Log HTTP requests/responses separately, visualize their concurrency and report logs/errors in context of a request



NPM version Build status Coverage status Dependency status Dev dependency status

HTTP logging middleware especially useful to unwind concurrent operations without losing the request context

HTTP logs in a terminal, visualizing server status codes, response times, debug information and errors for concurrent requests
Launch demo in your browser


$ npm install concurrency-logger


With koa

Basic usage

import Koa from 'koa';
import createLogger from 'concurrency-logger';

const app = new Koa;

// Logger is stateful as it contains information about concurrent requests
// Same instance needs to be reused across requests
const logger = createLogger(/* options */);


Log from middleware

// Log something in context to a specific request to trace it back easily -
// also when there are multiple concurrent requests
app.use(async (context, next) => {

    await next();

Attach more context to the log

const logger = createLogger({
    req: context => (
        context.originalUrl + '\n' +

Include localized timestamps

const logger = createLogger({
    timestamp: true

Write log to file

import { createWriteStream } from 'fs';

// To read log use program that interprets ANSI escape codes,
// e.g. cat or less -r
const log = createWriteStream('logs/requests.log');

const logger = createLogger({
    reporter: log

Adjust alert levels per method and response time

const logger = createLogger({
    getLevel: (responseTime, context) => {
              0 -  99ms: 0
            100 - 149ms: 1
            150 - 199ms: 2
            200 - 249ms: 3
            250 - 299ms: 4
            300 - 349ms: 5
            > 350ms    : 6

              0 - 149ms: 0
            150 - 225ms: 1
                   ... : ...

        let threshold = 50; // ms

        if (['POST', 'PUT'].includes(context.method)) {
            threshold *= 1.5;

        return Math.floor(responseTime / threshold) - 1;


import createLogger from 'concurrency-logger';

const logger = createLogger(/* options */);

(async () => {
    const context = {
        method: 'GET',
        originalUrl: '/'

    const next = async () => {
        await new Promise(resolve => setTimeout(resolve, 100));

        context.status = 200;

    try {
        await logger(context, next);
    } catch (error) {
        // Errors are passed through


Option Type Default Description Example
minSlots integer 1 Amount of space that is provisioned to display concurrent request lanes. Number of lanes will automatically scale up as the number of concurrent requests grow. 3
getLevel integer: function(responseTime: integer) responseTime => Math.floor(responseTime / 50) - 1 Map response time to alert level. Alert levels go from 0 (default color) to 6 (dark red). By default that means <100ms: 0, <150ms: 1 <200ms: 2, ..., >=350ms: 6. responseTime => Math.floor(responseTime / 100)
width integer, boolean(false) undefined If no width is provided, it will be dynamically read from process.stdout.columns. Pass in an integer to break all lines according to the specified fixed (terminal character) width. Pass in false if you want the lines not to break at all. 80, 132, false
timestamp boolean false Print localized timestamp for every requests. true, false
slim boolean false "Slim mode": don't use an extra character between request lanes to shrink width, but make them harder to separate visually. true, false
reporter writable stream process.stdout Specify a stream that handles the output lines. Write to terminal or stream to a log file, for example. Note that the lines contain ANSI color codes, so when streaming to a file you might need a program that can read those. E.g. less -r requests.log require('fs').createWriteStream('logs/requests.log')
req any: function(context: object) context => context.originalUrl Attach additional information to the request log line. context => context.originalUrl + '\n' + context.get('User-Agent')
res any: function(context: object) context => context.originalUrl Attach additional information to the response log line. context => context.originalUrl + '\n' + context.get('User-Agent')


Install development dependencies

$ npm install

Create new fixtures to test against

$ npm run create-fixtures

Manually review fixtures (you need a program that renders ANSI escape codes)

$ less -r test/fixtures/*

Run tests

$ npm test

Run code linter

$ npm run lint

Compile to ES5 from /src to /lib

$ npm run compile

Initialize demo project

$ git clone git@github.com:PabloSichert/concurrency-logger demo
$ cd demo
demo $ git checkout gh-pages
demo $ npm install

Build demo

demo $ npm run compile