Multiple loggers with same file transporter but with different labels

Hello together,

I have several modules (call them module A and module B) and they should share the same file transporter because I only want one single log file e.g. called somefile.log. Writing the logs to a file is my only transporter. Now I want the content of my log file to look like this:

2017-05-03T06:44:15.300Z - info: [module A] foo
2017-05-03T06:44:15.300Z - info: [module B] bar

I don’t want the JSON style here. Can you please tell me how I can achieve this?

When I define the file transport for each logger but set the filename in both transporters to the same string I get an error that the transport is already defined and I should change the file name:

winston.loggers.add('module A', { file: {
    filename: 'somefile.log',
    level: 'silly',
    label: 'module A',
    maxsize: 8,
    maxFiles: 3,
    json: false,
    prettyPrint: true,
    tailable: true,
    zippedArchive: true,
    handleExceptions: true,
    humanReadableUnhandledException: true,
    exitOnError: false,
  } });
  winston.loggers.add('module B', { file: {
    filename: 'somefile.log',
    level: 'silly',
    label: 'module B',
    maxsize: 8,
    maxFiles: 3,
    json: false,
    prettyPrint: true,
    tailable: true,
    zippedArchive: true,
    handleExceptions: true,
    humanReadableUnhandledException: true,
    exitOnError: false,
  } });

  const moduleAlogger = winston.loggers.get('module A');
  const moduleBlogger = winston.loggers.get('module B');
  moduleAlogger.info('foo');
  moduleBlogger.info('bar');

results in:

2017-05-03T06:49:30.641Z - error: uncaughtException: Transport already attached: file, assign a different name 

I didn’t understand the idea behind the labels per transporter because this would be redundant information when I am not allowed to write to the same file with different labels.

e.g. when I have two file transporters to different files I have in one file

2017-05-03T06:44:15.300Z - info: [module A] foo
2017-05-03T06:44:15.300Z - info: [module A] bar
.....

and in the other

2017-05-03T06:44:15.300Z - info: [module B] foo
2017-05-03T06:44:15.300Z - info: [module B] bar
.....

The label is totally useless here. What haven’t I understood when using labels in file transporters?

Thanks for your help.

Stefan

[omeraha]

So I see it is possible, but does winton handle well all the low-level details of multiple file handles writing to the same file? Does it sync to write calls, and make sure data is not overridden?

[IrisAmp]

Here is the solution I am using with v3.1.0. You can add the label format to the logger, and everything else you need to the transport.

In TypeScript:

import winston = require("winston");
import { resolve } from "path";

class Logger {
  public static getLogger(label: string): winston.Logger {
    if (!winston.loggers.has(label)) {
      winston.loggers.add(label, {
        transports: [Logger.consoleTransport, Logger.fileTransport],
        format: winston.format.label({ label }),
      });
    }
    return winston.loggers.get(label);
  }

  private static logFormatTemplate(i: { level: string, message: string, [key: string]: any }): string {
    return `${i.timestamp} ${i.level} [${i.label}] ${i.message}`;
  }

  private static readonly consoleTransport = new winston.transports.Console({
    format: winston.format.combine(
      winston.format.timestamp(),
      winston.format.cli(),
      winston.format.printf(Logger.logFormatTemplate),
    ),
  });

  private static readonly fileTransport = new winston.transports.File({
    format: winston.format.combine(
      winston.format.timestamp(),
      winston.format.printf(Logger.logFormatTemplate),
    ),
    filename: resolve("./myLogs.log"),
    level: "info"
  });
}

Logger.getLogger("A").info("Message with label: A");
Logger.getLogger("B").info("Message with label: B");

// 2019-01-01T20:20:57.845Z info [A] Message with label: A
// 2019-01-01T20:20:57.847Z info [B] Message with label: B

So you can pretty easily use this in all your different modules:

// ModuleA.ts
import { Logger } from "./Logger";
const log = Logger.getLogger("ModuleA");
log.info("Hello from ModuleA");

// ModuleB.ts
import { Logger } from "./Logger";
const log = Logger.getLogger("ModuleB");
log.info("Hello from ModuleB");

// myLogs.log
2019-01-01T20:35:58.855Z info [ModuleA] Hello from ModuleA
2019-01-01T20:35:58.884Z info [ModuleB] Hello from ModuleB