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## Motivation Currently, when using `tracing-subscriber`'s `Layer` API, it is necessary for each layer to manage its own storage for per-span data. Since subscribers are shared concurrently, this means that `Layer`s must also manage synchronization on this storage. This is easy to get wrong, and even when it's implemented correctly, having every layer synchronize separately adds a lot of overhead. Ideally, it should be possible for state stored by the subscriber to be exposed to `Layer`s. ## Solution: This branch *starts* on the project of a general-purpose span registry by adding a new trait, `LookupMetadata`. Subscribers may implement this trait to allow looking up a span's `Metadata` by ID. If a subscriber implements this trait, the `layer::Context` type will then allow any layers wrapping that subscriber to look up metadata by ID. The `FmtSubscriber` implements this trait. Future work will be able to an interface for `Subscriber`s to expose more span data, such as parents, children, and arbitrarily-typed extensions, to `Layer`s as well. Signed-off-by: Eliza Weisman <eliza@buoyant.io>
tracing-subscriber
Utilities for implementing and composing tracing subscribers.
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This project is licensed under the MIT license.
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Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tracing by you, shall be licensed as MIT, without any additional terms or conditions.