should remove targetFile when targetFile is existing?

[zoluzo]

when the current file should roll，the operation is
fs.move(sourceFilePath, targetFilePath, { overwrite: true });
this will delete target file if it exist.
But, in the mutli process app，when set disableClustering: true, all process will do the same operation. The first write log process will rename log.log to log.log.2020-01-01-01 and renew log.log. The next write log process will rename log.log that is renewed by first process to log.log.2020-01-01-01 too and this cause log loss.

So, when target file exists, should skip remove operation?
try { if (fs.existsSync(targetFilePath)) { return ; } await fs.move(sourceFilePath, targetFilePath, { overwrite: true }); } catch (e) { }

[lamweili]

This is tricky. While your proposed solution works to not have log loss, it results in another issue whereby the existing logs will continue growing (without rolling) till it runs out of disk space. That is another catastrophic event, as one would configure rolling to cater for a catered cyclic disk space usage.

[lamweili]

Originally posted by @nomiddlename in slack
Multiple processes writing to the same log file, and also attempting to rotate that log file, is going to cause weird behaviour. They have no way of communicating with each other, so each thinks it is the only one rotating the files. It would be better to maybe split the log files by process id or something?

Perhaps, if have to disableClustering=true, maybe the filename can include the process.pid?

index.js

const http = require('http');
const { fork } = require('child_process');
const path = require('path');

const taskPath = path.resolve(__dirname, './task.js');

const pidCount = 4;
const pids = {};
for (let i = 0; i < pidCount; i++) {
  (function () {
    let pro = fork(taskPath);
    pids[pro.pid] = pro.pid;

    pro.on('error', (err) => {
      pids[pro.pid] = null;
    });
    pro.on('exit', () => {
      pids[pro.pid] = null;
    });
  })();
}

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'application/json' });
  const resp = JSON.stringify(pids);
  res.end(resp);
});

process.stdout.write('[Server start on port 8000.]\n');
server.listen(8000);

task.js

const logger = require('./logger');

const pid = process.pid;
setInterval(() => {  
  logger.info(`I am ${pid}`);
}, 500);

process.stdout.write(`Subprocess ${pid} start\n`);

logger.js

const log4js = require('log4js');
log4js.configure({
  appenders: {
    out: { type: 'stdout' },
    app: { 
      type: 'dateFile', 
      filename: `logs/${process.pid}/app.log`, // logs/1234/app.log or logs/1234/app.2022-05-12-23:42:26.log
      pattern: 'yyyy-MM-dd-hh:mm:ss',
      alwaysIncludePattern: false,
      keepFileExt: true,
      numBackups: 5
    }
  },
  categories: {
    default: { appenders: [ 'out', 'app' ], level: 'trace' }
  },
  disableClustering: true
});

const infoLogger = log4js.getLogger();
function info(...params) {
  infoLogger.info(...params);
};

module.exports = {
  info
};