Interface TracingChannel<StoreType, ContextType>Experimental

ExperimentalasyncEnd

ExperimentalasyncStart

Experimentalend

Experimentalerror

Readonly ExperimentalhasSubscribers

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

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

if (channels.hasSubscribers) {
  // Do something
}
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Experimentalstart

subscribe

• subscribe(subscribers: TracingChannelSubscribers<ContextType>): void
  Experimental

  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
    },
  });
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  Parameters

  • subscribers: TracingChannelSubscribers<ContextType>

    Set of TracingChannel Channels subscribers

  Returns void

  Since

  v19.9.0

traceCallback

• traceCallback<ThisArg = any, Args extends any[] = any[], Result = any>(
      fn: (this: ThisArg, ...args: Args) => Result,
      position?: number,
      context?: ContextType,
      thisArg?: ThisArg,
      ...args: Args,
  ): Result
  Experimental

  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);
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  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;
  });
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  Type Parameters

  • ThisArg = any
  • Args extends any[] = any[]
  • Result = any

  Parameters

  • fn: (this: ThisArg, ...args: Args) => Result

    callback using function to wrap a trace around

  • Optionalposition: number

    Zero-indexed argument position of expected callback

  • Optionalcontext: ContextType

    Shared object to correlate trace events through

  • OptionalthisArg: ThisArg

    The receiver to be used for the function call

  • ...args: Args

    Optional arguments to pass to the function

  Returns Result

  The return value of the given function

  Since

  v19.9.0

tracePromise

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

  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',
  });
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  Type Parameters

  • ThisArg = any
  • Args extends any[] = any[]
  • Result = any

  Parameters

  • fn: (this: ThisArg, ...args: Args) => Promise<Result>

    Promise-returning function to wrap a trace around

  • Optionalcontext: ContextType

    Shared object to correlate trace events through

  • OptionalthisArg: ThisArg

    The receiver to be used for the function call

  • ...args: Args

    Optional arguments to pass to the function

  Returns Promise<Result>

  Chained from promise returned by the given function

  Since

  v19.9.0

traceSync

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

  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',
  });
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  Type Parameters

  • ThisArg = any
  • Args extends any[] = any[]
  • Result = any

  Parameters

  • fn: (this: ThisArg, ...args: Args) => Result

    Function to wrap a trace around

  • Optionalcontext: ContextType

    Shared object to correlate events through

  • OptionalthisArg: ThisArg

    The receiver to be used for the function call

  • ...args: Args

    Optional arguments to pass to the function

  Returns Result

  The return value of the given function

  Since

  v19.9.0

unsubscribe

• unsubscribe(subscribers: TracingChannelSubscribers<ContextType>): void
  Experimental

  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
    },
  });
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  Parameters

  • subscribers: TracingChannelSubscribers<ContextType>

    Set of TracingChannel Channels subscribers

  Returns void

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

  Since

  v19.9.0