I'm trying to check if a file exists, and if it doesn't, create the file.
self.checkFeedbackFile = function() {
// attempt to read the file - if it does not exist, create the file
var feedbackFile = fs.readFile('feedback.log', function (err, data) {
console.log("Checking that the file exists.");
});
if (feedbackFile === undefined) {
console.log("File does not exist. Creating a new file...");
}
}
I'm obviously very new to node. Been working in Ruby for a while, and I only have a little bit of experience in Javascript, so the concept of callbacks and async execution is quite foreign to me.
Right now my console is returning the following:
File does not exist. Creating a new file...
Sat Sep 29 2018 12:59:12 GMT-0400 (Eastern Daylight Time): Node server started on 127.0.0.1:3333 ...
Checking that the file exists.
In addition to not being sure how to do this, what is the ELI5 explanation for why console logs are printing out of order?
In your case the fs.readFile() method is called. It waits for the io to complete. However, the checkFeedbackFile() method continues to the if statement.
Would recommend that you use fs.stat to check if the file exists.
And fs.writeFileSync to write to the file a sync way.
self.checkFeedbackFile = function() {
// attempt to read the file - if it does not exist, create the file
fs.stat('feedback.log', function(err, data){
if(err){
console.log("File doesnt exist, creating a new file");
//Do Something
fs.writeFileSync('feedback.log',data);
}
}
}
Node.js is asycn, if you are coming in from C or Java, you are used to the this:
function main(){
1();
2();
3();
}
In C or Java the control will move to 2() only when 1() is finished. That is not the case with Node depending on what 1() is doing, if its doing anything in an async way, say IO, then 2() will be executed before 1() completes, and hence you see async methods taking a callback, which will be executed once the relevant function completes.
Would recommend taking a look at how Nodes Event loop works.
Ok. In your main function you have two console.logs.
console.log("Checking that the file exists."); is inside a callback. e
But the
console.log("File does not exist. Creating a new file...");
is just within a if block. so it's fired first
Because the console.log("Checking that the file exists."); code is dependent of the call of the readfile function. you wrap it in the callback give as a second argument to the readfile. one the operation of reading the file is completed the callback will be triggered with the result. and all other code which are at the same level as the readfile function will be execute as if the readfile function have already finish it execution. the call of readfile will not block the execution of all other code that come after that because you have provide some callback which will be execute when the opration is completed.
This behavior is diferent of the behavior of synchronous programming.
console.log('first');
console.log('second');
setTimeout(function(){
console.log('third');
}, 2000);
console.log('Fourth');
In the code provide above in the synchronous programming the execution will go line after line. To log the third text. the execution will wait 2 seconds but in non blocking programming (asynchronous) the Fourth text will be print before the execution of the console.log('third');
Background
I am working on a C# program which currently runs Node via Process.Start(). I am capturing the stdout and stderr from this child process and redirecting it for my own reasons. I am looking into replacing the invocation of Node.exe with a call to Edge.js instead. In order to be able to do this I must be able to reliably capture stdout and stderr from the Javascript running within Edge, and get the messages back into my C# application.
Approach 1
I'll describe this approach for completeness in case anybody recommends it :)
If the Edge process terminates, it is fairly easy to deal with this by simply declaring a msgs array and overwriting process.stdout.write and process.stderr.write with new functions that accumulate messages on that array, then at the end, simply return the msgs array. Example:
var msgs = [];
process.stdout.write = function (string) {
msgs.push({ stream: 'o', message : string });
};
process.stderr.write = function (string) {
msgs.push({ stream: 'e', message: string });
};
// Return to caller.
var result = { messages: msgs; ...other stuff... };
callback(null, result);
Obviously this only works if the Edge code terminates, and msgs may grow large in the worst case. However, it is likely to perform well because only one marshalling call is necessary to get all the messages back.
Approach 2
This is a little harder to explain. Instead of accumulating messages, we "hook" stdout and stderr using a delegate we send in from C#. In the C#, we create an object that we will pass into Edge, and that object has a property called stdoutHook:
dynamic payload = new ExpandoObject();
payload.stdoutHook = GetStdoutHook();
public Func<object, Task<object>> GetStdoutHook()
{
Func<object, Task<object>> hook = (message) =>
{
TheLogger.LogMessage((message as string).Trim());
return Task.FromResult<object>(null);
};
return hook;
}
I could really get away with an Action, but Edge appears to require the Func<object, Task<object>>, it won't proxy the function otherwise. Then, in the Javascript, we can detect that function and use it like this:
var func = Edge.Func(#"
return function(payload, callback) {
if (typeof (payload.stdoutHook) === 'function') {
process.stdout.write = payload.stdoutHook;
}
// do lots of stuff while stdout and stderr are hooked...
var what = require('whatever');
what.futz();
// terminate.
callback(null, result);
}");
dynamic result = func(payload).Result;
Questions
Q1. Both of these techniques seem to work, but is there a better way of doing this, something built-in to Edge perhaps that I have missed? Both solutions are invasive - they require some shim code to wrap the actual work that is to be done in Edge. This is not the end of the world, but it would be better if there was a non-invasive method.
Q2. In approach 2, where I have to return a task here
return Task.FromResult<object>(null);
it feels wrong to be returning an already completed "null task". But is there another way of writing this?
Q3. Do I need to be more rigorous in the Javascript code when hooking stdout and stderr? I note in double-edge.js there is this code, frankly I am not sure what is happening here, but it is quite a bit more complex than my crude overwriting of process.stdout.write :-)
// Fix #176 for GUI applications on Windows
try {
var stdout = process.stdout;
}
catch (e) {
// This is a Windows GUI application without stdout and stderr defined.
// Define process.stdout and process.stderr so that all output is discarded.
(function () {
var stream = require('stream');
var NullStream = function (o) {
stream.Writable.call(this);
this._write = function (c, e, cb) { cb && cb(); };
}
require('util').inherits(NullStream, stream.Writable);
var nullStream = new NullStream();
process.__defineGetter__('stdout', function () { return nullStream; });
process.__defineGetter__('stderr', function () { return nullStream; });
})();
}
Q1: There isn't anything built into Edge that would make capturing stdout or stderr of Node.js code automatic when calling Node from CLR. At some point I thought of writing an extension of Edge that would make marshaling Streams across CLR/V8 boundary easy. Under the hood it would be very similar to your Approach 2. It could be done as a standalone module on top of Edge.
Q2: Returning a completed task is very appropriate in this case. Your function has captured the Node.js output, processed it, and has in fact "completed" in that sense. Returning a task completed with Null is really a moral equivalent of returning from an Action.
Q3: The code you are pointing to is only relevant in Windows GUI applications, not Console applications. If you are writing a Console application, simply overriding write should suffice at the level of the Node.js code you pass to Edge.js. Note that the signature of write in Node allows an optional encoding parameter to be passed in. You seem to ignore it both in Approach 1 and 2. In particular in Approach 2 I would suggest wrapping the JavaScript proxy to C# callback into a JavaScript function that normalizes the parameters before assigning it to process.stdout.write. Otherwise Edge.js code may assume that the encoding parameter passed to a write call is a callback function which would follow the Edge.js calling convention.
I have a NodeJS application which uses the fs API to read files from a directory tree. I'm using the fs-walk module to walk the tree. For every sub directory encountered, the same function executes again to handle it. (I don't think this is recursion; rather, the same function is bound to an event which is fired each time a directory is handled.) Files are handled by a different function, which does stuff to them.
I'd like to execute arbitrary code once all files have been read without using synchronous or blocking code. I couldn't find any way to keep track of the number of files in a directory (to count down, for instance), nor could I find any attribute in fs.stat to indicate that the entire operation has completed.
Had anyone found a way to do this yet? I could find nothing in the node docs or on stack overflow.
After reviewing the fs-walk library a little closer, it looks like the third argument to the walk() method is actually a final callback. Internally they are using the async library, specifically async.whilst() and async.waterfall() methods which will execute the final callback when everything is complete.
I think the intention of the library creator is for that final callback to be executed when all async actions are completed. If that isn't working, you may want to file an issue in Github for it:
According to the code, you should be able to do:
var walk = require('fs-walk';
walk('/some/dir', someFileOrDirHandler, function(err) {
// This should be a final callback, if the first argument is present,
// then there was an error
if (err) {
/* handle it */
return;
}
// Getting here indicates success
});
As a compromise in performance, I ended up doing a total file count using a recursive function that accessed the file system synchronously. Using the total, I then accessed all the files asynchronously, decrementing the total each time. Once the total reached zero, I executed a function to handle all of the completed data.
var countAllFiles = new Promise(function (resolve, reject) {
var total = 0,
count = function (path) {
var contents = fs.readdirSync(path), file, name;
for (file in contents) {
if (!contents.hasOwnProperty(file)) continue;
name = path + '/' + contents[file];
if (fs.statSync(name).isDirectory())
count(name);
else
++total;
}
};
count('/path/to/tree/');
resolve(total);
}).then(function (total) {
walk.dirs('/path/to/tree/', handlerFunction, errorHandler);
// for every file, decrement total. Then, if it's zero, execute the code that
// depends on all the read/write operations being complete
});
I'm parsing a large amount of files using nodejs. In my process, I'm parsing audio files, video files and than the rest.
The function to parse files looks like this :
/**
* #param arr : array of files objects (path, ext, previous directory)
* #param cb : the callback when every object is parsed,
* objects are then throwed in a database
* #param others : the array beeing populated by matching objects
**/
var parseOthers = function(arr, cb, others) {
others = others === undefined ? [] : others;
if(arr.length == 0)
return cb(others); //should be a nextTick ?
var e = arr.shift();
//do some tests on the element and add it
others.push(e);
//Then call next tested callImediate and nextTick according
//to another stackoverflow questions with no success
return parseOthers(arr, cb, others);
});
Full code here (care it's a mess)
Now with about 3565 files (not so much) the script catch a "RangeError: Maximum call stack size exceeded" exception, with no trace.
What have I tried :
I've tried to debug it with node-inspector and node debug script, but it never hangs as if it was running without debugging (does debugging increase the stack ?).
I've tried with process.on('uncaughtException') to catch the exception with no success.
I've got no memory leak.
How may I found an exception trace ?
Edit 1
Increasing the --stack_size seams to work pretty well. Isn't there another way of preventing this ?
(about 1300 there)
Edit 2
According to :
$ node --v8-options | grep -B0 -A1 stack_size
The default stack size (in kBytes) is 984.
Edit 3
A few more explanations :
I'm never reading this type of files itselves
I'm working here on an array of paths, I don't parse folders recursively
I'm looking at the path and checking if it's already stored in the database
My guess is that the populated array becomes to big for nodejs, but memory looks fine and that's weird...
Most Stackoverflow situations are not easy or sometimes possible to debug. Even if you debug on the problem, you may not find the trigger.
But I can suggest you a way to share the task load easily (including the queue management):
JXcore (a multithreaded fork on Node.JS) would suit better in your case. Simply create a task pool and set a task method handling 1 file at a time. It will manage your queue 1 by 1 multithreaded.
var myTask = function ( args here )
{
logic here
}
for(var i=0;i<LIST_OF_THE_FILES;i++)
jxcore.tasks.addTask( myTask, paramshere, optional callback ...
OR in case the logic definition is out of the scope of a single method;
var myTask = function ( args here )
{
require('mytasketc.js').handleTask(args here);
}
for(var i=0;i<LIST_OF_THE_FILES;i++)
jxcore.tasks.addTask( myTask, paramshere, optional callback ...
Remarks
Every single thread has its own V8 memory limit.
The context among the threads are separated
Make sure the task method closes the file in the end
Link
You can find more on multithreaded Javascript tasks
You getting this error because of recursion. Reformat your code to do not use it, especially because this peace of code really don't need it. Here is just APPROXIMATE example, to show you how better to do it:
var parseElems = function(arr, cb) {
var result = [];
arr.forEach(function (el) {
//do some tests on the element (el)
result.push(el);
});
cb(result);
});
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,
}
},