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Health Endpoint Monitoring — Junior

At junior level, focus on this question:

What's the difference between "is this process alive" and "is this process ready to serve traffic," and why does conflating them cause incidents?

Two different questions

flowchart LR Liveness["LIVENESS:\nis the process running\nand not deadlocked/hung?"] Readiness["READINESS:\nis the process currently\nable to serve requests\nsuccessfully?"]
Check Answers Action if it fails
Liveness "Is this process fundamentally broken (hung, deadlocked, crashed internally) and needs to be restarted?" Restart the process/container
Readiness "Is this process currently able to handle requests successfully right now?" Stop routing traffic to it, but don't restart — it might recover on its own (e.g. a temporarily overloaded downstream dependency)

Why conflating them causes incidents

flowchart LR Scenario["A downstream database\nis temporarily slow"] --> BadCheck["Health check conflates\nliveness+readiness:\nchecks DB connectivity,\nfails, returns unhealthy"] BadCheck --> Restart["Orchestrator RESTARTS\nthe (perfectly healthy)\nprocess"] Restart --> Worse["Restarting doesn't fix the\nslow database, and now\nyou've also lost the\nprocess's warm caches/\nconnections - WORSE"]

If a single health check is used for both liveness and readiness, a temporary downstream issue (which should only affect readiness — "don't send me traffic right now") can trigger an unnecessary restart (liveness's action), which does nothing to fix the actual problem and adds the cost of cold-starting a process that wasn't actually broken.

🎓 Takeaway: liveness answers "should this specific process be restarted?" — readiness answers "should traffic be routed here right now?" These are different questions with different correct actions, and a single conflated health check routinely gets the response wrong for one of them.

Test yourself

  1. Why is "restart the process" the wrong response to a temporarily overloaded downstream dependency?
  2. Give an example of a condition that should fail readiness but NOT liveness.
  3. Give an example of a condition that should fail liveness (the process genuinely needs restarting).

Continue to middle.md.