Module tracing_core::callsite

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Callsites represent the source locations from which spans or events originate.

What Are Callsites?

Every span or event in tracing is associated with a Callsite. A callsite is a small static value that is responsible for the following:

  • Storing the span or event’s Metadata,
  • Uniquely identifying the span or event definition,
  • Caching the subscriber’s Interest1 in that span or event, to avoid re-evaluating filters.

Registering Callsites

When a span or event is recorded for the first time, its callsite registers itself with the global callsite registry. Registering a callsite calls the Subscriber::register_callsite method with that callsite’s Metadata on every currently active subscriber. This serves two primary purposes: informing subscribers of the callsite’s existence, and performing static filtering.

Callsite Existence

If a Subscriber implementation wishes to allocate storage for each unique span/event location in the program, or pre-compute some value that will be used to record that span or event in the future, it can do so in its register_callsite method.

Performing Static Filtering

The register_callsite method returns an Interest value, which indicates that the subscriber either always wishes to record that span or event, sometimes wishes to record it based on a dynamic filter evaluation, or never wishes to record it.

When registering a new callsite, the Interests returned by every currently active subscriber are combined, and the result is stored at each callsite. This way, when the span or event occurs in the future, the cached Interest value can be checked efficiently to determine if the span or event should be recorded, without needing to perform expensive filtering (i.e. calling the Subscriber::enabled method every time a span or event occurs).

Rebuilding Cached Interest

When a new Dispatch is created (i.e. a new subscriber becomes active), any previously cached Interest values are re-evaluated for all callsites in the program. This way, if the new subscriber will enable a callsite that was not previously enabled, the Interest in that callsite is updated. Similarly, when a subscriber is dropped, the interest cache is also re-evaluated, so that any callsites enabled only by that subscriber are disabled.

In addition, the rebuild_interest_cache function in this module can be used to manually invalidate all cached interest and re-register those callsites. This function is useful in situations where a subscriber’s interest can change, but it does so relatively infrequently. The subscriber may wish for its interest to be cached most of the time, and return Interest::always or Interest::never in its register_callsite method, so that its Subscriber::enabled method doesn’t need to be evaluated every time a span or event is recorded. However, when the configuration changes, the subscriber can call rebuild_interest_cache to re-evaluate the entire interest cache with its new configuration. This is a relatively costly operation, but if the configuration changes infrequently, it may be more efficient than calling Subscriber::enabled frequently.

Implementing Callsites

In most cases, instrumenting code using tracing should not require implementing the Callsite trait directly. When using the tracing crate’s macros or the #[instrument] attribute, a Callsite is automatically generated.

However, code which provides alternative forms of tracing instrumentation may need to interact with the callsite system directly. If instrumentation-side code needs to produce a Callsite to emit spans or events, the DefaultCallsite struct provided in this module is a ready-made Callsite implementation that is suitable for most uses. When possible, the use of DefaultCallsite should be preferred over implementing Callsite for user types, as DefaultCallsite may benefit from additional performance optimizations.

  1. Returned by the Subscriber::register_callsite method. ↩

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