Diagnostics Channel in Node.js: Managing Diagnostic Messages and Tracing

1. Introduction

The diagnostics_channel module enables the creation of named channels for publishing diagnostic messages. These channels can be subscribed to by handlers that consume diagnostic data.

2. Public API

• diagnostics_channel.channel(name): Create or retrieve a channel.
• diagnostics_channel.hasSubscribers(name): Check if a channel has active subscribers.
• diagnostics_channel.subscribe(name, handler): Register a handler to receive messages.
• diagnostics_channel.unsubscribe(name, handler): Remove a previously registered handler.

3. Channel Class

• channel.hasSubscribers: Check for active subscribers.
• channel.publish(message): Publish a message to subscribers.
• channel.subscribe(handler): Register a handler for the channel.
• channel.unsubscribe(handler): Remove a handler.
• channel.bindStore(store, transform): Bind context data to AsyncLocalStorage.
• channel.runStores(context, fn): Run a function within the bound storage context.

4. TracingChannel Class

TracingChannel is a collection of channels representing traceable actions. It simplifies event publishing for tracing application flow.

• traceSync(fn, context): Trace synchronous function execution.
• tracePromise(fn, context): Trace promise-based function execution.
• traceCallback(fn, position, context): Trace callback-based function execution.
• Channels include: start, end, asyncStart, asyncEnd, and error.

5. Built-in Channels

• Console: Events for console.log, console.error, etc.
• HTTP: Events for client and server requests/responses.
• HTTP/2: Events for client and server streams.
• Modules: Events for require() and import().
• NET: Events for TCP and pipe connections.

6. Example

const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');

channel.subscribe((message, name) => {
  console.log('Received:', message);
});

if (channel.hasSubscribers) {
  channel.publish({ some: 'data' });
}

Conclusion

The diagnostics_channel module provides a structured way to manage diagnostic data and trace application execution in Node.js. By leveraging Channel and TracingChannel, developers can monitor synchronous and asynchronous operations, capture errors, and integrate with AsyncLocalStorage for context propagation.