What's the right way to exit node.js script with a log message?

Question

I have a node.js script that does some logging to a file using WriteStream. On certain events I want to stop execution of the script, i.e. warn to log and exit immediately after that. Being asyncronious node.js does not allow us to do it straight forward like:

#!/usr/local/bin/node

var fs = require('fs');
var stream = fs.createWriteStream('delme.log', { flags: 'a' });

stream.write('Something bad happened\n');
process.exit(1);

Instead of appending a message to delme.log this script does nothing with the file. Handling 'exit' event and flushing doesn't work. The only way to write the last log message before exitting found so far is to wrap process.exit(1) in the setTimeout():

#!/usr/local/bin/node

var fs = require('fs');
var stream = fs.createWriteStream('delme.log', { flags: 'a' });

stream.write('Something bad happened\n');
setTimeout(function(){
  process.exit(1);
}, 30);

However in this form it doesn't stop the script execution immediately and the script will be running for some time after the critical event happened. So I'm wondering if there are other ways to exit a script with a log message?

fent · Accepted Answer · 2012-02-08 17:55:45Z

14

Since you want to block, and already are using a stream, you will probably want to handle the writing yourself.

var data = new Buffer('Something bad happened\n');
fs.writeSync(stream.fd, data, 0, data.length, stream.pos);
process.exit();

edited Feb 8, 2012 at 17:55

answered Feb 8, 2012 at 10:20

fent

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2 Comments

nab Over a year ago

Thanks! That's a variation of setTimeout() call, which does not block the script execution. If there's a code that should not be executed after exception this pattern might become a concern.

nab Over a year ago

In my particular case I use WriteStream for logging, so that the actual call would be log.error('Something bad happened') which formats the message with appropriate log level adding some other stuff. So it would be hard to implement this snippet without changing the logger interface. I'm posting my workaround as an alternative solution. Initial question is perfectly answered. Thanks!

zupa · Accepted Answer · 2013-01-11 21:19:36Z

5

Improved.

var fs = require('fs');
var stream = fs.createWriteStream('delme.log', {flags: 'a'});

// Gracefully close log
process.on('uncaughtException', function () {
    stream.write('\n'); // Make sure drain event will fire (queue may be empty!)
    stream.on('drain', function() {
        process.exit(1);
    });
});

// Any code goes here...
stream.write('Something bad happened\n');
throw new Error(SOMETHING_BAD);

The try-catch block works but it is ugly. Still, credits go @nab, I just prettified it.

answered Jan 11, 2013 at 21:19

zupa

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Comments

danmactough · Accepted Answer · 2012-02-08 12:24:31Z

4

I think this is the right way:

process.on('exit', function (){
  // You need to use a synchronous, blocking function, here.
  // Not streams or even console.log, which are non-blocking.
  console.error('Something bad happened\n');
});

answered Feb 8, 2012 at 12:24

danmactough

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Comments

nab · Accepted Answer · 2012-02-08 22:38:45Z

3

To flush all log messages to a file before exitting one might want to wrap a script execution in a try-catch block. Once something bad has happened, it's being logged and throws an exception that will be catched by the outer try from which it is safe to exit asynchronously:

#!/usr/local/bin/node

var fs = require('fs');
var stream = fs.createWriteStream('delme.log', { flags: 'a' });
var SOMETHING_BAD = 'Die now';

try {
  // Any code goes here...
  if (somethingIsBad) {
    stream.write('Something bad happened\n');
    throw new Error(SOMETHING_BAD);
  }
} catch (e) {
  if (e.message === SOMETHING_BAD) {
    stream.on('drain', function () {
      process.exit(1);
    });
  } else {
     throw e;
  }
}

answered Feb 8, 2012 at 22:38

nab

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Comments

user935712063 · Accepted Answer · 2012-02-08 10:38:26Z

2

I would advocate just writing to stderr in this event - e.g trivial example

console.error(util.inspect(exception));

and then let the supervising* process handle the log persistence. From my understanding nowadays you don't have to worry about stdout and stderr not flushing before node exits (although I did see the problematic opposite behavior in some of 0.2.x versions).

(*) For supervising process take your pick from supervisord, god, monit, forever, pswatch etc...

This also provides a clean path to use PaaS providers such as Heroku and dotcloud etc... let the infrastructure managing the logging

answered Feb 8, 2012 at 10:38

user935712063

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1 Comment

nab Over a year ago

I would prefer to solve it (somehow) programmicaly. I have a number of short running scripts of different kinds. Some of them just warn to STDERR, some log data to filesystem. What I need is a unified approach to exit a script with a log message without relying on external software. Anyway thanks for the hint :)

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