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pezkuwi-subxt/polkadot/node/gum/README.md
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Bernhard Schuster d631f1dea8 observability: tracing gum, automatically cross ref traceID (#5079)
* add some gum

* bump expander

* gum

* fix all remaining issues

* last fixup

* Update node/gum/proc-macro/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* change

* netowrk

* fixins

* chore

* allow optional fmt str + args, prep for expr as kv field

* tracing -> gum rename fallout

* restrict further

* allow multiple levels of field accesses

* another round of docs and a slip of the pen

* update ADR

* fixup lock fiel

* use target: instead of target=

* minors

* fix

* chore

* Update node/gum/README.md

Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>
2022-03-15 11:05:16 +00:00

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Markdown

# tracing-gum
"gum" to make `tracing::{warn,info,..}` and `mick-jaeger` stick together, to be
cross referenced in grafana with zero additional loc in the source code.
## Architecture Decision Record (ADR)
### Context
For cross referencing spans and logs in grafana loki and tempo, a shared
`traceID` or `TraceIdentifier` is required. All logs must be annotated with such
meta information.
In most cases `CandidateHash` is the primary identifier of the `jaeger::Span`
and hence the source from which the `traceID` is derived. For cases where it is
_not_ the primary identifier, a helper tag named `traceID` is added to those
spans (out of scope, this is already present as a convenience measure).
Log lines on the other hand side, use `warn!,info!,debug!,trace!,..` API
provided by the `tracing` crate. Many of these, contain a `candidate_hash`,
which is _not_ equivalent to the `traceID` (256bits vs 128bits), and hence must
be derived.
To achieve the cross ref, either all instances of `candidate_hash` could be
added or this could be approached more systematically by providing a `macro` to
automatically do so.
Related issues:
* <https://github.com/paritytech/polkadot/issues/5045>
### Decision
Adding approx. 2 lines per tracing line including a `candidate_hash` reference,
to derive the `TraceIdentifier` from that, and printing that as part of the
key-value section in the `tracing::*` macros. The visual overhead and friction
and required diligence to keep the 100s of `tracing::{warn!,info!,debug!,..}` up
is unreasonably high in the mid/long run. This is especially true, in the
context of more people joining the team. Hence a proc-macro is introduced
which abstracts this away, and does so automagically at the cost of
one-more-proc-macro in the codebase.
### Consequences
Minimal training/impact is required to name `CandidateHash` as `candidate_hash`
when providing to any of the log macros (`warn!`, `info!`, etc.).
The crate has to be used throughout the entire codebase to work consistently, to
disambiguate, the prefix `gum::` is used.
Feature parity with `tracing::{warn!,..}` is not desired. We want consistency
more than anything. All currently used features _are_ supported with _gum_ as
well.