ADR-0006: Telemetry — the SDK emits, the application exports

Status: Accepted (M1) · Date: 2026-07

Context

An SDK that installs a global tracing subscriber, metrics recorder, or OTLP exporter fights the host application for process-global state and drags heavy exporter dependencies into every build.

Decision

The SDK only emits through the ecosystem facades: a canton.rpc tracing span per RPC (method + transport fields) and metrics counters (canton_client_requests_total / canton_client_errors_total, labelled by method, transport, retriability). The application installs whatever subscriber/recorder it wants. The optional otel feature adds the two pieces that genuinely belong SDK-side: an OTLP span-exporter constructor (telemetry::otel::otlp_tracer) and automatic W3C trace-context injection into outgoing gRPC metadata and JSON headers.

Consequences

• Zero process-global state claimed by the SDK; no exporter deps in default builds.
• Per-endpoint success/error breakdowns come from the labels; “success” = requests − errors.
• Log level/format/destination are consumer-owned (standard Rust model), which is how the proposal’s “configurable logging” is satisfied.