Code Quality Rank: L5
Programming language: JavaScript
License: MIT License
Tags: Debugging / Profiling     Dtrace     Trace     Debug     Debugging     Tracing    
Latest version: v0.3.0

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Dynamic tracing for JavaScript, written in JavaScript, providing you insight into your live nodejs applications, at the process, machine, or cluster level.

Similar to systems like dtrace or ktap, the goal of dynamic tracing is to enable a rich set of debugging information in live processes, often in production in order to help discover the root of an issue. These libraries have very minimal overhead when disabled, and may be enabled externally when needed.

View the wiki for additional information and libraries which may be helpful.



$ npm install jstrace


$ npm install -g jstrace


  • dynamic tracing :)
  • local / remote execution support
  • minimal overhead when idle
  • flexible scripting capabilities
  • probe name filtering
  • pid, process title, and hostname filtering
  • remote messaging for map/reduce style reporting
  • multi-process support, inspect your cluster in realtime
  • binds to (may need this for firewalls)


  Usage: jstrace [options] <script>


    -h, --help             output usage information
    -V, --version          output the version number
    -p, --pid <pid>        trace with the given <pid>
    -t, --title <pattern>  trace with title matching <pattern>
    -H, --host <pattern>   trace with hostname matching <pattern>



Suppose for example you have probes set up to mark the start and end of an http request, you may want to quickly tap into the process and see which part of the request/response cycle is hindering latency.

This contrived example isn't very exciting, and only has two probes, but it illustrates the capabilites. We simply mark the start and end of the request, as well as providing the request id.

var trace = require('jstrace');
var http = require('http');

var ids = 0;

var server = http.createServer(function(req, res){
  var id = ++ids;

  trace('request:start', { id: id });

    res.end('hello world');
    trace('request:end', { id: id });
  }, Math.random() * 250 | 0);


Local analysis

jstrace-local analysis is performed by exporting a .local function; When you invoke .on() jstrace internally broadcasts this information to the remotes when they connect and filters probes accordingly. The data is transmitted as-is from the remote for analysis.

exports.local = function(traces){
  traces.on('request:*', function(trace){

Local analysis can be expensive since entire objects are transferred, if you need less information or would prefer to distribute the load you can use the remote analysis feature.

Remote analysis

Remote analysis serializes the .remote function to the target processes for remote execution. This can be great for reporting on data that would be too expensive to transfer over the wire to jstrace(1). For example suppose you just want to know the lengths of BLOBs sent to your API:

exports.remote = function(traces){
  traces.on('api:buffer', function(trace){

Note the use of console.log(), jstrace provides custom console.log(), console.error(), and console.dir() methods which report back to jstrace(1). You'll now receive something like the following:


When analysing a machine or cluster it's useful to know which machine did what, so the console.dir() method prefixes with the hostname, process title, and pid:

api-1/api/1234 >> 123132
api-1/api/1234 >> 3212
api-2/api/1200 >> 4324

Note that unlike .local you need to require() your modules from within the .remote function.

Remote cleanup

When you perform evented operations in your remote function, like setInterval() or listening for events on emitters other than traces, listen for the "cleanup" event in order to finish the trace script completely once the cli exits:

exports.remote = function(traces){
  traces.on('api:buffer', function(trace){
    // will automatically be cleaned up

  var id = setInterval(function(){

  traces.on('cleanup', function(){

Local & remote analysis

Local and remote methods may be used in tandem for map/reduce style reporting. Using traces.emit() in the .remote function you can transmit custom information back to jstrace for display or further analysis.

var bytes = require('bytes');

exports.remote = function(traces){
  traces.on('api:buffer', function(trace){
    traces.emit('buffer size', trace.buffer.length);

exports.local = function(traces){
  traces.on('buffer size', function(n){
    console.log('buffer %s', bytes(n));

It's worth noting that .on() in .local is used for both remote probe subscription, and events emitted by the .remote. Since they share this space you wouldn't want to emit similar names, for example traces.emit('api:buffer', ...) would be bad, since local would actually end up subscribing to the original trace instead of the data emitted. This is rarely an issue but something to be aware of.

Full analysis example

The jstrace(1) executable accepts a script which exports functions with trace patterns to match. These function names tell jstrace which traces to subscribe to. The trace object passed contains the information given to the in-processe trace() call, along with additional metadata such as .timestamp, .hostname, .pid, and .title.

We can use this data to add anything we like, here we're simply mapping the requset ids to output deltas between the two. Note that we export the function named .local, there are two functions supported by jstrace, however .local means that the trace objects are sent over the wire and analysis is performed local to jstrace(1).

var m = {};

exports.local = function(traces){
  traces.on('request:start', function(trace){
    m[trace.id] = trace.timestamp;

  traces.on('request:end', function(trace){
    var d = Date.now() - m[trace.id];
    console.log('%s -> %sms', trace.id, d);

To run the script just pass it to jstrace(1) and watch the output flow!

$ jstrace response-duration.js

298 -> 50ms
302 -> 34ms
299 -> 112ms
287 -> 184ms
289 -> 188ms
297 -> 124ms
286 -> 218ms
295 -> 195ms
300 -> 167ms
304 -> 161ms
307 -> 116ms
301 -> 206ms
305 -> 136ms
314 -> 19ms

Plotting distribution

Using node modules such as bars can aid in analysis, for exmaple plotting the distribution of response status codes over time.

var clear = require('clear');
var bars = require('bars');

var m = {};

exports.local = function(traces){
  traces.on('request:end', function(trace){
    m[trace.status] = m[trace.status] || 0;

  console.log(bars(m, { bar: '=', width: 30 }));
}, 1000);
  200 | ============================== | 6
  404 | ====================           | 4
  500 | ====================           | 4
  505 | ===============                | 3
  400 | ==========                     | 2
  201 | =====                          | 1

  201 | ============================== | 19
  500 | ===========================    | 17
  505 | =====================          | 13
  200 | ===================            | 12
  404 | ===================            | 12
  400 | =================              | 11

  500 | ============================== | 19
  201 | ========================       | 15
  200 | ===================            | 12
  404 | ===================            | 12
  505 | =================              | 11
  400 | ===========                    | 7


To reset the data per-interval tick all you'd have to do is add m = {}; at the end of the setInterval() callback to refresh the data!


Create realtime charts using chart to monitor changes over time:

var chart = require('chart');
var clear = require('clear');

var data = [];
var n = 0;

exports.local = function(traces){
  traces.on('request:end', function(trace){ n++ });

  n = 0;
}, 1000);


Naming probes

In general you should use ":" as a separator for pattern matching, and prefix with something relevant for your module, such as the module's name. Here are some examples:

  • express:request:start
  • express:socket:error
  • koa:request:start
  • koa:request:end
  • myapp:login
  • myapp:logout

You should also consider listing probe names with descriptions in your library or application readme file.

Dependency injection

If your library supports tracing, it's best that you do not add jstrace as a dependency, instead you should provide a trace option to let the user pass in jstrace if they wish. Some people call this "dependency injection". For example:

function MyLib(opts) {
  opts = opts || {};
  this.trace = opts.trace || function(){};
  this.trace('something', { some: 'data' });

The premise here is that the community should start instrumenting libraries with this functionality so that node becomes easier to profile, monitor, and debug. This is especially important for detecting latency issues across async boundaries, as they not necessarily CPU-bound and may not show up in profiles at all.

Trace object

The trace object sent to both local and remote subscription handlers.

  • timestamp timestamp at the time of invocation
  • hostname machine hostname
  • title process title
  • pid process id
  • name trace name
  • * all other properties given


  • TJ Holowaychuk
  • Julian Gruber



*Note that all licence references and agreements mentioned in the jstrace README section above are relevant to that project's source code only.