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Express middleware to track the request and response storing in the domain the operation, transactionId and correlator.

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These values can be used by a logging system, e.g. logops, to track all the log entries corresponding to a transaction (a request/response) by matching the transactionId or to track a flow (from one to multiple transactions) by matching the correlator.

The middleware follows this process:

  • In the request:

    • Get or initialize the tracking object in the domain. This object stores all the variables for tracking the request and response.
    • Set op (operation) in the tracking object. Its value is obtained from the middleware options, if available. Otherwise, it is set to null.
    • Set trans (transactionId) in the tracking object. Its value is a generated UUID that makes unique the request/response.
    • Set corr (correlator) in the tracking object. The middleware tries to get the correlator from the request using a correlator handler (based on HTTP header or JWT header depending on middleware options). If it is not possible to get a valid correlator, it reuses the transactionId as correlator.
  • In the response:

    • It uses the correlator handler to set the correlator in the response. The correlator handler based on HTTP headers adds a HTTP header (Unica-Correlator by default) with the correlator.


npm install express-tracking

Basic usage

var express = require('express'),
    expressLogging = require('express-tracking');
var app = express();


The express middleware may receive an object with optional settings:

Option Type Default Description
op String null Name of the operation (for logging purposes)
corrHeader String Unica-Correlator Name of the HTTP header with the correlator
isJwt Boolean false If the handler to get the correlator from the request and set the correlator in the response is based on JWT (obtained from the corr field in the JWT header) or on a HTTP header. Note that this option is ignored if corrHandler option is set.
corrHandler Function(corrHeader) null Function to get the correlator from the request and set the correlator in the response when the default implementations (header and JWT-based) are not enough.

Advanced usage

Enabling JWT-based correlator handler

The default correlator handler is based on the HTTP header: Unica-Correlator (unless modified with the option corrHeader). To use the other correlator header, based on the corr field of the JWT header:

app.use(expressTracking({isJwt: true}));

Using a custom correlator handler

The following sample implements a custom handler to get the correlator from the query parameter corr instead of an HTTP header.

var customCorrHandler = function(header) {
  return {
    getCorrelator: function(req, cb) {
      return cb(null, req.query.corr);
    setCorrelator: function(res, correlator) {
      res.set(header, correlator);
app.use(expressTracking({corrHandler: customCorrHandler}));

Full example

It is recommended to use this express middleware in combination with:

var express = require('express'),
    expressDomain = require('express-domaining'),
    expressTracking = require('express-tracking'),
    expressLogging = require('express-logging'),
    logger = require('logops');
logger.getContext = function() {
  return process.domain && process.domain.tracking;
var app = express();
app.use(expressTracking({op: 'test'}));
app.get('/test', function(req, res) {

After launching the previous server, each HTTP request generates 2 log entries to trace the request and response:

time=2015-06-25T14:34:55.847Z | lvl=INFO | corr=0ad79e44-95e9-48fe-aa17-19773ebe9056 | trans=0ad79e44-95e9-48fe-aa17-19773ebe9056 | op=test | msg=Request from ::1: GET /test
time=2015-06-25T14:34:55.848Z | lvl=INFO | corr=0ad79e44-95e9-48fe-aa17-19773ebe9056 | trans=0ad79e44-95e9-48fe-aa17-19773ebe9056 | op=test | msg=Response with status 200 in 1 ms.


v1.0.1 (11-02-2016)

  • Update Jwt Utils dependency

v1.0.0 (02-07-2015)

  • First version


Copyright 2015 Telefónica 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.