Observability should answer specific operational questions. Metrics show population behavior, traces follow paths and logs explain events.
Before you start
You should understand API requests, storage and basic capacity estimates. Begin with a concrete user action and its correctness requirement. Draw data flow and failure boundaries before selecting infrastructure; a technology name by itself does not explain why a design meets the requirement.
The practical goal is to reason through this situation: Trace an API request through a queue and worker using a stable operation identity. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Start with failure questions
Know what operators must diagnose during an incident.
Step 2: Choose complementary signals
Metrics summarize, traces attribute paths and logs explain events.
Step 3: Preserve safe correlation
Carry request or operation identity without recording unnecessary payloads.
Worked scenario
Trace an API request through a queue and worker using a stable operation identity.
An export request enters a queue and completes in a worker minutes later. A stable operation ID connects both phases, while queue-age metrics show population health. Logging every export row creates cost and exposure without establishing why the queue is slow; timing and saturation evidence answer that question more directly.
Common mistake
Collecting every payload increases cost and privacy risk without ensuring useful diagnosis.
Verify the behavior
Trace one failed operation and verify aggregate failure, latency and capacity signals.
Interview exercise
Define three useful signals.
Answer and reasoning
Track user-facing failure rate, latency distribution and saturation, with safe correlation for investigating representative failures.
Continue learning
Compare the scenario with the System Design interview questions and test your understanding with the System Design MCQs. For terminology and implementation details, consult the reference material.