@opentelemetry/context-async-hooks

What is @opentelemetry/context-async-hooks?

The @opentelemetry/context-async-hooks package is part of the OpenTelemetry project, which provides a set of APIs, libraries, agents, and instrumentation to create and manage telemetry data (metrics, logs, and traces) for cloud-native software. Specifically, this package leverages Node.js' async_hooks module to implement context propagation for asynchronous operations. This is crucial for tracing and logging in asynchronous applications, as it allows developers to maintain context across asynchronous boundaries.

What are @opentelemetry/context-async-hooks's main functionalities?

Context Management

This feature allows for the management of context across asynchronous operations. It uses the AsyncLocalStorage class to create a new context storage, run a function within this context, and retrieve values from this context later in the async flow.

const { AsyncLocalStorage } = require('@opentelemetry/context-async-hooks');
const asyncLocalStorage = new AsyncLocalStorage();

asyncLocalStorage.run(new Map(), () => {
  asyncLocalStorage.getStore().set('key', 'value');
  someAsyncOperation().then(() => {
    console.log(asyncLocalStorage.getStore().get('key')); // Outputs: 'value'
  });
});

Context Propagation

This feature demonstrates how to set up the AsyncLocalStorageContextManager to work with OpenTelemetry's API for global context propagation. This setup is essential for tracing asynchronous operations across different parts of an application.

const { propagation } = require('@opentelemetry/api');
const { AsyncLocalStorageContextManager } = require('@opentelemetry/context-async-hooks');

const contextManager = new AsyncLocalStorageContextManager();
propagation.setGlobalPropagator(contextManager);

// Now, the context is propagated across asynchronous calls automatically.

Other packages similar to @opentelemetry/context-async-hooks

cls-hooked

The cls-hooked package provides a similar functionality to @opentelemetry/context-async-hooks by using the async_hooks module to create context-like namespaces that are preserved across asynchronous calls. It differs in its API and is not specifically tailored for OpenTelemetry or tracing but can be used for general context propagation purposes.

async_hooks

While not a separate npm package, Node.js' built-in async_hooks module is the underlying technology that both @opentelemetry/context-async-hooks and cls-hooked rely on for tracking asynchronous operations. It provides the primitives for creating hooks that monitor the lifecycle of asynchronous operations, which can be used directly for custom context propagation implementations.

README

OpenTelemetry async_hooks-based Context Managers

This package provides two ContextManager implementations built on APIs from Node.js's async_hooks module. If you're looking for a ContextManager to use in browser environments, consider opentelemetry-context-zone or opentelemetry-context-zone-peer-dep.

The definition of the ContextManager interface and the problem it solves can be found here.

API

Two ContextManager implementations are exported:

• AsyncLocalStorageContextManager, based on AsyncLocalStorage
• AsyncHooksContextManager, based on AsyncHook

The former should be preferred over the latter as its implementation is substantially simpler than the latter's, and according to Node.js docs,

While you can create your own implementation [of AsyncLocalStorage] on top of [AsyncHook], AsyncLocalStorage should be preferred as it is a performant and memory safe implementation that involves significant optimizations that are non-obvious to implement.

AsyncLocalStorage is available in node ^12.17.0 || >= 13.10.0, however AsyncLocalStorageContextManager is not enabled by default for node <14.8.0 because of some important bugfixes which were introduced in v14.8.0 (e.g., nodejs/node#34573).

Limitations

It's possible that this package won't track context perfectly when used with certain packages. In particular, it inherits any bugs present in async_hooks. See here for known issues.

async_hooks is still seeing significant correctness and performance fixes, it's recommended to run the latest Node.js LTS release to benefit from said fixes.

Prior art

Context propagation is a big subject when talking about tracing in Node.js. If you want more information about it here are some resources:

• https://www.npmjs.com/package/continuation-local-storage (which was the old way of doing context propagation)
• Datadog's own implementation for their JavaScript tracer: here
• OpenTracing implementation: here
• Discussion about context propagation by the Node.js Diagnostics Working Group: here

Useful links

• For more information on OpenTelemetry, visit: https://opentelemetry.io/
• For more about OpenTelemetry JavaScript: https://github.com/open-telemetry/opentelemetry-js
• For help or feedback on this project, join us in GitHub Discussions

License

Apache 2.0 - See LICENSE for more information.