Background
I have a REST API using MongoDB, Node.js and Express that makes a request to my NoSQL DB and depending on different results, I want to differentiate which error I send the customer.
Problem
The current version of my code has a generic error handler and always sends the same error message to the client:
api.post("/Surveys/", (req, res) => {
const surveyJSON = req.body;
const sender = replyFactory(res);
Survey.findOne({_id: surveyJSON.id})
.then(doc => {
if(doc !== null)
throw {reason: "ObjectRepeated"};
//do stuff
return new Survey(surveyJSON).save();
})
.then(() => sender.replySuccess("Object saved with success!"))
.catch(error => {
/*
* Here I don't know if:
* 1. The object is repeated
* 2. There was an error while saving (eg. validation failed)
* 3. Server had a hiccup (500)
*/
sender.replyBadRequest(error);
});
});
This is a problem, because the client will always get the same error message, no matter what and I need error differentiation!
Research
I found a possible solution, based on the division of logic and error/response handling:
Handling multiple catches in promise chain
However, I don't understand a few things:
I don't see how, at least in my example, I can separate the logic from the response. The response will depend on the logic after all!
I would like to avoid error sub-classing and hierarchy. First because I don't use bluebird, and I can't subclass the error class the answer suggests, and second because I don't want my code with a billion different error classes with brittle hierarchies that will change in the future.
My idea, that I don't really like either
With this structure, if I want error differentiation, the only thing I can do is to detect an error occurred, build an object with that information and then throw it:
.then(doc => {
if(doc === null)
throw {reason: "ObjectNotFound"};
//do stuff
return doc.save();
})
.catch(error => {
if(error.reason === "ObjectNotFound")
sendJsonResponse(res, 404, err);
else if(error.reason === "Something else ")
sendJsonResponse(/*you get the idea*/);
else //if we don't know the reasons, its because the server likely crashed
sendJsonResponse(res, 500, err);
});
I personally don't find this solution particularly attractive because it means I will have a huge if then else chain of statements in my catch block.
Also, as mentioned in the previous post, generic error handlers are usually frowned upon (and for a good reason imo).
Questions
How can I improve this code?
Objectives
When I started this thread, I had two objectives in mind:
Having error differentiation
Avoid an if then else of doom in a generic catcher
I have now come up with two radically distinct solutions, which I now post here, for future reference.
Solution 1: Generic error handler with Errors Object
This solution is based on the solution from #Marc Rohloff, however, instead of having an array of functions and looping through each one, I have an object with all the errors.
This approach is better because it is faster, and removes the need for the if validation, meaning you actually do less logic:
const errorHandlers = {
ObjectRepeated: function(error){
return { code: 400, error };
},
SomethingElse: function(error){
return { code: 499, error };
}
};
Survey.findOne({
_id: "bananasId"
})
.then(doc => {
//we dont want to add this object if we already have it
if (doc !== null)
throw { reason: "ObjectRepeated", error:"Object could not be inserted because it already exists."};
//saving empty object for demonstration purposes
return new Survey({}).save();
})
.then(() => console.log("Object saved with success!"))
.catch(error => {
respondToError(error);
});
const respondToError = error => {
const errorObj = errorHandlers[error.reason](error);
if (errorObj !== undefined)
console.log(`Failed with ${errorObj.code} and reason ${error.reason}: ${JSON.stringify(errorObj)}`);
else
//send default error Obj, server 500
console.log(`Generic fail message ${JSON.stringify(error)}`);
};
This solution achieves:
Partial error differentiation (I will explain why)
Avoids an if then else of doom.
This solution only has partial error differentiation. The reason for this is because you can only differentiate errors that you specifically build, via the throw {reaon: "reasonHere", error: "errorHere"} mechanism.
In this example, you would be able to know if the document already exists, but if there is an error saving the said document (lets say, a validation one) then it would be treated as "Generic" error and thrown as a 500.
To achieve full error differentiation with this, you would have to use the nested Promise anti pattern like the following:
.then(doc => {
//we dont want to add this object if we already have it
if (doc !== null)
throw { reason: "ObjectRepeated", error:"Object could not be inserted because it already exists." };
//saving empty object for demonstration purposes
return new Survey({}).save()
.then(() => {console.log("great success!");})
.catch(error => {throw {reason: "SomethingElse", error}});
})
It would work... But I see it as a best practice to avoid anti-patterns.
Solution 2: Using ECMA6 Generators via co.
This solution uses Generators via the library co. Meant to replace Promises in near future with a syntax similar to async/await this new feature allows you to write asynchronous code that reads like synchronous (well, almost).
To use it, you first need to install co, or something similar like ogen. I pretty much prefer co, so that is what i will be using here instead.
const requestHandler = function*() {
const survey = yield Survey.findOne({
_id: "bananasId"
});
if (survey !== null) {
console.log("use HTTP PUT instead!");
return;
}
try {
//saving empty object for demonstration purposes
yield(new Survey({}).save());
console.log("Saved Successfully !");
return;
}
catch (error) {
console.log(`Failed to save with error: ${error}`);
return;
}
};
co(requestHandler)
.then(() => {
console.log("finished!");
})
.catch(console.log);
The generator function requestHandler will yield all Promises to the library, which will resolve them and either return or throw accordingly.
Using this strategy, you effectively code like you were coding synchronous code (except for the use of yield).
I personally prefer this strategy because:
Your code is easy to read and it looks synchronous (while still have the advantages of asynchronous code).
You do not have to build and throw error objects every where, you can simply send the message immediately.
And, you can BREAK the code flow via return. This is not possible in a promise chain, as in those you have to force a throw (many times a meaningless one) and catch it to stop executing.
The generator function will only be executed once passed into the library co, which then returns a Promise, stating if the execution was successful or not.
This solution achieves:
error differentiation
avoids if then else hell and generalized catchers (although you will use try/catch in your code, and you still have access to a generalized catcher if you need one).
Using generators is, in my opinion, more flexible and makes for easier to read code. Not all cases are cases for generator usage (like mpj suggests in the video) but in this specific case, I believe it to be the best option.
Conclusion
Solution 1: good classical approach to the problem, but has issues inherent to promise chaining. You can overcome some of them by nesting promises, but that is an anti pattern and defeats their purpose.
Solution 2: more versatile, but requires a library and knowledge on how generators work. Furthermore, different libraries will have different behaviors, so you should be aware of that.
I think a good improvement would be creating an error utility method that takes the error message as a parameter, then does all your ifs to try to parse the error (logic that does have to happen somewhere) and returns a formatted error.
function errorFormatter(errMsg) {
var formattedErr = {
responseCode: 500,
msg: 'Internal Server Error'
};
switch (true) {
case errMsg.includes('ObjectNotFound'):
formattedErr.responseCode = 404;
formattedErr.msg = 'Resource not found';
break;
}
return formattedErr;
}
I have a route that first need to query the database, then with the results, query another web service, then with that result render the page.
I have that flow worked out and am trying to figure out the error handling. Given that i talk to multiple service, I'm trying to massage the error before returning them to express.
Here is the structure of the code for the route:
Models.Episode.findById(request.params.episodeID)
.catch(function (error) {
throw (throwjs.notFound());
})
.then(function (episode) {
if (episode.getUser().id !== request.user.href) {
return next(throwjs.unauthorized("You do not have access to this podcast"));
}
return doSomeOtherAsyncStuff();
})
.then(function (queryResponse) {
renderPage();
})
.catch(function (error) {
next(error);
});
My problem is with the first catch. My goal in this catch is to repackage the error and stop the execution and send the error to express middleware.
With the way it is written above, the execution stops, but my express error handler are not called.
I tried rewriting the first catch as
.catch(function(error){
return next(error);
})
But that does not solve the issue. The only solution i found is to move the catch to the end. But then i lose context of the failure location.
Any clue as to what i'm doing wrong?
Thanks, olivier
I'd recommend taking a different approach so you don't have to rely on long running promise chains. With the following approach, you've decoupled your authorization and and validation to separate middleware, since they're not necessarily a concern of the actual episode handler itself. Plus this approach is more idiomatic to express.
An added bonus is that you're free to pass errors down to an error handler so you further decouple your errors from your route handlers.
function validateEpisode(req, res, next) {
Models.Episode
.findById(req.params.episodeID)
.then(function(episode) {
req.yourApp.episode = episode;
next() // everything's good
})
.catch(function(error) {
// would be better to pass error in next
// so you can have a general error handler
// do something with the actual error
next(throwjs.notFound());
});
}
function authUserByEpisode(req, res, next) {
if (req.yourApp.episode.getUser().id !== req.user.href) {
next(throwjs.unauthorized("You do not have access to this podcast"));
}
next(); // authorized
}
function episodeController(req, res) {
// do something with req.yourApp.episode
}
app.get('/episode/:id', validateEpisode, authUserByEpisode, episodeController)
Well after all, this is related to the throwjs framework I'm using and the fact that I'm using incorrectly
throw (throwjs.notFound());
should be
throw (new throwjs.notFound());
...
I am modifying a node.js library to support true asynchronous operations.
I am having troubles with Mocha and Chai to make this (a similar) test pass.
it('should throw an error', function() {
expect(function() {
process.nextTick(function() {
throw new Error('This is my error');
});
}).to.throw(Error);
});
The problem is - because of the nextTick - that the Error is thrown out of scope of it and besides the test failing, Mocha also outputs the below.
Uncaught Error: This is my error
What is the proper way to structure this test in order to make it succeed?
Hmm... in a full-fledged application what I'd do is probably use something like Sinon to check that the method that should throw an error has been called and is throwing.
In code where you cannot do this, then the following method would trap the exception:
var expect = require("chai").expect;
var domain = require("domain");
it('should throw an error', function(done) {
var d = domain.create();
d.on('error', function (err) {
// Exit the current domain.
d.exit();
// We must execute this code at the next tick.
process.nextTick(function () {
console.log(err); // Just to show something on the console.
expect(err instanceof Error).to.be.true;
done();
});
});
d.run(function () {
process.nextTick(function() {
throw new Error('This is my error');
});
});
});
This code creates a "domain" stored in d. A domain will emit error events on uncaught exceptions that happen in it so we run the test inside the domain (d.run(...)) we've created and wait for an exception to happen (d.on('error', ...). We check that it is an Error object. (In a real test, I'd also check the error message.) When we are finished we call done() to tell Mocha that the asynchronous test is over.
The handler for error events calls d.exit(). This is to make it so that Mocha can catch the error normally if the assertion (expect(err instanceof Error)...) turns out to fail. If we do not exit the domain, then the domain will trap the error. Also, the check itself must be performed on the next tick to be outside the d domain.
Is Using domain A Problem?
NO!
The documentation for domain comes with some warnings about shutting down operations once an uncaught exception is caught when running an ongoing process, like a server. Then the thing to do is to clean what can be cleaned and exit as soon as possible. However, using domain in a test does not differ from what Mocha is already doing. The way Mocha catches unhandled exceptions in asynchronous code is by using process.on('uncaughtException'. Upon catching an unhandled exception Mocha marks the current test as failed and continues. Yet the documentation regarding uncaughtException says "Don't use it, use domains instead. If you do use it, restart your application after every unhandled exception!"
Ergo, anyone who has a problem with using domain should not be using Mocha in the first place.
You're trying to catch an exception on the incorrect function, the container for the function that throws. Also, because the function is wrapped in nextTick it is executed in a different stack and therefore the exception cannot be captured (this is simply a JS thing unfortunately).
Try this instead:
it ('should throw an error', function (done) {
process.nextTick(function () {
var myFn = function () { throw new Error() };
expect(myFn).to.throw(Error);
// Tell mocha the test is complete
done();
});
});
Update: There is no proper way to structure this test to make it pass as you cannot catch the exception in this scenario. Perhaps update your code to use callbacks to handle errors:
function doSomethingUnsafe() {
try {
// Run code here that may cause exceptions...
callback(null, 'Woohoo! No errors!');
} catch (e) {
callback (e, null);
}
}
I just started trying out node.js a few days ago. I've realized that the Node is terminated whenever I have an unhandled exception in my program. This is different than the normal server container that I have been exposed to where only the Worker Thread dies when unhandled exceptions occur and the container would still be able to receive the request. This raises a few questions:
Is process.on('uncaughtException') the only effective way to guard against it?
Will process.on('uncaughtException') catch the unhandled exception during execution of asynchronous processes as well?
Is there a module that is already built (such as sending email or writing to a file) that I could leverage in the case of uncaught exceptions?
I would appreciate any pointer/article that would show me the common best practices for handling uncaught exceptions in node.js
Update: Joyent now has their own guide. The following information is more of a summary:
Safely "throwing" errors
Ideally we'd like to avoid uncaught errors as much as possible, as such, instead of literally throwing the error, we can instead safely "throw" the error using one of the following methods depending on our code architecture:
For synchronous code, if an error happens, return the error:
// Define divider as a syncrhonous function
var divideSync = function(x,y) {
// if error condition?
if ( y === 0 ) {
// "throw" the error safely by returning it
return new Error("Can't divide by zero")
}
else {
// no error occured, continue on
return x/y
}
}
// Divide 4/2
var result = divideSync(4,2)
// did an error occur?
if ( result instanceof Error ) {
// handle the error safely
console.log('4/2=err', result)
}
else {
// no error occured, continue on
console.log('4/2='+result)
}
// Divide 4/0
result = divideSync(4,0)
// did an error occur?
if ( result instanceof Error ) {
// handle the error safely
console.log('4/0=err', result)
}
else {
// no error occured, continue on
console.log('4/0='+result)
}
For callback-based (ie. asynchronous) code, the first argument of the callback is err, if an error happens err is the error, if an error doesn't happen then err is null. Any other arguments follow the err argument:
var divide = function(x,y,next) {
// if error condition?
if ( y === 0 ) {
// "throw" the error safely by calling the completion callback
// with the first argument being the error
next(new Error("Can't divide by zero"))
}
else {
// no error occured, continue on
next(null, x/y)
}
}
divide(4,2,function(err,result){
// did an error occur?
if ( err ) {
// handle the error safely
console.log('4/2=err', err)
}
else {
// no error occured, continue on
console.log('4/2='+result)
}
})
divide(4,0,function(err,result){
// did an error occur?
if ( err ) {
// handle the error safely
console.log('4/0=err', err)
}
else {
// no error occured, continue on
console.log('4/0='+result)
}
})
For eventful code, where the error may happen anywhere, instead of throwing the error, fire the error event instead:
// Definite our Divider Event Emitter
var events = require('events')
var Divider = function(){
events.EventEmitter.call(this)
}
require('util').inherits(Divider, events.EventEmitter)
// Add the divide function
Divider.prototype.divide = function(x,y){
// if error condition?
if ( y === 0 ) {
// "throw" the error safely by emitting it
var err = new Error("Can't divide by zero")
this.emit('error', err)
}
else {
// no error occured, continue on
this.emit('divided', x, y, x/y)
}
// Chain
return this;
}
// Create our divider and listen for errors
var divider = new Divider()
divider.on('error', function(err){
// handle the error safely
console.log(err)
})
divider.on('divided', function(x,y,result){
console.log(x+'/'+y+'='+result)
})
// Divide
divider.divide(4,2).divide(4,0)
Safely "catching" errors
Sometimes though, there may still be code that throws an error somewhere which can lead to an uncaught exception and a potential crash of our application if we don't catch it safely. Depending on our code architecture we can use one of the following methods to catch it:
When we know where the error is occurring, we can wrap that section in a node.js domain
var d = require('domain').create()
d.on('error', function(err){
// handle the error safely
console.log(err)
})
// catch the uncaught errors in this asynchronous or synchronous code block
d.run(function(){
// the asynchronous or synchronous code that we want to catch thrown errors on
var err = new Error('example')
throw err
})
If we know where the error is occurring is synchronous code, and for whatever reason can't use domains (perhaps old version of node), we can use the try catch statement:
// catch the uncaught errors in this synchronous code block
// try catch statements only work on synchronous code
try {
// the synchronous code that we want to catch thrown errors on
var err = new Error('example')
throw err
} catch (err) {
// handle the error safely
console.log(err)
}
However, be careful not to use try...catch in asynchronous code, as an asynchronously thrown error will not be caught:
try {
setTimeout(function(){
var err = new Error('example')
throw err
}, 1000)
}
catch (err) {
// Example error won't be caught here... crashing our app
// hence the need for domains
}
If you do want to work with try..catch in conjunction with asynchronous code, when running Node 7.4 or higher you can use async/await natively to write your asynchronous functions.
Another thing to be careful about with try...catch is the risk of wrapping your completion callback inside the try statement like so:
var divide = function(x,y,next) {
// if error condition?
if ( y === 0 ) {
// "throw" the error safely by calling the completion callback
// with the first argument being the error
next(new Error("Can't divide by zero"))
}
else {
// no error occured, continue on
next(null, x/y)
}
}
var continueElsewhere = function(err, result){
throw new Error('elsewhere has failed')
}
try {
divide(4, 2, continueElsewhere)
// ^ the execution of divide, and the execution of
// continueElsewhere will be inside the try statement
}
catch (err) {
console.log(err.stack)
// ^ will output the "unexpected" result of: elsewhere has failed
}
This gotcha is very easy to do as your code becomes more complex. As such, it is best to either use domains or to return errors to avoid (1) uncaught exceptions in asynchronous code (2) the try catch catching execution that you don't want it to. In languages that allow for proper threading instead of JavaScript's asynchronous event-machine style, this is less of an issue.
Finally, in the case where an uncaught error happens in a place that wasn't wrapped in a domain or a try catch statement, we can make our application not crash by using the uncaughtException listener (however doing so can put the application in an unknown state):
// catch the uncaught errors that weren't wrapped in a domain or try catch statement
// do not use this in modules, but only in applications, as otherwise we could have multiple of these bound
process.on('uncaughtException', function(err) {
// handle the error safely
console.log(err)
})
// the asynchronous or synchronous code that emits the otherwise uncaught error
var err = new Error('example')
throw err
Following is a summarization and curation from many different sources on this topic including code example and quotes from selected blog posts. The complete list of best practices can be found here
Best practices of Node.JS error handling
Number1: Use promises for async error handling
TL;DR: Handling async errors in callback style is probably the fastest way to hell (a.k.a the pyramid of doom). The best gift you can give to your code is using instead a reputable promise library which provides much compact and familiar code syntax like try-catch
Otherwise: Node.JS callback style, function(err, response), is a promising way to un-maintainable code due to the mix of error handling with casual code, excessive nesting and awkward coding patterns
Code example - good
doWork()
.then(doWork)
.then(doError)
.then(doWork)
.catch(errorHandler)
.then(verify);
code example anti pattern – callback style error handling
getData(someParameter, function(err, result){
if(err != null)
//do something like calling the given callback function and pass the error
getMoreData(a, function(err, result){
if(err != null)
//do something like calling the given callback function and pass the error
getMoreData(b, function(c){
getMoreData(d, function(e){
...
});
});
});
});
});
Blog quote: "We have a problem with promises"
(From the blog pouchdb, ranked 11 for the keywords "Node Promises")
"…And in fact, callbacks do something even more sinister: they deprive us of the stack, which is something we usually take for granted in programming languages. Writing code without a stack is a lot like driving a car without a brake pedal: you don’t realize how badly you need it, until you reach for it and it’s not there. The whole point of promises is to give us back the language fundamentals we lost when we went async: return, throw, and the stack. But you have to know how to use promises correctly in order to take advantage of them."
Number2: Use only the built-in Error object
TL;DR: It pretty common to see code that throws errors as string or as a custom type – this complicates the error handling logic and the interoperability between modules. Whether you reject a promise, throw exception or emit error – using Node.JS built-in Error object increases uniformity and prevents loss of error information
Otherwise: When executing some module, being uncertain which type of errors come in return – makes it much harder to reason about the coming exception and handle it. Even worth, using custom types to describe errors might lead to loss of critical error information like the stack trace!
Code example - doing it right
//throwing an Error from typical function, whether sync or async
if(!productToAdd)
throw new Error("How can I add new product when no value provided?");
//'throwing' an Error from EventEmitter
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
//'throwing' an Error from a Promise
return new promise(function (resolve, reject) {
DAL.getProduct(productToAdd.id).then((existingProduct) =>{
if(existingProduct != null)
return reject(new Error("Why fooling us and trying to add an existing product?"));
code example anti pattern
//throwing a String lacks any stack trace information and other important properties
if(!productToAdd)
throw ("How can I add new product when no value provided?");
Blog quote: "A string is not an error"
(From the blog devthought, ranked 6 for the keywords “Node.JS error object”)
"…passing a string instead of an error results in reduced interoperability between modules. It breaks contracts with APIs that might be performing instanceof Error checks, or that want to know more about the error. Error objects, as we’ll see, have very interesting properties in modern JavaScript engines besides holding the message passed to the constructor.."
Number3: Distinguish operational vs programmer errors
TL;DR: Operations errors (e.g. API received an invalid input) refer to known cases where the error impact is fully understood and can be handled thoughtfully. On the other hand, programmer error (e.g. trying to read undefined variable) refers to unknown code failures that dictate to gracefully restart the application
Otherwise: You may always restart the application when an error appear, but why letting ~5000 online users down because of a minor and predicted error (operational error)? the opposite is also not ideal – keeping the application up when unknown issue (programmer error) occurred might lead unpredicted behavior. Differentiating the two allows acting tactfully and applying a balanced approach based on the given context
Code example - doing it right
//throwing an Error from typical function, whether sync or async
if(!productToAdd)
throw new Error("How can I add new product when no value provided?");
//'throwing' an Error from EventEmitter
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
//'throwing' an Error from a Promise
return new promise(function (resolve, reject) {
DAL.getProduct(productToAdd.id).then((existingProduct) =>{
if(existingProduct != null)
return reject(new Error("Why fooling us and trying to add an existing product?"));
code example - marking an error as operational (trusted)
//marking an error object as operational
var myError = new Error("How can I add new product when no value provided?");
myError.isOperational = true;
//or if you're using some centralized error factory (see other examples at the bullet "Use only the built-in Error object")
function appError(commonType, description, isOperational) {
Error.call(this);
Error.captureStackTrace(this);
this.commonType = commonType;
this.description = description;
this.isOperational = isOperational;
};
throw new appError(errorManagement.commonErrors.InvalidInput, "Describe here what happened", true);
//error handling code within middleware
process.on('uncaughtException', function(error) {
if(!error.isOperational)
process.exit(1);
});
Blog Quote: "Otherwise you risk the state"
(From the blog debugable, ranked 3 for the keywords "Node.JS uncaught exception")
"…By the very nature of how throw works in JavaScript, there is almost never any way to safely “pick up where you left off”, without leaking references, or creating some other sort of undefined brittle state. The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, you might have many connections open, and it is not reasonable to abruptly shut those down because an error was triggered by someone else. The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker"
Number4: Handle errors centrally, through but not within middleware
TL;DR: Error handling logic such as mail to admin and logging should be encapsulated in a dedicated and centralized object that all end-points (e.g. Express middleware, cron jobs, unit-testing) call when an error comes in.
Otherwise: Not handling errors within a single place will lead to code duplication and probably to errors that are handled improperly
Code example - a typical error flow
//DAL layer, we don't handle errors here
DB.addDocument(newCustomer, (error, result) => {
if (error)
throw new Error("Great error explanation comes here", other useful parameters)
});
//API route code, we catch both sync and async errors and forward to the middleware
try {
customerService.addNew(req.body).then(function (result) {
res.status(200).json(result);
}).catch((error) => {
next(error)
});
}
catch (error) {
next(error);
}
//Error handling middleware, we delegate the handling to the centrzlied error handler
app.use(function (err, req, res, next) {
errorHandler.handleError(err).then((isOperationalError) => {
if (!isOperationalError)
next(err);
});
});
Blog quote: "Sometimes lower levels can’t do anything useful except propagate the error to their caller"
(From the blog Joyent, ranked 1 for the keywords “Node.JS error handling”)
"…You may end up handling the same error at several levels of the stack. This happens when lower levels can’t do anything useful except propagate the error to their caller, which propagates the error to its caller, and so on. Often, only the top-level caller knows what the appropriate response is, whether that’s to retry the operation, report an error to the user, or something else. But that doesn’t mean you should try to report all errors to a single top-level callback, because that callback itself can’t know in what context the error occurred"
Number5: Document API errors using Swagger
TL;DR: Let your API callers know which errors might come in return so they can handle these thoughtfully without crashing. This is usually done with REST API documentation frameworks like Swagger
Otherwise: An API client might decide to crash and restart only because he received back an error he couldn’t understand. Note: the caller of your API might be you (very typical in a microservices environment)
Blog quote: "You have to tell your callers what errors can happen"
(From the blog Joyent, ranked 1 for the keywords “Node.JS logging”)
…We’ve talked about how to handle errors, but when you’re writing a new function, how do you deliver errors to the code that called your function? …If you don’t know what errors can happen or don’t know what they mean, then your program cannot be correct except by accident. So if you’re writing a new function, you have to tell your callers what errors can happen and what they mea
Number6: Shut the process gracefully when a stranger comes to town
TL;DR: When an unknown error occurs (a developer error, see best practice number #3)- there is uncertainty about the application healthiness. A common practice suggests restarting the process carefully using a ‘restarter’ tool like Forever and PM2
Otherwise: When an unfamiliar exception is caught, some object might be in a faulty state (e.g an event emitter which is used globally and not firing events anymore due to some internal failure) and all future requests might fail or behave crazily
Code example - deciding whether to crash
//deciding whether to crash when an uncaught exception arrives
//Assuming developers mark known operational errors with error.isOperational=true, read best practice #3
process.on('uncaughtException', function(error) {
errorManagement.handler.handleError(error);
if(!errorManagement.handler.isTrustedError(error))
process.exit(1)
});
//centralized error handler encapsulates error-handling related logic
function errorHandler(){
this.handleError = function (error) {
return logger.logError(err).then(sendMailToAdminIfCritical).then(saveInOpsQueueIfCritical).then(determineIfOperationalError);
}
this.isTrustedError = function(error)
{
return error.isOperational;
}
Blog quote: "There are three schools of thoughts on error handling"
(From the blog jsrecipes)
…There are primarily three schools of thoughts on error handling: 1. Let the application crash and restart it. 2. Handle all possible errors and never crash. 3. Balanced approach between the two
Number7: Use a mature logger to increase errors visibility
TL;DR: A set of mature logging tools like Winston, Bunyan or Log4J, will speed-up error discovery and understanding. So forget about console.log.
Otherwise: Skimming through console.logs or manually through messy text file without querying tools or a decent log viewer might keep you busy at work until late
Code example - Winston logger in action
//your centralized logger object
var logger = new winston.Logger({
level: 'info',
transports: [
new (winston.transports.Console)(),
new (winston.transports.File)({ filename: 'somefile.log' })
]
});
//custom code somewhere using the logger
logger.log('info', 'Test Log Message with some parameter %s', 'some parameter', { anything: 'This is metadata' });
Blog quote: "Lets identify a few requirements (for a logger):"
(From the blog strongblog)
…Lets identify a few requirements (for a logger):
1. Time stamp each log line. This one is pretty self explanatory – you should be able to tell when each log entry occured.
2. Logging format should be easily digestible by humans as well as machines.
3. Allows for multiple configurable destination streams. For example, you might be writing trace logs to one file but when an error is encountered, write to the same file, then into error file and send an email at the same time…
Number8: Discover errors and downtime using APM products
TL;DR: Monitoring and performance products (a.k.a APM) proactively gauge your codebase or API so they can auto-magically highlight errors, crashes and slow parts that you were missing
Otherwise: You might spend great effort on measuring API performance and downtimes, probably you’ll never be aware which are your slowest code parts under real world scenario and how these affects the UX
Blog quote: "APM products segments"
(From the blog Yoni Goldberg)
"…APM products constitutes 3 major segments:1. Website or API monitoring – external services that constantly monitor uptime and performance via HTTP requests. Can be setup in few minutes. Following are few selected contenders: Pingdom, Uptime Robot, and New Relic
2. Code instrumentation – products family which require to embed an agent within the application to benefit feature slow code detection, exceptions statistics, performance monitoring and many more. Following are few selected contenders: New Relic, App Dynamics
3. Operational intelligence dashboard – these line of products are focused on facilitating the ops team with metrics and curated content that helps to easily stay on top of application performance. This is usually involves aggregating multiple sources of information (application logs, DB logs, servers log, etc) and upfront dashboard design work. Following are few selected contenders: Datadog, Splunk"
The above is a shortened version - see here more best practices and examples
You can catch uncaught exceptions, but it's of limited use. See http://debuggable.com/posts/node-js-dealing-with-uncaught-exceptions:4c933d54-1428-443c-928d-4e1ecbdd56cb
monit, forever or upstart can be used to restart node process when it crashes. A graceful shutdown is best you can hope for (e.g. save all in-memory data in uncaught exception handler).
nodejs domains is the most up to date way of handling errors in nodejs. Domains can capture both error/other events as well as traditionally thrown objects. Domains also provide functionality for handling callbacks with an error passed as the first argument via the intercept method.
As with normal try/catch-style error handling, is is usually best to throw errors when they occur, and block out areas where you want to isolate errors from affecting the rest of the code. The way to "block out" these areas are to call domain.run with a function as a block of isolated code.
In synchronous code, the above is enough - when an error happens you either let it be thrown through, or you catch it and handle there, reverting any data you need to revert.
try {
//something
} catch(e) {
// handle data reversion
// probably log too
}
When the error happens in an asynchronous callback, you either need to be able to fully handle the rollback of data (shared state, external data like databases, etc). OR you have to set something to indicate that an exception has happened - where ever you care about that flag, you have to wait for the callback to complete.
var err = null;
var d = require('domain').create();
d.on('error', function(e) {
err = e;
// any additional error handling
}
d.run(function() { Fiber(function() {
// do stuff
var future = somethingAsynchronous();
// more stuff
future.wait(); // here we care about the error
if(err != null) {
// handle data reversion
// probably log too
}
})});
Some of that above code is ugly, but you can create patterns for yourself to make it prettier, eg:
var specialDomain = specialDomain(function() {
// do stuff
var future = somethingAsynchronous();
// more stuff
future.wait(); // here we care about the error
if(specialDomain.error()) {
// handle data reversion
// probably log too
}
}, function() { // "catch"
// any additional error handling
});
UPDATE (2013-09):
Above, I use a future that implies fibers semantics, which allow you to wait on futures in-line. This actually allows you to use traditional try-catch blocks for everything - which I find to be the best way to go. However, you can't always do this (ie in the browser)...
There are also futures that don't require fibers semantics (which then work with normal, browsery JavaScript). These can be called futures, promises, or deferreds (I'll just refer to futures from here on). Plain-old-JavaScript futures libraries allow errors to be propagated between futures. Only some of these libraries allow any thrown future to be correctly handled, so beware.
An example:
returnsAFuture().then(function() {
console.log('1')
return doSomething() // also returns a future
}).then(function() {
console.log('2')
throw Error("oops an error was thrown")
}).then(function() {
console.log('3')
}).catch(function(exception) {
console.log('handler')
// handle the exception
}).done()
This mimics a normal try-catch, even though the pieces are asynchronous. It would print:
1
2
handler
Note that it doesn't print '3' because an exception was thrown that interrupts that flow.
Take a look at bluebird promises:
https://github.com/petkaantonov/bluebird
Note that I haven't found many other libraries other than these that properly handle thrown exceptions. jQuery's deferred, for example, don't - the "fail" handler would never get the exception thrown an a 'then' handler, which in my opinion is a deal breaker.
I wrote about this recently at http://snmaynard.com/2012/12/21/node-error-handling/. A new feature of node in version 0.8 is domains and allow you to combine all the forms of error handling into one easier manage form. You can read about them in my post.
You can also use something like Bugsnag to track your uncaught exceptions and be notified via email, chatroom or have a ticket created for an uncaught exception (I am the co-founder of Bugsnag).
One instance where using a try-catch might be appropriate is when using a forEach loop. It is synchronous but at the same time you cannot just use a return statement in the inner scope. Instead a try and catch approach can be used to return an Error object in the appropriate scope. Consider:
function processArray() {
try {
[1, 2, 3].forEach(function() { throw new Error('exception'); });
} catch (e) {
return e;
}
}
It is a combination of the approaches described by #balupton above.
I would just like to add that Step.js library helps you handle exceptions by always passing it to the next step function. Therefore you can have as a last step a function that check for any errors in any of the previous steps. This approach can greatly simplify your error handling.
Below is a quote from the github page:
any exceptions thrown are caught and passed as the first argument to
the next function. As long as you don't nest callback functions inline
your main functions this prevents there from ever being any uncaught
exceptions. This is very important for long running node.JS servers
since a single uncaught exception can bring the whole server down.
Furthermore, you can use Step to control execution of scripts to have a clean up section as the last step. For example if you want to write a build script in Node and report how long it took to write, the last step can do that (rather than trying to dig out the last callback).
Catching errors has been very well discussed here, but it's worth remembering to log the errors out somewhere so you can view them and fix stuff up.
Bunyan is a popular logging framework for NodeJS - it supporst writing out to a bunch of different output places which makes it useful for local debugging, as long as you avoid console.log.
In your domain's error handler you could spit the error out to a log file.
var log = bunyan.createLogger({
name: 'myapp',
streams: [
{
level: 'error',
path: '/var/tmp/myapp-error.log' // log ERROR to this file
}
]
});
This can get time consuming if you have lots of errors and/or servers to check, so it could be worth looking into a tool like Raygun (disclaimer, I work at Raygun) to group errors together - or use them both together.
If you decided to use Raygun as a tool, it's pretty easy to setup too
var raygunClient = new raygun.Client().init({ apiKey: 'your API key' });
raygunClient.send(theError);
Crossed with using a tool like PM2 or forever, your app should be able to crash, log out what happened and reboot without any major issues.
After reading this post some time ago I was wondering if it was safe to use domains for exception handling on an api / function level. I wanted to use them to simplify exception handling code in each async function I wrote. My concern was that using a new domain for each function would introduce significant overhead. My homework seems to indicate that there is minimal overhead and that performance is actually better with domains than with try catch in some situations.
http://www.lighthouselogic.com/#/using-a-new-domain-for-each-async-function-in-node/
If you want use Services in Ubuntu(Upstart): Node as a service in Ubuntu 11.04 with upstart, monit and forever.js
getCountryRegionData: (countryName, stateName) => {
let countryData, stateData
try {
countryData = countries.find(
country => country.countryName === countryName
)
} catch (error) {
console.log(error.message)
return error.message
}
try {
stateData = countryData.regions.find(state => state.name === stateName)
} catch (error) {
console.log(error.message)
return error.message
}
return {
countryName: countryData.countryName,
countryCode: countryData.countryShortCode,
stateName: stateData.name,
stateCode: stateData.shortCode,
}
},