🛈 Note: This is pre-release documentation for the upcoming tracing 0.2.0 ecosystem.

For the release documentation, please see docs.rs, instead.

pub trait Collect: 'static {
Show 14 methods fn enabled(&self, metadata: &Metadata<'_>) -> bool;
fn new_span(&self, span: &Attributes<'_>) -> Id;
fn record(&self, span: &Id, values: &Record<'_>);
fn record_follows_from(&self, span: &Id, follows: &Id);
fn event(&self, event: &Event<'_>);
fn enter(&self, span: &Id);
fn exit(&self, span: &Id);
fn current_span(&self) -> Current; fn register_callsite(
        metadata: &'static Metadata<'static>
    ) -> Interest { ... }
fn max_level_hint(&self) -> Option<LevelFilter> { ... }
fn clone_span(&self, id: &Id) -> Id { ... }
fn drop_span(&self, _id: Id) { ... }
fn try_close(&self, id: Id) -> bool { ... }
unsafe fn downcast_raw(&self, id: TypeId) -> Option<NonNull<()>> { ... }
Expand description

Trait representing the functions required to collect trace data.

Crates that provide implementations of methods for collecting or recording trace data should implement the Collect interface. This trait is intended to represent fundamental primitives for collecting trace events and spans — other libraries may offer utility functions and types to make collector implementations more modular or improve the ergonomics of writing collectors.

A collector is responsible for the following:

  • Registering new spans as they are created, and providing them with span IDs. Implicitly, this means the collector may determine the strategy for determining span equality.
  • Recording the attachment of field values and follows-from annotations to spans.
  • Filtering spans and events, and determining when those filters must be invalidated.
  • Observing spans as they are entered, exited, and closed, and events as they occur.

When a span is entered or exited, the collector is provided only with the ID with which it tagged that span when it was created. This means that it is up to the collector to determine whether and how span data — the fields and metadata describing the span — should be stored. The new_span function is called when a new span is created, and at that point, the collector may choose to store the associated data if it will be referenced again. However, if the data has already been recorded and will not be needed by the implementations of enter and exit, the collector may freely discard that data without allocating space to store it.

Overriding default impls

Some trait methods on Collect have default implementations, either in order to reduce the surface area of implementing Collect, or for backward-compatibility reasons. However, many collectors will likely want to override these default implementations.

The following methods are likely of interest:

  • register_callsite is called once for each callsite from which a span event may originate, and returns an Interest value describing whether or not the collector wishes to see events or spans from that callsite. By default, it calls enabled, and returns Interest::always() if enabled returns true, or Interest::never() if enabled returns false. However, if the collector’s interest can change dynamically at runtime, it may want to override this function to return Interest::sometimes(). Additionally, collectors which wish to perform a behaviour once for each callsite, such as allocating storage for data related to that callsite, can perform it in register_callsite.
  • clone_span is called every time a span ID is cloned, and try_close is called when a span ID is dropped. By default, these functions do nothing. However, they can be used to implement reference counting for spans, allowing collectors to free storage for span data and to determine when a span has closed permanently (rather than being exited). Collectors which store per-span data or which need to track span closures should override these functions together.

Required methods

Returns true if a span or event with the specified metadata would be recorded.

By default, it is assumed that this filter needs only be evaluated once for each callsite, so it is called by register_callsite when each callsite is registered. The result is used to determine if the collector is always interested or never interested in that callsite. This is intended primarily as an optimization, so that expensive filters (such as those involving string search, et cetera) need not be re-evaluated.

However, if the collector’s interest in a particular span or event may change, or depends on contexts only determined dynamically at runtime, then the register_callsite method should be overridden to return Interest::sometimes. In that case, this function will be called every time that span or event occurs.

Visit the construction of a new span, returning a new span ID for the span being constructed.

The provided Attributes contains any field values that were provided when the span was created. The collector may pass a visitor to the Attributesrecord method to record these values.

IDs are used to uniquely identify spans and events within the context of a collector, so span equality will be based on the returned ID. Thus, if the collector wishes for all spans with the same metadata to be considered equal, it should return the same ID every time it is given a particular set of metadata. Similarly, if it wishes for two separate instances of a span with the same metadata to not be equal, it should return a distinct ID every time this function is called, regardless of the metadata.

Note that the collector is free to assign span IDs based on whatever scheme it sees fit. Any guarantees about uniqueness, ordering, or ID reuse are left up to the collector implementation to determine.

Record a set of values on a span.

This method will be invoked when value is recorded on a span. Recording multiple values for the same field is possible, but the actual behaviour is defined by the collector implementation.

Keep in mind that a span might not provide a value for each field it declares.

The collector is expected to provide a visitor to the Record’s record method in order to record the added values.


“foo = 3” will be recorded when record is called on the Attributes passed to new_span. Since values are not provided for the bar and baz fields, the span’s Metadata will indicate that it has those fields, but values for them won’t be recorded at this time.

let mut span = span!("my_span", foo = 3, bar, baz);

// `Collector::record` will be called with a `Record`
// containing "bar = false"
span.record("bar", &false);

// `Collector::record` will be called with a `Record`
// containing "baz = "a string""
span.record("baz", &"a string");

Adds an indication that span follows from the span with the id follows.

This relationship differs somewhat from the parent-child relationship: a span may have any number of prior spans, rather than a single one; and spans are not considered to be executing inside of the spans they follow from. This means that a span may close even if subsequent spans that follow from it are still open, and time spent inside of a subsequent span should not be included in the time its precedents were executing. This is used to model causal relationships such as when a single future spawns several related background tasks, et cetera.

If the collector has spans corresponding to the given IDs, it should record this relationship in whatever way it deems necessary. Otherwise, if one or both of the given span IDs do not correspond to spans that the collector knows about, or if a cyclical relationship would be created (i.e., some span a which proceeds some other span b may not also follow from b), it may silently do nothing.

Records that an Event has occurred.

This method will be invoked when an Event is constructed by the Event’s dispatch method. For example, this happens internally when an event macro from tracing is called.

The key difference between this method and record is that record is called when a value is recorded for a field defined by a span, while event is called when a new event occurs.

The provided Event struct contains any field values attached to the event. The collector may pass a visitor to the Event’s record method to record these values.

Records that a span has been entered.

When entering a span, this method is called to notify the collector that the span has been entered. The collector is provided with the span ID of the entered span, and should update any internal state tracking the current span accordingly.

Records that a span has been exited.

When exiting a span, this method is called to notify the collector that the span has been exited. The collector is provided with the span ID of the exited span, and should update any internal state tracking the current span accordingly.

Exiting a span does not imply that the span will not be re-entered.

Returns a type representing this collector’s view of the current span.

If collectors track a current span, they should return Current::new if the thread from which this method is called is inside a span, or Current::none if the thread is not inside a span.

Provided methods

Registers a new callsite with this collector, returning whether or not the collector is interested in being notified about the callsite.

By default, this function assumes that the collector’s filter represents an unchanging view of its interest in the callsite. However, if this is not the case, collectors may override this function to indicate different interests, or to implement behaviour that should run once for every callsite.

This function is guaranteed to be called at least once per callsite on every active collector. The collector may store the keys to fields it cares about in order to reduce the cost of accessing fields by name, preallocate storage for that callsite, or perform any other actions it wishes to perform once for each callsite.

The collector should then return an Interest, indicating whether it is interested in being notified about that callsite in the future. This may be Always indicating that the collector always wishes to be notified about the callsite, and its filter need not be re-evaluated; Sometimes, indicating that the collector may sometimes care about the callsite but not always (such as when sampling), or Never, indicating that the collector never wishes to be notified about that callsite. If all active collectors return Never, a callsite will never be enabled unless a new collector expresses interest in it.

Collectors which require their filters to be run every time an event occurs or a span is entered/exited should return Interest::sometimes. If a collector returns Interest::sometimes, then its’ enabled method will be called every time an event or span is created from that callsite.

For example, suppose a sampling collector is implemented by incrementing a counter every time enabled is called and only returning true when the counter is divisible by a specified sampling rate. If that collector returns Interest::always from register_callsite, then the filter will not be re-evaluated once it has been applied to a given set of metadata. Thus, the counter will not be incremented, and the span or event that corresponds to the metadata will never be enabled.

Collectors that need to change their filters occasionally should call rebuild_interest_cache to re-evaluate register_callsite for all callsites.

Similarly, if a Collector has a filtering strategy that can be changed dynamically at runtime, it would need to re-evaluate that filter if the cached results have changed.

A collector which manages fanout to multiple other collectors should proxy this decision to all of its child collectors, returning Interest::never only if all such children return Interest::never. If the set of collectors to which spans are broadcast may change dynamically, the collector should also never return Interest::Never, as a new collector may be added that is interested.


This function may be called again when a new collector is created or when the registry is invalidated.

If a collector returns Interest::never for a particular callsite, it may still see spans and events originating from that callsite, if another collector expressed interest in it.

Returns the highest verbosity level that this Collector will enable, or None, if the collector does not implement level-based filtering or chooses not to implement this method.

If this method returns a Level, it will be used as a hint to determine the most verbose level that will be enabled. This will allow spans and events which are more verbose than that level to be skipped more efficiently. collectors which perform filtering are strongly encouraged to provide an implementation of this method.

If the maximum level the collector will enable can change over the course of its lifetime, it is free to return a different value from multiple invocations of this method. However, note that changes in the maximum level will only be reflected after the callsite Interest cache is rebuilt, by calling the callsite::rebuild_interest_cache function. Therefore, if the collector will change the value returned by this method, it is responsible for ensuring that rebuild_interest_cache is called after the value of the max level changes.

Notifies the collector that a span ID has been cloned.

This function is guaranteed to only be called with span IDs that were returned by this collector’s new_span function.

Note that the default implementation of this function this is just the identity function, passing through the identifier. However, it can be used in conjunction with try_close to track the number of handles capable of entering a span. When all the handles have been dropped (i.e., try_close has been called one more time than clone_span for a given ID), the collector may assume that the span will not be entered again. It is then free to deallocate storage for data associated with that span, write data from that span to IO, and so on.

For more unsafe situations, however, if id is itself a pointer of some kind this can be used as a hook to “clone” the pointer, depending on what that means for the specified pointer.

👎 Deprecated since 0.1.2:

use Collector::try_close instead

This method is deprecated.

Using drop_span may result in collectors composed using tracing-subscriber crate’s Subscriber trait from observing close events. Use try_close instead.

The default implementation of this function does nothing.

Notifies the collector that a span ID has been dropped, and returns true if there are now 0 IDs that refer to that span.

Higher-level libraries providing functionality for composing multiple collector implementations may use this return value to notify any “layered” collectors that this collector considers the span closed.

The default implementation of this method calls the collector’s drop_span method and returns false. This means that, unless the collector overrides the default implementation, close notifications will never be sent to any layered collectors. In general, if the collector tracks reference counts, this method should be implemented, rather than drop_span.

This function is guaranteed to only be called with span IDs that were returned by this collector’s new_span function.

It’s guaranteed that if this function has been called once more than the number of times clone_span was called with the same id, then no more handles that can enter the span with that id exist. This means that it can be used in conjunction with clone_span to track the number of handles capable of entering a span. When all the handles have been dropped (i.e., try_close has been called one more time than clone_span for a given ID), the collector may assume that the span will not be entered again, and should return true. It is then free to deallocate storage for data associated with that span, write data from that span to IO, and so on.

Note: since this function is called when spans are dropped, implementations should ensure that they are unwind-safe. Panicking from inside of a try_close function may cause a double panic, if the span was dropped due to a thread unwinding.

If self is the same type as the provided TypeId, returns an untyped NonNull pointer to that type. Otherwise, returns None.

If you wish to downcast a Collector, it is strongly advised to use the safe API provided by downcast_ref instead.

This API is required for downcast_raw to be a trait method; a method signature like downcast_ref (with a generic type parameter) is not object-safe, and thus cannot be a trait method for Collector. This means that if we only exposed downcast_ref, Collector implementations could not override the downcasting behavior

This method may be overridden by “fan out” or “chained” collector implementations which consist of multiple composed types. Such collectors might allow downcast_raw by returning references to those component if they contain components with the given TypeId.


The downcast_ref method expects that the pointer returned by downcast_raw points to a valid instance of the type with the provided TypeId. Failure to ensure this will result in undefined behaviour, so implementing downcast_raw is unsafe.


Returns true if this Collector is the same type as T.

Returns some reference to this Collector value if it is of type T, or None if it isn’t.

Implementations on Foreign Types