Documentation Index
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The scope manager provides explicit control over which span is active in any given execution context. In most cases you do not need to use it directly—tracer.trace() activates the span automatically. Use the scope manager only for advanced scenarios where the default async context propagation breaks down (e.g., custom timer queues, event emitters, or third-party async schedulers).
Access the scope manager via:
const scope = tracer.scope()
scope.active()
Returns the active Span in the current execution context, or null if there is none.
scope.active(): Span | null
const tracer = require('dd-trace').init()
const scope = tracer.scope()
const span = scope.active()
if (span) {
span.setTag('custom.tag', 'value')
}
scope.activate(span, fn)
Activates a span for the duration of the provided function. All code executed synchronously or asynchronously from within fn—including setTimeout, setInterval, and promise continuations—will see span as the active span.
scope.activate<T>(span: Span, fn: (...args: any[]) => T): T
const tracer = require('dd-trace').init()
const scope = tracer.scope()
const log = console.log
const requestSpan = tracer.startSpan('web.request')
const promise = Promise.resolve()
scope.activate(requestSpan, () => {
log(scope.active()) // requestSpan because in new scope
someFunction() // requestSpan because called in scope
setTimeout(() => {
log(scope.active()) // requestSpan because setTimeout called in scope
})
promise.then(() => {
log(scope.active()) // requestSpan because then() called in scope
})
})
function someFunction () {
log(scope.active())
}
log(scope.active()) // null
someFunction() // null because called outside the scope
fn is forwarded as the return value of scope.activate().
scope.bind(target, [span])
Binds a function or promise to a specific span (or to the currently active span if no span is provided). The binding is permanent—the bound function always runs in the context of that span regardless of when or where it is called.
scope.bind<T extends (...args: any[]) => void>(fn: T, span?: Span | null): T
scope.bind<T>(fn: Promise<T>, span?: Span | null): Promise<T>
span argument | Result |
|---|
A Span instance | Function is always bound to that span. |
Omitted / undefined | Function is bound to the span active at the time bind() is called. |
null | Function is explicitly bound to no span. |
const tracer = require('dd-trace').init()
const scope = tracer.scope()
const log = console.log
const outerSpan = tracer.startSpan('web.request')
scope.activate(outerSpan, () => {
const innerSpan = tracer.startSpan('web.middleware')
const boundToInner = scope.bind(() => {
log(scope.active())
}, innerSpan)
const boundToOuter = scope.bind(() => {
log(scope.active())
})
boundToInner() // innerSpan because explicitly bound
boundToOuter() // outerSpan because implicitly bound at bind() call time
})
When to use the scope manager directly
Use scope.activate() or scope.bind() when:
- You maintain an internal queue of callbacks and execute them on a timer (the scope from when the callback was enqueued needs to be preserved).
- You use an event emitter whose listeners are called outside the original async context.
- You use a custom async scheduler that does not integrate with Node.js
AsyncLocalStorage.
tracer.trace() calls scope.activate() internally. If you use tracer.trace(), the span is already active for all code inside the callback—no additional scope management is needed.
Example: preserving context across an event emitter
const tracer = require('dd-trace').init()
const scope = tracer.scope()
const EventEmitter = require('events')
const emitter = new EventEmitter()
tracer.trace('web.request', (span) => {
// Bind the listener to the current span before registering it
emitter.on('data', scope.bind((chunk) => {
// scope.active() === span here, even if emitter fires outside this callback
span.setTag('data.length', chunk.length)
}))
})
// Even though this fires outside the tracer.trace callback, the bound
// listener still sees the correct span.
emitter.emit('data', Buffer.alloc(256))
