Question

I’m trying to use node core’s cluster feature.

I would like the stdout and stderr streams to output to a file, one for each worker id.

Something much like the following:

var fs          = require('fs'), 
    env         = process.env,
    workerId    = env.NODE_WORKER_ID || env.NODE_UNIQUE_ID;

process.stdout =  fs.createWriteStream(__dirname + '/app#' + workerId + '.log', {
    encoding: 'utf8'
});

Unfortunately, it does not seem to rewrite process.stdout like this.

Is there a way to achieve this, or should this be done differently? Currently when I run my cluster I am getting all output from all processes in one console, which is extremely messy.

Answer 1

I ended up doing the following:

    //create a new stdout file stream
    var stdoutFS = fs.createWriteStream(stdoutFile, {
        encoding: 'utf8',
        flags   : 'a+'
    });

    //create a new stderr file stream
    var stderrFS = fs.createWriteStream(stderrFile, {
        encoding: 'utf8',
        flags   : 'a+'
    });

    //pipe stdout to a worker file
    var unhookStdout = hookWriteStream(stdout, function(string, encoding, fd) {
        stdoutFS.write(string, encoding || 'utf8');
    });
    console.log('\n\nPrepared new stdout hook to worker file.');

    //pipe stderr to a worker file
    var unhookStderr = hookWriteStream(stderr, function(string, encoding, fd) {
        stderrFS.write(string, encoding || 'utf8');
    });
    console.log('Prepared new stderr hook to worker file.');

    //unhook when things go wrong
    stdoutFS.once('close', function() {
        unhookStdout();
        console.log('Unhooked stdout.');
    });
    stdoutFS.once('error', function(err) {
        unhookStdout();
        console.error('Error: Unhooked stdout due to error %j.', err);
    });
    stderrFS.once('close', function() {
        unhookStderr();
        console.log('Unhooked stderr.');
    });
    stderrFS.once('error', function(err) {
        unhookStderr();
        console.error('Error: Unhooked stderr due to error %j.', err);
    });

});

function hookWriteStream(stream, callback) {
    var oldWrite = stream.write;

    stream.write = (function(write) {
        return function(string, encoding, fd) {
            write.apply(stream, arguments);
            callback(string, encoding, fd);
        };
    })(stream.write);

    return function() {
        stream.write = oldWrite;
    };
}

It may not be very elegant, but so far this is the best solution I’ve found.