Telemetry & Observability Conventions

If the project has a dedicated telemetry/observability spec or conventions document, read it first — it takes precedence over the generic guidance below.

Log Levels

Instrumentation Depth

Not all code deserves the same instrumentation level. Choose based on the crate's role:

• Hot-path code (serialization, framing, tight loops): trace! only, no spans. Spans add overhead that matters here.
• Type definition crates (pure data types, no logic): trace! only if anything at all.
• Core business logic (servers, handlers, state machines): Full instrumentation — #[instrument] + spans.
• Client-facing code (SDKs, CLI flows): Full instrumentation for debuggability.

#[instrument] Rules

• Use on async functions that represent logical operations or flow steps
• Skip on hot-path synchronous functions (serialization, encoding, tight loops)
• Always skip sensitive fields: tokens, keys, passwords, raw payloads
• Include identifying fields that aid correlation (IDs, resource names)
• Set appropriate level — default is INFO, use level = "debug" or level = "trace" for noisy functions

Structured Logging

Always use named fields, never string interpolation:

// Good: named fields — searchable, parseable
trace!(payload_bytes = payload.len(), frame_bytes = buf.len(), "frame encoded");

// Bad: string interpolation — opaque to log aggregators
trace!("frame encoded, payload={}, frame={}", payload.len(), buf.len());

Span Design

For request/message processing pipelines, use a three-tier span pattern:

• Inbound span: one per received message/request (message type, size, sender ID)
• Process span: business logic processing (operation type, affected resources)
• Outbound span: one per response/forwarded message (recipient, payload size)

This gives visibility into where time is spent and enables per-hop latency analysis.

General Principles

• Prefer tracing over log — structured spans enable distributed tracing
• Log at the point of decision, not at every intermediate step
• Include enough context to diagnose without reproducing: IDs, sizes, error details
• Never log secrets, tokens, keys, or raw user data — even at trace! level
• Use Display for user-facing context, Debug for developer diagnostics