Trace
The full timeline of one request across every service it touched, drawn as nested bars so you can see which step ate the time.
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What it is
A trace is the record of one request from the moment it arrives to the moment it answers, assembled from spans. Each span is one timed step (the HTTP handler, an auth call, three database queries, a payment API hop) and carries the shared trace ID plus a pointer to its parent. Draw the spans as nested bars on a shared clock and you get the picture everyone actually wants: where the 4 seconds went, which step was waiting on which, and what ran in parallel versus in sequence.
Reach for a trace when the metric already told you something is slow or failing and you need to know where. Metrics say 'p95 checkout latency doubled', traces say 'the coupon service adds 600ms and gets called four times'. The stitching happens by propagating a traceparent header on every outbound call, which OpenTelemetry does for you once the auto-instrumentation is in place. Jaeger, Tempo, Honeycomb, and the commercial APM tools all render the same waterfall.
Gotcha: traces break quietly. One service that drops the traceparent header (a hand-rolled fetch wrapper, a queue hop, a background job) turns the rest of the journey into orphan traces that look like separate requests, and the gap never announces itself. The other trap is sampling: at 1 percent head sampling, the slow request you came to investigate was probably thrown away, so use tail sampling to keep the errors and the slow ones.
Ask AI for it
Instrument this service with OpenTelemetry tracing: install the auto instrumentation for the HTTP server, the outbound HTTP client, and the database driver, and export spans over OTLP. Add manual spans around the slow business logic, with attributes for route, tenant, and cache hit/miss, and record exceptions on the span when a step fails. Make sure the traceparent header is propagated on every outbound call, including background jobs and queue messages, so the trace survives service hops. Use tail-based sampling that always keeps errors and any trace slower than 1 second, and samples the rest at 5 percent.