Part of custom Bunyan setup with Logstash output, as used by HD-Sydsvenskan

bunyan-setup.js

const bunyan = require('bunyan');
const bunyanExpressSerializer = require('bunyan-express-serializer');

const LogstashStream = require('./logstash-stream');

const streams = [
  {
    level: 'trace',
    type: 'raw',
    stream: new LogstashStream(outStream)
  }
];

const logger = bunyan.createLogger({
    name: 'app-name',
    streams,
    serializers: {
      err: bunyan.stdSerializers.err,
      req: bunyanExpressSerializer
    }
});

logstash-stream.js

'use strict';

const bunyan = require('bunyan');
const safeCycles = bunyan.safeCycles;

const levels = {
  10: 'trace',
  20: 'debug',
  30: 'info',
  40: 'warn',
  50: 'error',
  60: 'fatal'
};

const LogstashConverter = function (stream) {
  this.stream = stream;
};

LogstashConverter.prototype.write = function (rec) {
  const logstashEntry = {
    '@timestamp': rec.time,
    '@version': 1,
    level: levels[rec.level],
    message: rec.msg,
    type: 'log',
    application: rec.name,
    job: rec.name
  };

  let skipLogPoint;

  if (rec.req) {
    const req = rec.req;
    const res = rec.res || {};
    const headers = req.headers || {};
    const status = res.statusCode;

    logstashEntry.method = req.method;
    logstashEntry.host = headers.host;
    logstashEntry.path = req.url;
    logstashEntry.status = status;
    logstashEntry.user_agent = headers['user-agent'];
    logstashEntry.content_length = headers['content-length'];
    logstashEntry.content_type = headers['content-type'];
    logstashEntry.xff = headers['x-forwarded-for'];
    logstashEntry.referer = headers['referer'];

    if (!status || typeof status !== 'number' || ['error', 'fatal'].includes(logstashEntry.level)) {
      // Leave alone
    } else if (status < 400 && logstashEntry.level === 'info') {
      skipLogPoint = true;
    } else if (status < 500) {
      logstashEntry.level = 'warn';
    } else {
      logstashEntry.level = 'error';
    }

    delete rec.req;
    delete rec.res;
  }

  const output = Object.assign({}, rec, logstashEntry);

  // remove duplicate fields
  delete output.name;
  delete output.time;
  delete output.msg;

  // Remove internal bunyan fields that won't mean anything outside of
  // a bunyan context.
  delete output.v;

  if (!skipLogPoint) {
    const str = JSON.stringify(output, safeCycles()) + '\n';
    this.stream.write(str);
  }
};

module.exports = LogstashConverter;