TracingChannel - diagnostics_channel - Node

class TracingChannel

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
  },
});