownGracefully(function () { process.exit(1); }); }); • Wait for any existing requests, sockets, etc to be dealt with • Close any open resources, connections, etc

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Big Combined Example server = http.createServer(...); function reportError(err, cb) { console.error(err); Raven.captureException(err, cb); } function shutDownGracefully(err, cb) { // quit accepting connections, clean up any other resources server.close(function () { // could also wait for all connections: server._connections reportError(err, cb) }); } process.on('uncaughtException', function (err) { process.send('offline'); shutDownGracefully(function () { process.exit(1); }); }); server.listen(80, function () { if (process.send) process.send('online'); });

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What NOT to do with a global catch-all: • Just log the error and carry on • Keep the process running indeﬁnitely • Try to recover in any way: it's too late! • Try to centralize handling of operational errors into one place

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Other global error mechanisms • process.on('uncaughtException') • process.on('unhandledRejection') • Currently non-fatal, warning starting in Node 7 • Future: fatal, will cause process exit • Domains: application code shouldn't need them

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Recap: overall 1. Follow the guiding principles 2. Know and use different mechanisms for effective handling 3. Have a global catch-all for the errors you couldn't handle 4. Use a process manager so shutting down is no big deal 5. Accept when it's time to pack up shop, clean up, shut down

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Related things I didn't go into • Run-to-completion semantics & the event loop • V8 stacktrace API • The "callback contract" • Asynchronous stacktraces • Cluster module & process managers • Domains • async_hooks (!!!!!)

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Some related links • https://sentry.io/for/node/ • https://www.joyent.com/node-js/production/design/errors • https://nodejs.org/api/errors.html

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Thank you! • Slides available at GitHub.com/LewisJEllis/node-error-talk • I'm Lewis J Ellis: @lewisjellis on Twitter and GitHub