Overview#
Following is a simple overview of the public API.
import diagnostics_channel from 'node:diagnostics_channel';
const channel = diagnostics_channel.channel('my-channel');
function onMessage(message, name) {
}
diagnostics_channel.subscribe('my-channel', onMessage);
if (channel.hasSubscribers) {
channel.publish({
some: 'data',
});
}
diagnostics_channel.unsubscribe('my-channel', onMessage);const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');
function onMessage(message, name) {
}
diagnostics_channel.subscribe('my-channel', onMessage);
if (channel.hasSubscribers) {
channel.publish({
some: 'data',
});
}
diagnostics_channel.unsubscribe('my-channel', onMessage);
diagnostics_channel.hasSubscribers(name)#
Added in: v15.1.0, v14.17.0
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')) {
}const diagnostics_channel = require('node:diagnostics_channel');
if (diagnostics_channel.hasSubscribers('my-channel')) {
}
diagnostics_channel.tracingChannel(nameOrChannels)#
Added in: v19.9.0, v18.19.0
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');
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'),
});const diagnostics_channel = require('node:diagnostics_channel');
const channelsByName = diagnostics_channel.tracingChannel('my-channel');
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'),
});
Class: Channel#
Added in: v15.1.0, v14.17.0
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
diagnostics_channel.channel(name), constructing a channel directly
with new Channel(name) is not supported.
channel.hasSubscribers#
Added in: v15.1.0, v14.17.0
- Returns: <boolean> If there are active subscribers
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) {
}const diagnostics_channel = require('node:diagnostics_channel');
const channel = diagnostics_channel.channel('my-channel');
if (channel.hasSubscribers) {
}
channel.bindStore(store[, transform])#
Added in: v19.9.0, v18.19.0
store <AsyncLocalStorage> The store to which to bind the context data
transform <Function> Transform context data before setting the store context
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 };
});const diagnostics_channel = require('node:diagnostics_channel');
const { AsyncLocalStorage } = require('node:async_hooks');
const store = new AsyncLocalStorage();
const channel = diagnostics_channel.channel('my-channel');
channel.bindStore(store, (data) => {
return { data };
});
channel.unbindStore(store)#
Added in: v19.9.0, v18.19.0
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);const diagnostics_channel = require('node:diagnostics_channel');
const { AsyncLocalStorage } = require('node:async_hooks');
const store = new AsyncLocalStorage();
const channel = diagnostics_channel.channel('my-channel');
channel.bindStore(store);
channel.unbindStore(store);
channel.runStores(context, fn[, thisArg[, ...args]])#
Added in: v19.9.0, v18.19.0
context <any> Message to send to subscribers and bind to stores
fn <Function> Handler to run within the entered storage context
thisArg <any> The receiver to be used for the function call.
...args <any> Optional arguments to pass to the function.
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();
});const diagnostics_channel = require('node:diagnostics_channel');
const { AsyncLocalStorage } = require('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();
});
Class: TracingChannel#
Added in: v19.9.0, v18.19.0
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.
diagnostics_channel.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.
tracingChannel.subscribe(subscribers)#
Added in: v19.9.0, v18.19.0
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) {
},
end(message) {
},
asyncStart(message) {
},
asyncEnd(message) {
},
error(message) {
},
});const diagnostics_channel = require('node:diagnostics_channel');
const channels = diagnostics_channel.tracingChannel('my-channel');
channels.subscribe({
start(message) {
},
end(message) {
},
asyncStart(message) {
},
asyncEnd(message) {
},
error(message) {
},
});
tracingChannel.unsubscribe(subscribers)#
Added in: v19.9.0, v18.19.0
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) {
},
end(message) {
},
asyncStart(message) {
},
asyncEnd(message) {
},
error(message) {
},
});const diagnostics_channel = require('node:diagnostics_channel');
const channels = diagnostics_channel.tracingChannel('my-channel');
channels.unsubscribe({
start(message) {
},
end(message) {
},
asyncStart(message) {
},
asyncEnd(message) {
},
error(message) {
},
});
tracingChannel.traceSync(fn[, context[, thisArg[, ...args]]])#
Added in: v19.9.0, v18.19.0
fn <Function> Function to wrap a trace around
context <Object> Shared object to correlate events through
thisArg <any> The receiver to be used for the function call
...args <any> Optional arguments to pass to the function
- Returns: <any> The return value of the given function
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(() => {
}, {
some: 'thing',
});const diagnostics_channel = require('node:diagnostics_channel');
const channels = diagnostics_channel.tracingChannel('my-channel');
channels.traceSync(() => {
}, {
some: 'thing',
});
tracingChannel.tracePromise(fn[, context[, thisArg[, ...args]]])#
Added in: v19.9.0, v18.19.0
fn <Function> Promise-returning function to wrap a trace around
context <Object> Shared object to correlate trace events through
thisArg <any> The receiver to be used for the function call
...args <any> Optional arguments to pass to the function
- Returns: <Promise> Chained from promise returned by the given function
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 () => {
}, {
some: 'thing',
});const diagnostics_channel = require('node:diagnostics_channel');
const channels = diagnostics_channel.tracingChannel('my-channel');
channels.tracePromise(async () => {
}, {
some: 'thing',
});
tracingChannel.traceCallback(fn[, position[, context[, thisArg[, ...args]]]])#
Added in: v19.9.0, v18.19.0
fn <Function> callback using function to wrap a trace around
position <number> Zero-indexed argument position of expected callback
(defaults to last argument if undefined is passed)
context <Object> Shared object to correlate trace events through (defaults
to {} if undefined is passed)
thisArg <any> The receiver to be used for the function call
...args <any> arguments to pass to the function (must include the callback)
- Returns: <any> The return value of the given function
Trace a callback-receiving function call. The callback is expected to follow
the error as first arg convention typically used. 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 or the first argument passed to
the callback is set. 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.traceCallback((arg1, callback) => {
callback(null, 'result');
}, 1, {
some: 'thing',
}, thisArg, arg1, callback);const diagnostics_channel = require('node:diagnostics_channel');
const channels = diagnostics_channel.tracingChannel('my-channel');
channels.traceCallback((arg1, callback) => {
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.
import diagnostics_channel from 'node:diagnostics_channel';
import { AsyncLocalStorage } from 'node:async_hooks';
const channels = diagnostics_channel.tracingChannel('my-channel');
const myStore = new AsyncLocalStorage();
channels.start.bindStore(myStore, (data) => {
const span = new Span(data);
data.span = span;
return span;
});
channels.asyncStart.bindStore(myStore, (data) => {
return data.span;
});const diagnostics_channel = require('node:diagnostics_channel');
const { AsyncLocalStorage } = require('node:async_hooks');
const channels = diagnostics_channel.tracingChannel('my-channel');
const myStore = new AsyncLocalStorage();
channels.start.bindStore(myStore, (data) => {
const span = new Span(data);
data.span = span;
return span;
});
channels.asyncStart.bindStore(myStore, (data) => {
return data.span;
});
TracingChannel Channels#
A TracingChannel is a collection of several diagnostics_channels representing
specific points in the execution lifecycle of a single traceable action. The
behavior is split into five diagnostics_channels consisting of start,
end, asyncStart, asyncEnd, and error. A single traceable action will
share the same event object between all events, this can be helpful for
managing correlation through a weakmap.
These event objects will be extended with result or error values when
the task "completes". In the case of a synchronous task the result will be
the return value and the error will be anything thrown from the function.
With callback-based async functions the result will be the second argument
of the callback while the error will either be a thrown error visible in the
end event or the first callback argument in either of the asyncStart or
asyncEnd events.
To ensure only correct trace graphs are formed, events should only be published
if subscribers are present prior to starting the trace. Subscriptions which are
added after the trace begins should not receive future events from that trace,
only future traces will be seen.
Tracing channels should follow a naming pattern of:
tracing:module.class.method:start or tracing:module.function:start
tracing:module.class.method:end or tracing:module.function:end
tracing:module.class.method:asyncStart or tracing:module.function:asyncStart
tracing:module.class.method:asyncEnd or tracing:module.function:asyncEnd
tracing:module.class.method:error or tracing:module.function:error
start(event)#
- Name:
tracing:${name}:start
The start event represents the point at which a function is called. At this
point the event data may contain function arguments or anything else available
at the very start of the execution of the function.
end(event)#
- Name:
tracing:${name}:end
The end event represents the point at which a function call returns a value.
In the case of an async function this is when the promise returned not when the
function itself makes a return statement internally. At this point, if the
traced function was synchronous the result field will be set to the return
value of the function. Alternatively, the error field may be present to
represent any thrown errors.
It is recommended to listen specifically to the error event to track errors
as it may be possible for a traceable action to produce multiple errors. For
example, an async task which fails may be started internally before the sync
part of the task then throws an error.
asyncStart(event)#
- Name:
tracing:${name}:asyncStart
The asyncStart event represents the callback or continuation of a traceable
function being reached. At this point things like callback arguments may be
available, or anything else expressing the "result" of the action.
For callbacks-based functions, the first argument of the callback will be
assigned to the error field, if not undefined or null, and the second
argument will be assigned to the result field.
For promises, the argument to the resolve path will be assigned to result
or the argument to the reject path will be assign to error.
It is recommended to listen specifically to the error event to track errors
as it may be possible for a traceable action to produce multiple errors. For
example, an async task which fails may be started internally before the sync
part of the task then throws an error.
asyncEnd(event)#
- Name:
tracing:${name}:asyncEnd
The asyncEnd event represents the callback of an asynchronous function
returning. It's not likely event data will change after the asyncStart event,
however it may be useful to see the point where the callback completes.
error(event)#
- Name:
tracing:${name}:error
The error event represents any error produced by the traceable function
either synchronously or asynchronously. If an error is thrown in the
synchronous portion of the traced function the error will be assigned to the
error field of the event and the error event will be triggered. If an error
is received asynchronously through a callback or promise rejection it will also
be assigned to the error field of the event and trigger the error event.
It is possible for a single traceable function call to produce errors multiple
times so this should be considered when consuming this event. For example, if
another async task is triggered internally which fails and then the sync part
of the function then throws and error two error events will be emitted, one
for the sync error and one for the async error.