Logging — DLT base library with swappable backends¶

Requirements for the workspace-wide logging facade and its default DLT backend. The chapter introduces two new crates — taktora-log (the facade) and taktora-log-dlt (the DLT backend) — and documents how they coexist with the existing taktora-executor-tracing.

The design rationale, alternatives considered, and reference deployment context (COVESA dlt-daemon, AUTOSAR R20-11) live in the companion design doc docs/superpowers/specs/2026-05-18-taktora-log-dlt-design.md in the repository root.

The umbrella is split into eight capability-cluster sub-features. Each sub-feature :satisfies: an umbrella; each req :satisfies: exactly one capability-cluster feature. Each sub-feature has its own page (see the toctree): the facade (taktora-log facade crate (FEAT_0071)), the backend-swap surface (Backend-swap surface (FEAT_0073)), the tracing bridge (tracing-log bridge for exis... (FEAT_0078)), the DLT backend (taktora-log-dlt DLT-protoco... (FEAT_0072)), structured fields (Structured key-value fields... (FEAT_0074)), runtime log-level control (Runtime per-context log-lev... (FEAT_0075)), the offline ring buffer (Offline ring buffer with re... (FEAT_0076)), and the console dev fallback (Console dev fallback (FEAT_0077)).

Top-level umbrella¶

Feature: Shared logging base library FEAT_0070

status: approved

is satisfied by: FEAT_0072, FEAT_0071

is refined by: ARCH_0072, ADR_0087, ADR_0089, QG_0018, QG_0019, QG_0020, QG_0021, CON_0024, CON_0026, CON_0027

A workspace-wide logging surface used by every taktora crate and by any downstream connector. The umbrella satisfies two competing forces simultaneously:

Vehicle integrators want DLT. The base library must speak AUTOSAR Diagnostic Log and Trace natively to a co-located COVESA dlt-daemon so taktora’s events surface in the same DLT Viewer / dlt-tui / backend-upload pipeline as everything else on the ECU.
Non-vehicle integrators do not want DLT. Bench rigs, dev machines, CI, and third-party experiments must be able to swap DLT for log4rs / env_logger / a bespoke logger without touching any caller site.

The resolution is to commit to the rust-native log crate as the workspace logging facade (per log crate as workspace logg... (CON_0024)) and ship a DLT backend behind it (per taktora-log-dlt DLT-protoco... (FEAT_0072)). This mirrors the embassy-rs/embassy posture for std targets — log is the facade, the backend is chosen at process init.

The umbrella decomposes into the capability clusters below.

Requirements at a glance¶

Used filter:

ID	Title	Status	Satisfies
FEAT_0071	taktora-log facade crate	approved	FEAT_0070
FEAT_0072	taktora-log-dlt DLT-protocol backend	approved	FEAT_0070
FEAT_0073	Backend-swap surface	approved	FEAT_0071
FEAT_0074	Structured key-value fields mapped to DLT verbose args	approved	FEAT_0072
FEAT_0075	Runtime per-context log-level control	approved	FEAT_0072
FEAT_0076	Offline ring buffer with reconnect drain	approved	FEAT_0072
FEAT_0077	Console dev fallback	approved	FEAT_0071
FEAT_0078	tracing-log bridge for existing tracing emitters	approved	FEAT_0071
REQ_0800	Single facade for all taktora crates	implemented	FEAT_0071
REQ_0801	taktora-log re-exports log macros	implemented	FEAT_0071
REQ_0802	LogSink trait defines backend extension surface	implemented	FEAT_0071
REQ_0803	One-shot init builder selects the backend	implemented	FEAT_0071
REQ_0804	Integrator may install any log::Log implementation	implemented	FEAT_0073
REQ_0805	tracing-log bridge installed at init	implemented	FEAT_0078
REQ_0806	AUTOSAR Classic DLT R20-11 encoding via dlt-core	implemented	FEAT_0072
REQ_0807	UDS (default) and TCP transports to a local dlt-daemon	implemented	FEAT_0072
REQ_0808	4-character DLT App ID and Context ID per emitting crate	implemented	FEAT_0072
REQ_0809	log::kv pairs encoded as DLT verbose arguments	implemented	FEAT_0074
REQ_0810	Set-Log-Level and Set-Default-Log-Level control messages	implemented	FEAT_0075
REQ_0811	Production default level is INFO	implemented	FEAT_0075
REQ_0812	Emission shall not block the calling thread	implemented	FEAT_0076
REQ_0813	ERROR and FATAL emission shall not heap-allocate	approved	FEAT_0076
REQ_0814	Bounded in-memory ring buffers records while daemon is down	implemented	FEAT_0076
REQ_0815	Drop-oldest overflow policy with summary record on reconnect	approved	FEAT_0076
REQ_0816	Console fallback installed when no daemon and no other logger	approved	FEAT_0077