Logging — architecture (arc42)

Architecture documentation for the workspace-wide logging facade and its default DLT backend (see Logging — DLT base library with swappable backends), structured per the arc42 template and encoded with sphinx-needs using the useblocks “x-as-code” arc42 directive types. Mirrors the structure of CANopen device-driver codegen — architecture (arc42) for diff-friendly review.

Each architectural element :refines: or :implements: a parent requirement so the trace is preserved end-to-end.

This chapter is split across pages (see the toctree): the framing sections §1–§2 (goals and constraints) live here on the index; the context-and-scope flows and the building-block decomposition (§3–§4) live in Context, scope, and building blocks; the runtime data-flow and control-plane sequences live in Runtime view; and the architecture decisions (§5) live in Architecture decisions.

1. Introduction and goals

The chapter’s reason-to-exist is a single logging surface that all taktora crates emit through, with DLT as the default backend and a clear swap path for ``log4rs`` / ``env_logger`` / bespoke loggers. Vehicle integrators get AUTOSAR-spec’d DLT to a co-located dlt-daemon; non-vehicle integrators replace the backend at process boot without touching any caller site.

Quality goals capture the qualities the architecture is optimised for.

Quality Goal: Backend decoupling (single facade, replaceable backend) QG_0018
status: open
refines: FEAT_0070

Every taktora crate shall emit through one stable facade — the log crate — so the concrete backend can be replaced without touching any caller. The facade is non-negotiable; the backend is a deployment choice. This is what lets a CI run with env_logger use the same business code that a vehicle ECU runs with DLT.
Quality Goal: DLT-ecosystem observability QG_0019
status: open
refines: FEAT_0070

taktora’s events shall surface in the standard COVESA DLT ecosystem — DLT Viewer, dlt-tui, dlt-daemon gateway-mode aggregation, backend upload — without requiring custom adapters on the consumer side. Wire-level compatibility with AUTOSAR Classic DLT R20-11 is the contract.
Quality Goal: Low-overhead, non-blocking hot path QG_0020
status: open
refines: FEAT_0070

Emitting a log record shall not block the calling thread, shall not allocate on the producer side for ERROR / FATAL records, and shall not require coordinated state between producer and flusher beyond a bounded SPSC/SPMC queue. Overload manifests as the documented drop policy, never as a stall in the executor.
Quality Goal: Dev-friendly fallback (no daemon required) QG_0021
status: open
refines: FEAT_0070

Local development and CI must work without a running dlt-daemon. The console-formatted fallback (per Console dev fallback (FEAT_0077)) is the default behaviour when no daemon socket is configured, so newcomers see log output the first time they cargo run.

2. Constraints

Constraint: log crate as workspace logging facade CON_0024
status: open
refines: FEAT_0070

The workspace logging facade shall be the log crate (rust-lang/log, v0.4.21 or newer for kv support). The tracing crate remains in the workspace for its span model in taktora-log facade crate (BB_0090) and is bridged into log via the tracing-log consumer (per tracing-log bridge installe... (REQ_0805)). No third facade is introduced.
Constraint: No build-time dependency on libdlt CON_0025
status: open
refines: FEAT_0072

taktora-log-dlt shall not depend on the C libdlt library at build time. No libdlt-sys / dlt-sys / dlt-rs / dlt_log (rusty-projects) in the default Cargo graph. The DLT codec is pure Rust via esrlabs/dlt-core (per Pure-Rust DLT via dlt-core;... (ADR_0088)). Integrators with bit-for-bit libdlt parity needs bring their own LogSink impl behind Backend-swap surface (FEAT_0073).
Constraint: std required, no_std out of scope CON_0026
status: open
refines: FEAT_0070

Both taktora-log and taktora-log-dlt shall require std. no_std support is out of scope for this round — taktora’s targets are all std platforms. A future MCU connector needing DLT will get its own spec covering defmt-style emission with a host-side adapter.
Constraint: Logging is QM CON_0027
status: open
refines: FEAT_0070

Logging is treated as QM. No tsr is committed against taktora-log or taktora-log-dlt. The freedom-from- interference posture is carried as Assumptions of Use on the integrator side (see Integrator provides a DLT d... (AOU_0010) through Reboot persistence is daemo... (AOU_0015)). Integrators who need a certified path bring their own backend behind Backend-swap surface (FEAT_0073); the safety case is then theirs.