Answer one question with each of the three signals

short · 45 min · Objective 3.3

Task

Instrument a small multi-service application with all three signals, then pose one question that only each signal can answer. The exercise proves why you need all three rather than a bigger dashboard.

Steps

  1. Run three services where A calls B and B calls C. Add an artificial delay of a few hundred milliseconds inside C only.
  2. Collect metrics: request rate, error rate and latency per service. Record in lab/signals.md what the metrics tell you about WHERE the latency is, and be honest about the limit.
  3. Collect logs centrally with a correlation ID propagated from A through to C. Show a single request's path through the logs.
  4. Add tracing and capture one trace showing the span timings. Record which hop consumed the time and how long it took you to find out compared with the log approach.
  5. Write three questions in lab/signals.md, one answerable only by each signal, with the answer and the evidence.

Verify

grep -Eci 'correlation' lab/signals.md
grep -Eci 'span|trace' lab/signals.md
grep -Ec '[0-9]+ ?(ms|millisecond)' lab/signals.md
grep -Eci 'cannot|only|limit' lab/signals.md

All four non-zero. The last one is the point: a file that does not record what metrics could NOT tell you has not demonstrated why traces exist.

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