Node.js Diagnostics


Somewhere in March, I was going through the Node.js Github repository, and the Node.js mentorship program caught my attention. I was immediately drawn to this program, mainly because this would give me an opportunity to get seriously involved with the V8 JavaScript engine and I quickly submitted my application. Thanks to the Outreachy program, I am now so comfortable and confident to contribute to Open Source.

Screen Shot 2018-09-09 at 12.26.33
An excerpt from my application to the Mentorship program

🎉 In the last week of July I got an email saying that I was accepted into the Beta phase of this program 🎉! I was paired with my mentor, Stephen Belanger from the Node.js Diagnostics Working Group. We had our first kick-off meeting in the first week of August and I have been hearing a lot about Async Hooks lately 🤔.

In this post, I shall try to summarise the Node.js Diagnostics Working Group, and their core domains. It took me a month to finally trace, profile, analyse and debug through this Working Group’s main features!



Node.js is a JavaScript run-time environment. This run-time environment includes everything you need to execute a program written in JavaScript.

In the pre-Node.js era, one had to rely on a browser to execute JavaScript code. Node.js turned JavaScript from something you could only run in the browser to something you could run on your machine as a standalone application.

V8 JavaScript engine

Node.js runs on the V8 JavaScript runtime engine.

  • V8 is the same JavaScript engine that powers the Google Chrome browser.
  • It is an open source runtime engine
  • And is written in C++

The Node.js Diagnostics Working Group

The Diagnostics Working Group has 4 core domains:

  • Tracing
  • Profiling
  • Heap & Memory Analysis
  • Step Debugging
WhatsApp Image 2018-09-09 at 14.33.03
Node.js Diagnostics Working Group

Heap & Memory Analysis and Step Debugging are post-mortem diagnostic activities and are out of scope of this post. In the remainder of this post, I shall briefly describe – Tracing, Async Hooks & Profiling.


Tracing is a technique used to understand what goes on in a running software system. The software used for tracing is called a tracer, which is conceptually similar to a tape recorder.

Tracing is often compared to logging. However, tracers and loggers are two different tools, serving two different purposes. Tracers are designed to record much lower-level events that occur much more frequently than log messages, often in the range of thousands per second, with very little execution overhead.

The list of recorded events inside a trace file can be read manually like a log file for the maximum level of detail, but it is generally much more interesting to perform application-specific analyses to produce reduced statistics and graphs that are useful to resolve a given problem.

Screen Shot 2018-09-09 at 12.50.19
Node.js tracing

Async Hooks

WhatsApp Image 2018-09-09 at 22.41.42
Async Hooks cartoon

The libuv module provides asynchronous capabilities to the V8 engine. Without libuv NodeJS is just a synchronous JavaScript\C++ execution.

async_hooks API makes it easier to track asynchronous requests and their callback activities. Async Hooks are used to correlate the async behaviour with V8.

The async_hooks module provides an API to register callbacks tracking the lifetime of asynchronous resources created inside a Node.js application. It can be accessed using:

const async_hooks = require('async_hooks');


Profiling is suitable to identify where performance is lost in a given software. The profiler outputs a profile, a statistical summary of observed events, which you may use to discover which functions took the most time to execute.

V8 has a sampling profiler called V8 profiler which is intended for capturing and analyzing CPU profiles and heap snapshots for your Node.js applications.

Profiling for Node.js mainly comprises of the following:

  • CPU Profiling – helps to understand what keeps the CPU busy
  • Heap Profiling – records the state of the JS heap and profiles heap memory usage statistics

Next steps

I am still trying to understand things around the Diagnostics universe at a more granular level. I hope to experiment more with the v8-profiler and async_hooks and consider novel ways to connect the two to capture multiple profiles and connect them together.

One thought on “Node.js Diagnostics

Add yours

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Blog at

Up ↑

%d bloggers like this: