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class

diagnostics_channel.TracingChannel

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.