Oct 8, 2018 · 4 min read


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.

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
The 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.

Node.js tracing

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.

Tracing is a form of logging. Logging just implies a sequence of messages, while tracing additionally imposes some form of structure. Often “logging” is used to refer to the act of actually storing the data, while “tracing” is used to refer to the production of the data being handed to whatever is consuming/recording that data.

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.

Async Hooks

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.

Profiling is also a form of logging. Rather than tracing, which is typically recorded chronologically, profiling is typically sampling-based, so it takes a snapshot and walks through the structure, recording nodes in the tree-walk.

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.

Node.js Collection

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Node.js Collection

Community-curated content for the millions of Node.js users.

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