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Use /ready to decide whether an Express instance should receive traffic, and /live to decide whether its process should be restarted. In Kubernetes, a failed readiness probe removes a Pod from the matching Service endpoints without restarting it; a failed liveness probe can trigger a container restart. Keeping those signals separate helps contain an interruption without turning overload or an upstream outage into a restart cascade.
What readiness and liveness mean during an outage
| Check | Question it answers | Effect of failure in Kubernetes |
|---|---|---|
Readiness (/ready) |
Can this instance safely serve requests now? | Kubernetes marks the Pod unready and removes it from matching Service endpoints. The container is not restarted just because readiness failed. |
Liveness (/live) |
Can this process still make progress? | After the configured failure tolerance is exceeded, Kubernetes restarts the container. |
| Startup probe | Has initialization completed enough for normal checks to begin? | Until startup succeeds, Kubernetes does not run readiness or liveness probes. If startup fails beyond its configured threshold, the container is killed and handled according to its restart policy. |
Express describes health checks as a way for a load balancer to decide whether an application instance is healthy and can accept requests. Kubernetes gives that decision two distinct operational outcomes: traffic exclusion for readiness and restart for liveness. See Express health checks and graceful shutdown and Kubernetes probe behavior.
Choose what each Express endpoint tests
Make readiness reflect ability to serve
Return success only when the instance can safely handle the traffic the Service sends it. Depending on the application, readiness may depend on completed initialization or a temporary inability to serve. If the check fails, Kubernetes can stop routing normal Service traffic to that Pod while the process remains available for recovery and subsequent checks.
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Keep liveness narrow
Use liveness for a process that is running but cannot make progress, such as one stuck in a deadlock. Do not make every dependency outage or ordinary period of high load fail liveness. Restarting many healthy-but-busy processes, or a fleet whose upstream dependency is unavailable, can add load and prolong recovery. Kubernetes specifically warns that poorly designed liveness probes can cause cascading failures.
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Keep handlers quick and predictable
Health handlers should respond promptly and avoid doing expensive work. The Node.js Reference Architecture example returns HTTP 200 with JSON status ok from its Express management endpoints. It recommends implementing management endpoints before metric instrumentation so that health checks are not counted as ordinary instrumented application requests. See the Node.js Reference Architecture health-check example.
Match Express paths to Kubernetes probe paths
The endpoint names are a convention, not a Kubernetes requirement. The example below uses /ready and /live; change both the Express routes and manifest if choosing different names. The reference architecture uses /readyz and /livez, illustrating the same requirement: each configured HTTP probe path must exist and return the intended status.
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const express = require('express');
const app = express();
app.get('/ready', (req, res) => {
// Return success only when this instance can safely serve traffic.
res.status(200).json({ status: 'ok' });
});
app.get('/live', (req, res) => {
// Keep this a narrow check that the process can still make progress.
res.status(200).json({ status: 'ok' });
});
app.listen(3000);This is a minimal route shape, not a complete readiness policy: replace the unconditional readiness success with the application’s real serving condition. In Kubernetes, configure matching paths and the port your container actually listens on:
containers:
- name: app
ports:
- containerPort: 3000
readinessProbe:
httpGet:
path: /ready
port: 3000
livenessProbe:
httpGet:
path: /live
port: 3000For HTTP probes, Kubernetes treats status codes from 200 through 399, inclusive, as success. A route returning a redirect may therefore pass the probe; use an intentional, direct health response to make the signal clear. Configuration details and examples are in Kubernetes probe configuration documentation.
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Use a startup probe for slow initialization
If an application needs substantial time to initialize, configure a startup probe rather than weakening liveness for its entire lifetime. Kubernetes waits for startup success before running readiness and liveness checks. A startup failure beyond its configured threshold causes the container to be killed and subject to its restart policy.
Size the startup allowance for the application’s realistic worst-case startup behavior. Kubernetes documents the allowance as the product of failureThreshold and periodSeconds; do not copy sample values without checking them against observed startup times. The current Kubernetes probe concepts page lists defaults of periodSeconds: 10, timeoutSeconds: 1, and failureThreshold: 3. These are version-sensitive defaults, not universal tuning recommendations. See the current probe concepts documentation.
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Reconstruct an interruption from probe and service evidence
Health endpoints show what the application reported at a moment in time; Kubernetes probe events show how the orchestrator acted on that signal. Neither alone identifies why users experienced errors. Build the incident timeline by aligning evidence from the same period:
- Probe successes and failures, including the probe type and configured thresholds.
- Pod readiness transitions and whether the Pod was available through the matching Service endpoints.
- Container restarts and their timing relative to probe failures.
- Request errors and latency, to establish when service symptoms began and eased.
- Dependency and initialization evidence, to distinguish an application-process failure from inability to serve or an upstream problem.
For example, a readiness failure without a restart is consistent with traffic exclusion while the process remains running. Repeated liveness failures followed by restarts point to a different Kubernetes response, but the timeline still needs application and dependency evidence to establish the underlying cause. Probe state records the decision made by Kubernetes; it is not, by itself, a root-cause diagnosis.
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Common design mistakes to avoid
- One check for both purposes: A dependency-sensitive check used for liveness can restart a fleet during an upstream incident. Keep the traffic-serving decision in readiness and reserve liveness for unrecoverable process failure.
- Path mismatch: If Express exposes
/readybut the manifest probes/readyz, the configured check does not reach the intended handler. Keep names aligned. - Readiness that never changes: An unconditional success response does not protect traffic when the instance cannot serve. Tie readiness to a genuine serving condition.
- Startup allowance copied without context: A window too short for real startup can trigger unnecessary kills before normal probes begin. Base the threshold on realistic startup behavior.
- Reading a failed readiness probe as a restart: Readiness failure removes the Pod from Service endpoints; liveness failure is the probe that can restart its container.
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