diagnostics_channel

The node:diagnostics_channel module provides an API to create named channels to report arbitrary message data for diagnostics purposes.

It can be accessed using:

import diagnostics_channel from 'node:diagnostics_channel';

It is intended that a module writer wanting to report diagnostics messages will create one or many top-level channels to report messages through. Channels may also be acquired at runtime but it is not encouraged due to the additional overhead of doing so. Channels may be exported for convenience, but as long as the name is known it can be acquired anywhere.

If you intend for your module to produce diagnostics data for others to consume it is recommended that you include documentation of what named channels are used along with the shape of the message data. Channel names should generally include the module name to avoid collisions with data from other modules.

Usage in Deno

import * as mod from "node:diagnostics_channel";

class Channel

Usage in Deno

import { Channel } from "node:diagnostics_channel";

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.

Constructors

Channel(name: string | symbol)

new

Type Parameters

StoreType = unknown

ContextType = StoreType

Properties

hasSubscribers: boolean

readonly

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
}

name: string | symbol

readonly

Methods

bindStore(

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 };
});

publish(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',
});

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' })
});

subscribe(onMessage: ChannelListener): void

deprecated

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';

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

channel.subscribe((message, name) => {
  // Received data
});

unbindStore(store: any): void

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);

unsubscribe(onMessage: ChannelListener): void

deprecated

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

import diagnostics_channel from 'node:diagnostics_channel';

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

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

channel.subscribe(onMessage);

channel.unsubscribe(onMessage);

class TracingChannel

implements TracingChannelCollection

unstable

Usage in Deno

import { TracingChannel } from "node:diagnostics_channel";

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.

Type Parameters

StoreType = unknown

ContextType extends object = { }

Properties

asyncEnd: Channel<StoreType, ContextType>

asyncStart: Channel<StoreType, ContextType>

end: Channel<StoreType, ContextType>

error: Channel<StoreType, ContextType>

start: Channel<StoreType, ContextType>

Methods

subscribe(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
  },
});

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;
});

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.

import diagnostics_channel from 'node:diagnostics_channel';

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

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

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.

import diagnostics_channel from 'node:diagnostics_channel';

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

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

unsubscribe(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
  },
});

function channel

Usage in Deno

import { channel } from "node:diagnostics_channel";

channel(name: string | symbol): Channel

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');

Parameters

name: string | symbol

The channel name

Return Type

Channel

The named channel object

function hasSubscribers

Usage in Deno

import { hasSubscribers } from "node:diagnostics_channel";

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
}

Parameters

name: string | symbol

The channel name

Return Type

boolean

If there are active subscribers

function subscribe

Usage in Deno

import { subscribe } from "node:diagnostics_channel";

subscribe(

name: string | symbol,

onMessage: ChannelListener,
): void

import diagnostics_channel from 'node:diagnostics_channel';

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

function tracingChannel

unstable

Usage in Deno

import { tracingChannel } from "node:diagnostics_channel";

tracingChannel<

StoreType = unknown,

ContextType extends object = StoreType extends object ? 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'),
});

Type Parameters

StoreType = unknown

ContextType extends object = StoreType extends object ? StoreType : object

Parameters

nameOrChannels: string | TracingChannelCollection<StoreType, ContextType>

Channel name or object containing all the TracingChannel Channels

Return Type

TracingChannel<StoreType, ContextType>

Collection of channels to trace with

function unsubscribe

Usage in Deno

import { unsubscribe } from "node:diagnostics_channel";

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);

Parameters

name: string | symbol

The channel name

onMessage: ChannelListener

The previous subscribed handler to remove

Return Type

boolean

true if the handler was found, false otherwise.

interface TracingChannelCollection

Usage in Deno

import { type TracingChannelCollection } from "node:diagnostics_channel";

Type Parameters

StoreType = unknown

ContextType = StoreType

Properties

start: Channel<StoreType, ContextType>

end: Channel<StoreType, ContextType>

asyncStart: Channel<StoreType, ContextType>

asyncEnd: Channel<StoreType, ContextType>

error: Channel<StoreType, ContextType>

interface TracingChannelSubscribers

Usage in Deno

import { type TracingChannelSubscribers } from "node:diagnostics_channel";

Type Parameters

ContextType extends object

Properties

start: (message: ContextType) => void

end: (message: ContextType & { error?: unknown; result?: unknown; }) => void

asyncStart: (message: ContextType & { error?: unknown; result?: unknown; }) => void

asyncEnd: (message: ContextType & { error?: unknown; result?: unknown; }) => void

error: (message: ContextType & { error: unknown; }) => void

type alias ChannelListener

Usage in Deno

import { type ChannelListener } from "node:diagnostics_channel";

Definition

(

message: unknown,

name: string | symbol,
) => void