Simple and performant nodejs JSON logger

  • logops

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2082.1.22 years ago10 years agoMinified + gzip package size for logops in KB


Really simple and performant JSON logger for node.js.
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npm install logops

Basic usage

var logger = require('logops');

//plain strings
logger.debug('This is an example');
// {"time":"2015-12-22T16:31:39.220Z","lvl":"DEBUG","msg":"This is an example"}

//util.format support
logger.info('Request %s %d %j', 'is', 5, {key: 'value'}, 'guy');
// {"time":"2015-12-22T16:31:56.184Z","lvl":"INFO","msg":"Request is 5 {\"key\":\"value\"} guy"}

//properties in the log trace
logger.warn({ip: ''}, 'Something went wrong');
// {"ip":"","time":"2015-12-22T16:33:17.002Z","lvl":"WARN","msg":"Something went wrong"}

//special case: error instance to print error info (and stack traces)...
logger.error(new TypeError('String required'));
/* {"time":"2015-12-22T16:36:39.650Z","lvl":"ERROR",
 *  "err":{"message":"String required","name":"TypeError","constructor":"TypeError","stack":"TypeError: String required\n    at...",
 *  "msg":"TypeError: String required"} */

//... or specify the message
logger.fatal(new Error('Out of memory'), 'SYSTEM UNSTABLE. BYE');
/* {"time":"2015-12-22T16:45:36.468Z","lvl":"FATAL",
 *  "err":{"message":"Out of memory","name":"Error","constructor":"Error","stack":"Error: Out of memory\n    at...",
 *  "msg":"SYSTEM UNSTABLE. BYE"} */

  • If you give an object as the first argument, you will print its properties but not a String representation of it. logger.info(req) will set all req properties in the final json. `logger.info({a:'guy'}) =>
  • The pattern logger.error(err) is very common. This API embraces the requirenment, and makes an special management of it. But getting an error stack trace is not cheap. It only will be get and printed when log.error or log.fatal is used, so you can use logger.info(new Error('User Not Found')); to not print useless stackstraces for your bussiness logic errors. You can override it, btw

  • With the rest of arguments is just like calling console.log. It will be serialized as the trace message. Easy to remember.

Context support

Logops supports using global properties that will be merged with the specific ones defined in the call. Simply override the logger.getContext method to let the logger get it. See logops.child to see how to also create loggers with context
var logger = require('logops'),
    hostname = require('os').hostname();

logger.getContext = function getContext() {
  return {
    hostname: hostname,
    pid: process.pid

logger.info({app: 'server'}, 'Startup');
// {"hostname":"host.local","pid":35502,"app":"server","time":"2015-12-23T11:47:25.862Z","lvl":"INFO","msg":"Startup"}

Logger Level

You can set the logging level at any time. All the disabled logging methods are replaced by a noop, so there is not any performance penalty at production using an undesired level
var logger = require('logops');

// {String} level one of the following values ['DEBUG', 'INFO', 'WARN', 'ERROR', 'FATAL']

You can also set the logging level using the LOGOPS_LEVEL environment variable:

You can get the logging level using the getLevel() function of the logger:
currentLevel = logger.getLevel();

Trace format

This library incorporates two flavors of trace formatting:
  • "json": writes logs as JSON. This is the DEFAULT in v1.0.0
  • "dev": for development. Used with 'de-facto' NODEENV variable is set to 'development'
  • "pipe": writes logs separating fields with pipes. DEPRECATED in v1.0.0

logger.format = logger.formatters.json;
logger.info({key:'value'}, 'This is an example: %d', 5);
// {"key":"value","time":"2015-12-23T11:55:27.041Z","lvl":"INFO","msg":"This is an example: 5"}

logger.format = logger.formatters.dev;
logger.info({key:'value'}, 'This is an example: %d', 5);
// INFO  This is an example: 5 { key: 'value' }

logger.format = logger.formatters.pipe; //DEPRECATED in v1.0.0
logger.info({key:'value'}, 'This is an example: %d', 5);
// time=2015-12-23T11:57:24.879Z | lvl=INFO | corr=n/a | trans=n/a | op=n/a | msg=This is an example: 5

You can also set the format specifying the formatter with LOGOPS_FORMAT environment variable:
export LOGOPS_FORMAT=json
# export LOGOPS_FORMAT=dev

Child Loggers

You can create an specialized logger for a part of your app with bound static context/properties. The child logger will inherit its parent config: level, format, stream and context. If the parent logger has a context returned by parent.getContext(), the conflicting child logger context will take precedence
let child = logger.child({component: 'client'});
// {"component":"client","time":"2015-12-23T11:47:25.862Z","lvl":"INFO","msg":"Startup"}

TIP: Using with express/connect You can create a simply middleware to add a logger to every request with something like
app.use(function(req, res, next) {
  req.logger = logger.child({
    requestId: uuid.v4()
So your req.logger will log the requestId to allow correlation of traces in your server traces.
Note: setting child.getContext property, will override the context used to create the logger and its merge with its parent one. So you can use it to create a context free logger

Advanced Usage

Trace format

You can override the format function and manage by yourself the formatting taking into account your own environment variables by overriding the logger.format function

Don't print specific properties with dev format

Omit some boring/repeated/always-the-same context properties from being logged with the dev formatter:
logger.format = logger.formatters.dev;
logger.getContext = () => ({ pid: process.pid });
logger.info({key:'value', ip:''}, 'This is an example: %d', 5);
// INFO  This is an example: 5 { pid: 123342, key: 'value', ip: '' }

// Specify the context fields to omit as an array
logger.formatters.dev.omit = ['pid', 'ip'];

logger.info({key:'value', ip:''}, 'This is an example: %d', 5);
// INFO  This is an example: 5 { key: 'value' }

Don't print Error Stack traces

Set logger.formatters.stacktracesWith array with the error levels that will print stacktraces. Default is stacktracesWith: ['ERROR', 'FATAL']

Writing to files

This library writes by default to process.stdout, the safest, fastest and easy way to manage logs. It's how you execute your app when you define how to manage logs.
This approach is also compatible with logrotate as this is how many servers and PaaS manage the logs. Therefore you don't need to put anything in your source code relative to logs, and all is done at execution time depending on the deployment.
Recommended execution: Pipelining the stdout to tee). With this configuration, you will not fail when the disk is full
# write all traces to out.log
set -o pipefail
node index.js | tee -a out.log > /dev/null

# write error and fatal traces to error.log and all traces to out.log (using json formatter)
set -o pipefail
LOGOPS_FORMAT=json node index.js | tee >(grep -a -F -e '"lvl":"ERROR"' -e '"lvl":"FATAL"' > error.log) > out.log

You can also write logs and fail miserably stopping your app when the disk is full by doing
node index.js > out.log

Please read carefully in the node documentation how the stdout/stderr stream behaves regarding synchronous/asynchronous writing
If you want to pipe the output stream to any other stream in your source code, or even write to files (not recommended), you can override the stream used by this library
var logger = require('logops');
logger.stream = new MyOtherSuperStreamThatDoesGreatThingsExceptWriteToDisk();


This project was created initially for logging using the now deprecated pipe format, used internally at Telefonica by some logging infrastructure deployments. Now we are switching to a new one one, based on documents and a NoSQL infrastructure, where the JSON format is the one that fits best. We got inspired by the wonderful bunyan project and made some little adjustments in our API to be compliant with it, to reduce developer learning curve, make our preexisting code compatible and keep (or even improve) its great performance.


A very basic benchmark with the most common use case has been setup to compare with bunyan
Running on a MAC OS X Yosemite, 2,5 GHz Intel Core i5, 8 GB 1333 MHz DDR3, SSD disk, node 6.10.0
$ cd benchmark; npm start
> benchmarklogops@1.0.0 tee /Users/javier/Documents/Proyectos/logops/benchmark
> node index.js | tee -a out.log > /dev/null

logops x 70,675 ops/sec ±11.89% (65 runs sampled)
bunyan x 81,981 ops/sec ±4.76% (70 runs sampled)
Basic logging: Fastest is bunyan,logops
logops x 67,169,402 ops/sec ±2.79% (80 runs sampled)
bunyan x 5,774,822 ops/sec ±5.74% (75 runs sampled)
Disabled logging: Fastest is logops

> benchmarklogops@1.0.0 file /Users/javier/Documents/Proyectos/logops/benchmark
> node index.js > out.log

logops x 37,479 ops/sec ±5.69% (76 runs sampled)
bunyan x 36,211 ops/sec ±2.72% (77 runs sampled)
Basic logging: Fastest is logops
logops x 70,740,515 ops/sec ±1.71% (82 runs sampled)
bunyan x 6,324,283 ops/sec ±2.68% (78 runs sampled)
Disabled logging: Fastest is logops

> benchmarklogops@1.0.0 null /Users/javier/Documents/Proyectos/logops/benchmark
> node index.js > /dev/null

logops x 49,509 ops/sec ±4.92% (77 runs sampled)
bunyan x 47,759 ops/sec ±4.34% (69 runs sampled)
Basic logging: Fastest is logops
logops x 68,293,618 ops/sec ±2.64% (80 runs sampled)
bunyan x 6,232,825 ops/sec ±2.42% (81 runs sampled)
Disabled logging: Fastest is logops


Copyright 2014, 2015 Telefonica Investigación y Desarrollo, S.A.U
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.