UpdatedAug 5, 2026

Koa Health Checks

Question Variations

"What is the difference between liveness and readiness in Koa?"
"Why fail readiness before calling `server.close()`?"
"Should a health check make an unbounded database query?"

Why This Is Asked

Health endpoints determine whether orchestrators restart a process or send it traffic. This question tests whether you understand readiness, liveness, and graceful shutdown from an HTTP service’s perspective.

Key Concepts

Liveness detects an unhealthy process and should be lightweight.
Readiness determines whether the instance can accept new traffic.
Readiness should fail during initialization and draining.
Dependency probes should be bounded and selected intentionally.

Question Variations

“What is the difference between liveness and readiness in Koa?”
“Why fail readiness before calling server.close()?”
“Should a health check make an unbounded database query?”

Answers by Technology

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Expected Answer (Koa 3.2.1 / Node.js 18+)

Expose a lightweight liveness endpoint to show the process is running and a readiness endpoint to show the instance may receive traffic. Set readiness false during initialization and before graceful shutdown starts. Keep dependency probes focused and bounded: an unbounded fan-out can turn a downstream incident into a restart storm.

Why It Matters

Accurate probe semantics keep load balancers from routing traffic to processes that are starting or draining.

Code Example

import Koa, { Context } from 'koa';

const app = new Koa(); let ready = false;
app.use((ctx: Context) => {
  if (ctx.path === '/livez') { ctx.status = 200; ctx.body = { ok: true }; return; }
  if (ctx.path === '/readyz') { ctx.status = ready ? 200 : 503; ctx.body = { ready }; return; }
  ctx.status = 404;
});
ready = true; app.listen(3000);

Common Mistakes

Using a deep database check for liveness: Transient dependency outages can restart healthy processes.
Leaving readiness true while stopping: New requests arrive during drain.

Follow-up Questions

When should readiness be true? (Answer: After required initialization completes.)
Why keep liveness cheap? (Answer: It should detect a wedged process without amplifying failures.)