Error handling with node.js streams

Question

What's the correct way to handle errors with streams? I already know there's an 'error' event you can listen on, but I want to know some more details about arbitrarily complicated situations.

For starters, what do you do when you want to do a simple pipe chain:

input.pipe(transformA).pipe(transformB).pipe(transformC)...

And how do you properly create one of those transforms so that errors are handled correctly?

More related questions:

• when an error happens, what happens to the 'end' event? Does it never get fired? Does it sometimes get fired? Does it depend on the transform/stream? What are the standards here?
• are there any mechanisms for propogating errors through the pipes?
• do domains solve this problem effectively? Examples would be nice.
• do errors that come out of 'error' events have stack traces? Sometimes? Never? is there a way to get one from them?

Tier

This is not trivial. Promise frameworks make it a lot simpler – Ghazi

Unfortunately promises/futures can't really help you with streams... – Tier

Answer 1

transform

Transform streams are both readable and writeable, and thus are really good 'middle' streams. For this reason, they are sometimes referred to as through streams. They are similar to a duplex stream in this way, except they provide a nice interface to manipulate the data rather than just sending it through. The purpose of a transform stream is to manipulate the data as it is piped through the stream. You may want to do some async calls, for example, or derive a couple of fields, remap some things, etc.

---

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For how to create a transform stream see here and here. All you have to do is :

1. include the stream module
2. instantiate ( or inherit from) the Transform class
3. implement a _transform method which takes a (chunk, encoding, callback).

The chunk is your data. Most of the time you won't need to worry about encoding if you are working in objectMode = true. The callback is called when you are done processing the chunk. This chunk is then pushed on to the next stream.

If you want a nice helper module that will enable you to do through stream really really easily, I suggest through2.

For error handling, keep reading.

pipe

In a pipe chain, handling errors is indeed non-trivial. According to this thread .pipe() is not built to forward errors. So something like ...

var a = createStream();
a.pipe(b).pipe(c).on('error', function(e){handleError(e)});

... would only listen for errors on the stream c. If an error event was emitted on a, that would not be passed down and, in fact, would throw. To do this correctly:

var a = createStream();
a.on('error', function(e){handleError(e)})
.pipe(b)
.on('error', function(e){handleError(e)})
.pipe(c)
.on('error', function(e){handleError(e)});

Now, though the second way is more verbose, you can at least keep the context of where your errors happen. This is usually a good thing.

One library I find helpful though if you have a case where you only want to capture the errors at the destination and you don't care so much about where it happened is event-stream.

end

When an error event is fired, the end event will not be fired (explicitly). The emitting of an error event will end the stream.

domains

In my experience, domains work really well most of the time. If you have an unhandled error event (i.e. emitting an error on a stream without a listener), the server can crash. Now, as the above article points out, you can wrap the stream in a domain which should properly catch all errors.

var d = domain.create();
 d.on('error', handleAllErrors);
 d.run(function() {
     fs.createReadStream(tarball)
       .pipe(gzip.Gunzip())
       .pipe(tar.Extract({ path: targetPath }))
       .on('close', cb);
 });

• the above code sample is from this post

The beauty of domains is that they will preserve the stack traces. Though event-stream does a good job of this as well.

For further reading, check out the stream-handbook¹. Pretty in depth, but super useful and gives some great links to lots of helpful modules.

^{1: Note: this link points to archive.org as the original GitHub repo was deleted around August, 2022.}

Douro

That's really great info, thanks! Could you add a little bit about why you'd want to create a transform stream and why it relates to my question? – Tier

Sure - though I figured it related since you asked about it ; ) – Douro

Post on this by isaccs on Google Groups- nodejs: groups.google.com/d/msg/nodejs/lJYT9hZxFu0/L59CFbqWGyYJ (not grokbase) – Bettyannbettye

This answer is written perfectly. I'm going to investigate the domain suggestion -- it appears to be the kind of solution I was looking for. – Sheng

Note that you do not need to wrap the .on('error') handler in an anonymous function i.e. a.on('error', function(e){handleError(e)}) can just be a.on('error', handleError) – Jaborandi

"The emitting of an error event will end the stream" So, does it mean that I can't really recover from a streaming error? – Unravel

This isn't true (anymore!?): When an error event is fired, the end event will not be fired (explicitly). The emitting of an error event will end the stream. ... see bennadel.com/blog/…. Relying on posts like that I had to explicitly stream.end() my passthrough stream to end it after emitting some error event. And due to that I got an end event, too. – Centralize

The domain module is being deprecated: nodejs.org/api/domain.html – Zackzackariah

Answer 2

If you are using node >= v10.0.0 you can use stream.pipeline and stream.finished.

For example:

const { pipeline, finished } = require('stream');

pipeline(
  input, 
  transformA, 
  transformB, 
  transformC, 
  (err) => {
    if (err) {
      console.error('Pipeline failed', err);
    } else {
      console.log('Pipeline succeeded');
    }
});


finished(input, (err) => {
  if (err) {
    console.error('Stream failed', err);
  } else {
    console.log('Stream is done reading');
  }
});

See this github PR for more discussion.

Cabana

Why would you use finished though, when pipeline already has a callback? – Brunhilde

You may want to handle errors differently between the pipeline and individual streams. – Cabana

Can pipeline be used on duplex streams? If it used on duplex streams, do we need to create 2 pipelines? I have 2 TCP sockets I need to connect to each other, does it make sense to create 2 pipelines? – Fusible

Thanks, pipeline() is exactly what I was looking for. – Drinkable

Answer 3

domains are deprecated. you dont need them.

for this question, distinctions between transform or writable are not so important.

mshell_lauren's answer is great, but as an alternative you can also explicitly listen for the error event on each stream you think might error. and reuse the handler function if you prefer.

var a = createReadableStream()
var b = anotherTypeOfStream()
var c = createWriteStream()

a.on('error', handler)
b.on('error', handler)
c.on('error', handler)

a.pipe(b).pipe(c)

function handler (err) { console.log(err) }

doing so prevents the infamous uncaught exception should one of those stream fire its error event

Hubing

lol have fun handling 3 different error events and pray that whoever wrote the 3 different streaming libs implemented error handling correctly – Phocis

@Alex Mills 1) What is the problem of handling 3 events, and why are they "different", when their type is the same -- error, one may as well then settle with the fact that each event is distinct; 2) what streaming libs are written above, other than native Node.js functionality? and 3) why does it matter how they handle events internally, when this obviously allows anyone to attach additional error handlers on top of whatever is already there? – Wrongful

But are the other streams also closed if there is an error in one stream? – Macias

Answer 4

Errors from the whole chain can be propagated to the rightmost stream using a simple function:

function safePipe (readable, transforms) {
    while (transforms.length > 0) {
        var new_readable = transforms.shift();
        readable.on("error", function(e) { new_readable.emit("error", e); });
        readable.pipe(new_readable);
        readable = new_readable;
    }
    return readable;
}

which can be used like:

safePipe(readable, [ transform1, transform2, ... ]);

Carrnan

Answer 5

.on("error", handler) only takes care of Stream errors but if you are using custom Transform streams, .on("error", handler) don't catch the errors happening inside _transform function. So one can do something like this for controlling application flow :-

this keyword in _transform function refers to Stream itself, which is an EventEmitter. So you can use try catch like below to catch the errors and later on pass them to the custom event handlers.

// CustomTransform.js
CustomTransformStream.prototype._transform = function (data, enc, done) {
  var stream = this
  try {
    // Do your transform code
  } catch (e) {
    // Now based on the error type, with an if or switch statement
    stream.emit("CTError1", e)
    stream.emit("CTError2", e)
  }
  done()
}

// StreamImplementation.js
someReadStream
  .pipe(CustomTransformStream)
  .on("CTError1", function (e) { console.log(e) })
  .on("CTError2", function (e) { /*Lets do something else*/ })
  .pipe(someWriteStream)

This way, you can keep your logic and error handlers separate. Also , you can opt to handle only some errors and ignore others.

UPDATE
Alternative: RXJS Observable

Jolly

Answer 6

Use multipipe package to combinate several streams into one duplex stream. And handle errors in one place.

const pipe = require('multipipe')

// pipe streams
const stream = pipe(streamA, streamB, streamC) 


// centralized error handling
stream.on('error', fn)

Aeroneurosis

Answer 7

Use Node.js pattern by creating a Transform stream mechanics and calling its callback done with an argument in order to propagate the error:

var transformStream1 = new stream.Transform(/*{objectMode: true}*/);

transformStream1.prototype._transform = function (chunk, encoding, done) {
  //var stream = this;

  try {
    // Do your transform code
    /* ... */
  } catch (error) {
    // nodejs style for propagating an error
    return done(error);
  }

  // Here, everything went well
  done();
}

// Let's use the transform stream, assuming `someReadStream`
// and `someWriteStream` have been defined before
someReadStream
  .pipe(transformStream1)
  .on('error', function (error) {
    console.error('Error in transformStream1:');
    console.error(error);
    process.exit(-1);
   })
  .pipe(someWriteStream)
  .on('close', function () {
    console.log('OK.');
    process.exit();
  })
  .on('error', function (error) {
    console.error(error);
    process.exit(-1);
   });

Kinsella

Hmm, so you're saying if all stream processors were built like this, errors would propagate? – Tier

Thank you. for stream transformer this seems to be the correct way. The error gets propagated by the stram.pipeline which can be very easily wrapped in a promise. – Conga

Answer 8

const http = require('http');
const fs = require('fs');
const server = http.createServer();

server.on('request',(req,res)=>{
    const readableStream = fs.createReadStream(__dirname+'/README.md');
    const writeableStream = fs.createWriteStream(__dirname+'/assets/test.txt');
    readableStream
    .on('error',()=>{
        res.end("File not found")
    })
    .pipe(writeableStream)
    .on('error',(error)=>{
        console.log(error)
        res.end("Something went to wrong!")
    })
    .on('finish',()=>{
        res.end("Done!")
    })
})

server.listen(8000,()=>{
    console.log("Server is running in 8000 port")
})

Importance

I'm pretty sure this does not properly catch errors that happen when piping to writeableStream. – Tier

@BT , I have edited my post accroding to your cooments and I believe now we can handle the error whenever we will get some error during piping to writeableStream. – Importance

Answer 9

Try catch won't capture the errors that occurred in the stream because as they are thrown after the calling code has already exited. you can refer to the documentation:

https://nodejs.org/dist/latest-v10.x/docs/api/errors.html

Canea

Thanks, but this doesn't answer the question at all. – Tier

Giving me a 40 page document is not helpful. What do you think I should refer to in that giant page? Also, have you read my question? My question is not "does try catch work with streams?" I'm already well aware that try-catch won't work with asynchronous errors, eg ones from stream processing pipelines. – Tier