Unlock Node.js Async Hooks to Trace Asynchronous Calls Seamlessly

After the release of Node.js v8.1.0, the async_hooks module was introduced as an experimental feature that can be used directly, providing a major benefit for those struggling to trace asynchronous calls and perform business tracing.

Although many early developers have been watching this feature, this article offers a brief introduction to its purpose.

In the era of asynchronous structures such as promise and yield, developers often needed to embed tracing code at various points of async execution, which could require invasive hacks, module hijacking, or even modifying the require system, breaking original logic. Earlier attempts like the domain module and the third‑party async-listener module never became standards and fell out of use. Consequently, monitoring business requests or tracing RPC calls in Node.js applications remained difficult.

The official inclusion and support of async_hooks now offers a promising solution.

As the name suggests, async_hooks means “asynchronous hooks”. It can automatically perform AOP‑like operations during async calls, requiring only that the hook be enabled before the business logic starts, after which it triggers automatically throughout the process.

The basic hooks fire at initialization, before the call, after the call, and when the object is destroyed. A simple example is shown below.

var asyncHooks = require('async_hooks');
var hooks = {
  init: init,
  before: before,
  after: after,
  destroy: destroy
};
var asyncHook = asyncHooks.createHook(hooks);

Defining the functions for these hooks is the challenging part; current examples are simple demos, and no complex modules leveraging this feature have emerged yet.

Of course, async_hooks provides additional methods; refer to the original documentation for full details.