Node.js将控制台错误传递到另一个程序（使其异步）

Question

From Expressjs documentation :

To keep your app purely asynchronous, you'd still want to pipe console.err() to another program

Qestions:

1. Is it enough to run my node app with stdout and stderr redirect to not block event loop ? Like this: node app 2>&1 | tee logFile node app 2>&1 | tee logFile ?

2. If ad.1 answer is true, then how to achieve non-blocking logging while using Winston or Bunyan? They have some built in mechanism to achieve this or they just save data to specific file wasting cpu time of current Node.js process ? Or maybe to achieve trully async logging they should pipe data to child process that performs "save to file" (is it still performance positive?) ? Can anyone explain or correct me if my way of thinking is just wrong?

3. Edited part : I can assume that piping data from processes A, B, ...etc to process L is cheaper for this specific processes (A, B, ...) than writing it to file (or sending over network). To the point: I am designing logger for application that uses nodejs cluster. Briefly - one of processes (L) will handle data streams from others, (A, B, ...). Process L will queue messages (for example line by line or some other special separator) and log it one by one into file, db or anywhere else. Advantage of this approach is reducing load of processes that can spent more time on doing their job. One more thing - assumption is to simplify usage of this library so user will only include this logger without any additional interaction (stream redirection) via shell. Do you think this solution makes sense? Maybe you know a library that already doing this?

Answer 1

Let's set up some ground level first...

Writing to a terminal screen ( console.log() etc.), writing to a file ( fs.writeFile() , fs.writeFileSync() etc.) or sending data to a stream process.stdout.write(data) etc.) will always "block the event loop". Why? Because some part of those functions is always written in JavaScript. The minimum amount of work needed by these functions would be to take the input and hand it over to some native code, but some JS will always be executed.

And since JS is involved, it will inevitably "block" the event loop because JavaScript code is always executed on a single thread no matter what.

Is this a bad thing...?

No. The amount of time required to process some log data and send it over to a file or a stream is quite low and does not have significant impact on performance.

When would this be a bad thing, then...?

You can hurt your application by doing something generally called a "synchronous" I/O operation - that is, writing to a file and actually not executing any other JavaScript code until that write has finished . When you do this, you hand all the data to the underlying native code and while theoretically being able to continue doing other work in JS space, you intentionally decide to wait until the native code responds back to you with the results. And that will "block" your event loop , because these I/O operations can take much much longer than executing regular code (disks/networks tend to be the slowest part of a computer).

Now, let's get back to writing to stdout / stderr .

From Node.js' docs:

process.stdout and process.stderr differ from other Node.js streams in important ways:

1. They are used internally by console.log() and console.error(), respectively.
2. They cannot be closed (end() will throw).
3. They will never emit the 'finish' event.
4. Writes may be synchronous depending on what the stream is connected to and whether the system is Windows or POSIX:
   • Files: synchronous on Windows and POSIX
   • TTYs (Terminals): asynchronous on Windows, synchronous on POSIX
   • Pipes (and sockets): synchronous on Windows, asynchronous on POSIX

I am assuming we are working with POSIX systems below.

In practice, this means that when your Node.js' output streams are not piped and are sent directly to the TTY, writing something to the console will block the event loop until the whole chunk of data is sent to the screen. However, if we redirect the output streams to something else (a process, a file etc.) now when we write something to the console Node.js will not wait for the completion of the operation and continue executing other JavaScript code while it writes the data to that output stream .

In practice, we get to execute more JavaScript in the same time period .

With this information you should be able to answer all your questions yourself now:

1. You do not need to redirect the stdout / stderr of your Node.js process if you do not write anything to the console, or you can redirect only one of the streams if you do not write anything to the other one. You may redirect them anyway, but if you do not use them you will not gain any performance benefit.
2. If you configure your logger to write the log data to a stream then it will not block your event loop too much (unless some heavy processing is involved).

If you care this much about your app's performance, do not use Winston or Bunyan for logging - they are extremely slow. Use pino instead - see the benchmarks in their readme.

Answer 2

To answer (1) we can dive into the Express documentation, you will see a link to the Node.js documentation for Console, which links to the Node documentation on the process I/O . There it describes how process.stdout and process.stderr behaves:

process.stdout and process.stderr differ from other Node.js streams in important ways:

1. They are used internally by console.log() and console.error(), respectively.
2. They cannot be closed (end() will throw).
3. They will never emit the 'finish' event.
4. Writes may be synchronous depending on what the stream is connected to and whether the system is Windows or POSIX:
   • Files: synchronous on Windows and POSIX
   • TTYs (Terminals): asynchronous on Windows, synchronous on POSIX
   • Pipes (and sockets): synchronous on Windows, asynchronous on POSIX

With that we can try to understand what will happen with node app 2>&1 | tee logFile node app 2>&1 | tee logFile :

• Stdout and stderr is piped to a process tee
• tee writes to both the terminal and the file logFile.

The important part here is that stdout and stderr is piped to a process , which means that it should be asynchronous.

Regarding (2) it would depend on how you configured Bunyan or Winston:

• Winston has the concept of Transports, which essentially allows you to configure where the log will go. If you want asynchronous logs, you should use any logger other than the Console Transport . Using the File Transport should be ok, as it should create a file stream object for this and that is asynchronous, and won't block the Node process.

• Bunyan has a similar configuration option: Streams . According to their doc, it can accept any stream interface. As long as you avoid using the process.stdout and process.stderr streams here you should be ok.