DocsReferenceGuidesBlog
Discord logo
Discord
GitHub logo
Bun

Node.js module

diagnostics_channel

The 'node:diagnostics_channel' module provides a mechanism to create named channels to report arbitrary messages and data within an application. It enables instrumenting libraries and user code without introducing heavy dependencies.

Listeners can subscribe to channels to receive structured diagnostic events, making it useful for logging, tracing, and performance monitoring in production environments.

Works in Bun

Fully implemented.

  • class Channel<StoreType = unknown, ContextType = StoreType>

    The class Channel represents an individual named channel within the data pipeline. It is used to track subscribers and to publish messages when there are subscribers present. It exists as a separate object to avoid channel lookups at publish time, enabling very fast publish speeds and allowing for heavy use while incurring very minimal cost. Channels are created with channel, constructing a channel directly with new Channel(name) is not supported.

    • readonly hasSubscribers: boolean

      Check if there are active subscribers to this channel. This is helpful if the message you want to send might be expensive to prepare.

      This API is optional but helpful when trying to publish messages from very performance-sensitive code.

      import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

if (channel.hasSubscribers) {
  // There are subscribers, prepare and publish message
}
    • readonly name: string | symbol
    • store: AsyncLocalStorage<StoreType>,
      transform?: (context: ContextType) => StoreType
      ): void;

      When channel.runStores(context, ...) is called, the given context data will be applied to any store bound to the channel. If the store has already been bound the previous transform function will be replaced with the new one. The transform function may be omitted to set the given context data as the context directly.

      import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';

const store = new AsyncLocalStorage();

const channel = diagnostics_channel.channel('my-channel');

channel.bindStore(store, (data) => {
  return { data };
});
      @param store

      The store to which to bind the context data

      @param transform

      Transform context data before setting the store context

    • message: unknown
      ): void;

      Publish a message to any subscribers to the channel. This will trigger message handlers synchronously so they will execute within the same context.

      import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

channel.publish({
  some: 'message',
});
      @param message

      The message to send to the channel subscribers

    • runStores(): void;

      Applies the given data to any AsyncLocalStorage instances bound to the channel for the duration of the given function, then publishes to the channel within the scope of that data is applied to the stores.

      If a transform function was given to channel.bindStore(store) it will be applied to transform the message data before it becomes the context value for the store. The prior storage context is accessible from within the transform function in cases where context linking is required.

      The context applied to the store should be accessible in any async code which continues from execution which began during the given function, however there are some situations in which context loss may occur.

      import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';

const store = new AsyncLocalStorage();

const channel = diagnostics_channel.channel('my-channel');

channel.bindStore(store, (message) => {
  const parent = store.getStore();
  return new Span(message, parent);
});
channel.runStores({ some: 'message' }, () => {
  store.getStore(); // Span({ some: 'message' })
});
    • store: AsyncLocalStorage<StoreType>
      ): boolean;

      Remove a message handler previously registered to this channel with channel.bindStore(store).

      import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';

const store = new AsyncLocalStorage();

const channel = diagnostics_channel.channel('my-channel');

channel.bindStore(store);
channel.unbindStore(store);
      @param store

      The store to unbind from the channel.

      @returns

      true if the store was found, false otherwise.

  • class TracingChannel<StoreType = unknown, ContextType extends object = {}>

    The class TracingChannel is a collection of TracingChannel Channels which together express a single traceable action. It is used to formalize and simplify the process of producing events for tracing application flow. tracingChannel is used to construct a TracingChannel. As with Channel it is recommended to create and reuse a single TracingChannel at the top-level of the file rather than creating them dynamically.

    • asyncEnd: Channel<StoreType, ContextType>
    • asyncStart: Channel<StoreType, ContextType>
    • end: Channel<StoreType, ContextType>
    • error: Channel<StoreType, ContextType>
    • readonly hasSubscribers: boolean

      true if any of the individual channels has a subscriber, false if not.

      This is a helper method available on a TracingChannel instance to check if any of the TracingChannel Channels have subscribers. A true is returned if any of them have at least one subscriber, a false is returned otherwise.

      const diagnostics_channel = require('node:diagnostics_channel');

const channels = diagnostics_channel.tracingChannel('my-channel');

if (channels.hasSubscribers) {
  // Do something
}
    • start: Channel<StoreType, ContextType>
    • subscribers: TracingChannelSubscribers<ContextType>
      ): void;

      Helper to subscribe a collection of functions to the corresponding channels. This is the same as calling channel.subscribe(onMessage) on each channel individually.

      import diagnostics_channel from 'node:diagnostics_channel';

const channels = diagnostics_channel.tracingChannel('my-channel');

channels.subscribe({
  start(message) {
    // Handle start message
  },
  end(message) {
    // Handle end message
  },
  asyncStart(message) {
    // Handle asyncStart message
  },
  asyncEnd(message) {
    // Handle asyncEnd message
  },
  error(message) {
    // Handle error message
  },
});
      @param subscribers

      Set of TracingChannel Channels subscribers

    • traceCallback<Fn extends (this: any, ...args: any[]) => any>(
      fn: Fn,
      position?: number,
      context?: ContextType,
      thisArg?: any,
      ...args: Parameters<Fn>
      ): void;

      Trace a callback-receiving function call. This will always produce a start event and end event around the synchronous portion of the function execution, and will produce a asyncStart event and asyncEnd event around the callback execution. It may also produce an error event if the given function throws an error or the returned promise rejects. This will run the given function using channel.runStores(context, ...) on the start channel which ensures all events should have any bound stores set to match this trace context.

      The position will be -1 by default to indicate the final argument should be used as the callback.

      import diagnostics_channel from 'node:diagnostics_channel';

const channels = diagnostics_channel.tracingChannel('my-channel');

channels.traceCallback((arg1, callback) => {
  // Do something
  callback(null, 'result');
}, 1, {
  some: 'thing',
}, thisArg, arg1, callback);

      The callback will also be run with channel.runStores(context, ...) which enables context loss recovery in some cases.

      To ensure only correct trace graphs are formed, events will only be published if subscribers are present prior to starting the trace. Subscriptions which are added after the trace begins will not receive future events from that trace, only future traces will be seen.

      import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';

const channels = diagnostics_channel.tracingChannel('my-channel');
const myStore = new AsyncLocalStorage();

// The start channel sets the initial store data to something
// and stores that store data value on the trace context object
channels.start.bindStore(myStore, (data) => {
  const span = new Span(data);
  data.span = span;
  return span;
});

// Then asyncStart can restore from that data it stored previously
channels.asyncStart.bindStore(myStore, (data) => {
  return data.span;
});
      @param fn

      callback using function to wrap a trace around

      @param position

      Zero-indexed argument position of expected callback

      @param context

      Shared object to correlate trace events through

      @param thisArg

      The receiver to be used for the function call

      @param args

      Optional arguments to pass to the function

      @returns

      The return value of the given function

    • tracePromise<ThisArg = any, Args extends any[] = any[]>(
      fn: (this: ThisArg, ...args: Args) => Promise<any>,
      context?: ContextType,
      thisArg?: ThisArg,
      ...args: Args
      ): void;

      Trace a promise-returning function call. This will always produce a start event and end event around the synchronous portion of the function execution, and will produce an asyncStart event and asyncEnd event when a promise continuation is reached. It may also produce an error event if the given function throws an error or the returned promise rejects. This will run the given function using channel.runStores(context, ...) on the start channel which ensures all events should have any bound stores set to match this trace context.

      To ensure only correct trace graphs are formed, events will only be published if subscribers are present prior to starting the trace. Subscriptions which are added after the trace begins will not receive future events from that trace, only future traces will be seen.

      import diagnostics_channel from 'node:diagnostics_channel';

const channels = diagnostics_channel.tracingChannel('my-channel');

channels.tracePromise(async () => {
  // Do something
}, {
  some: 'thing',
});
      @param fn

      Promise-returning function to wrap a trace around

      @param context

      Shared object to correlate trace events through

      @param thisArg

      The receiver to be used for the function call

      @param args

      Optional arguments to pass to the function

      @returns

      Chained from promise returned by the given function

    • traceSync<ThisArg = any, Args extends any[] = any[]>(
      fn: (this: ThisArg, ...args: Args) => any,
      context?: ContextType,
      thisArg?: ThisArg,
      ...args: Args
      ): void;

      Trace a synchronous function call. This will always produce a start event and end event around the execution and may produce an error event if the given function throws an error. This will run the given function using channel.runStores(context, ...) on the start channel which ensures all events should have any bound stores set to match this trace context.

      To ensure only correct trace graphs are formed, events will only be published if subscribers are present prior to starting the trace. Subscriptions which are added after the trace begins will not receive future events from that trace, only future traces will be seen.

      import diagnostics_channel from 'node:diagnostics_channel';

const channels = diagnostics_channel.tracingChannel('my-channel');

channels.traceSync(() => {
  // Do something
}, {
  some: 'thing',
});
      @param fn

      Function to wrap a trace around

      @param context

      Shared object to correlate events through

      @param thisArg

      The receiver to be used for the function call

      @param args

      Optional arguments to pass to the function

      @returns

      The return value of the given function

    • subscribers: TracingChannelSubscribers<ContextType>
      ): void;

      Helper to unsubscribe a collection of functions from the corresponding channels. This is the same as calling channel.unsubscribe(onMessage) on each channel individually.

      import diagnostics_channel from 'node:diagnostics_channel';

const channels = diagnostics_channel.tracingChannel('my-channel');

channels.unsubscribe({
  start(message) {
    // Handle start message
  },
  end(message) {
    // Handle end message
  },
  asyncStart(message) {
    // Handle asyncStart message
  },
  asyncEnd(message) {
    // Handle asyncEnd message
  },
  error(message) {
    // Handle error message
  },
});
      @param subscribers

      Set of TracingChannel Channels subscribers

      @returns

      true if all handlers were successfully unsubscribed, and false otherwise.

  • function channel(
    name: string | symbol

    This is the primary entry-point for anyone wanting to publish to a named channel. It produces a channel object which is optimized to reduce overhead at publish time as much as possible.

    import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');
    @param name

    The channel name

    @returns

    The named channel object

  • function hasSubscribers(
    name: string | symbol
    ): boolean;

    Check if there are active subscribers to the named channel. This is helpful if the message you want to send might be expensive to prepare.

    This API is optional but helpful when trying to publish messages from very performance-sensitive code.

    import diagnostics_channel from 'node:diagnostics_channel';

if (diagnostics_channel.hasSubscribers('my-channel')) {
  // There are subscribers, prepare and publish message
}
    @param name

    The channel name

    @returns

    If there are active subscribers

  • function subscribe(
    name: string | symbol,
    onMessage: ChannelListener
    ): void;

    Register a message handler to subscribe to this channel. This message handler will be run synchronously whenever a message is published to the channel. Any errors thrown in the message handler will trigger an 'uncaughtException'.

    import diagnostics_channel from 'node:diagnostics_channel';

diagnostics_channel.subscribe('my-channel', (message, name) => {
  // Received data
});
    @param name

    The channel name

    @param onMessage

    The handler to receive channel messages

  • function tracingChannel<StoreType = unknown, ContextType extends object = StoreType extends object ? StoreType<StoreType> : object>(
    nameOrChannels: string | TracingChannelCollection<StoreType, ContextType>
    ): TracingChannel<StoreType, ContextType>;

    Creates a TracingChannel wrapper for the given TracingChannel Channels. If a name is given, the corresponding tracing channels will be created in the form of tracing:${name}:${eventType} where eventType corresponds to the types of TracingChannel Channels.

    import diagnostics_channel from 'node:diagnostics_channel';

const channelsByName = diagnostics_channel.tracingChannel('my-channel');

// or...

const channelsByCollection = diagnostics_channel.tracingChannel({
  start: diagnostics_channel.channel('tracing:my-channel:start'),
  end: diagnostics_channel.channel('tracing:my-channel:end'),
  asyncStart: diagnostics_channel.channel('tracing:my-channel:asyncStart'),
  asyncEnd: diagnostics_channel.channel('tracing:my-channel:asyncEnd'),
  error: diagnostics_channel.channel('tracing:my-channel:error'),
});
    @param nameOrChannels

    Channel name or object containing all the TracingChannel Channels

    @returns

    Collection of channels to trace with

  • function unsubscribe(
    name: string | symbol,
    onMessage: ChannelListener
    ): boolean;

    Remove a message handler previously registered to this channel with subscribe.

    import diagnostics_channel from 'node:diagnostics_channel';

function onMessage(message, name) {
  // Received data
}

diagnostics_channel.subscribe('my-channel', onMessage);

diagnostics_channel.unsubscribe('my-channel', onMessage);
    @param name

    The channel name

    @param onMessage

    The previous subscribed handler to remove

    @returns

    true if the handler was found, false otherwise.

Type definitions