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Node Renderer Health and Readiness Endpoints

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The node renderer ships built-in, opt-in probe endpoints so container orchestrators (Kubernetes, ECS, Docker Compose, Control Plane) can check renderer liveness and readiness without custom Fastify code:

  • GET /health — liveness. Returns 200 with { "status": "ok" } whenever the process can answer, i.e. the event loop is responsive. It intentionally checks no dependencies (no bundle, Rails, or license state), so a transient dependency issue never restarts the container.
  • GET /ready — readiness. Returns 200 with { "status": "ready" } only when the renderer can actually serve render requests: the worker answering the probe is online and at least one server bundle has been compiled into its VM pool. Until then it returns 503 with { "status": "waiting_for_bundle" }, because a renderer with zero bundles responds 410 to render requests until the Rails client uploads one.

Both endpoints return status-only JSON bodies — no runtime versions, file paths, or license details — so leaving them reachable exposes nothing sensitive. Like /info, they are plain GET routes outside the authenticated render and asset endpoints and do not require the renderer password (orchestrator probes cannot carry it). Keep the renderer on localhost or private networking as usual; see Network Security.

Enabling the Endpoints​

The endpoints are off by default. Enable them with the enableHealthEndpoints config option or the RENDERER_ENABLE_HEALTH_ENDPOINTS environment variable (true, TRUE, yes, YES, or 1):

The 1 alias is scoped to RENDERER_ENABLE_HEALTH_ENDPOINTS; other node-renderer boolean environment variables keep their existing parsing behavior.

// renderer/node-renderer.js
const { reactOnRailsProNodeRenderer } = require('react-on-rails-pro-node-renderer');

reactOnRailsProNodeRenderer({
  enableHealthEndpoints: true,
  // ... your other options
});

Or, without changing the launch file:

RENDERER_ENABLE_HEALTH_ENDPOINTS=true node renderer/node-renderer.js

Verify locally (note --http2-prior-knowledge — see the next section for why):

curl -s --http2-prior-knowledge http://localhost:3800/health
# => {"status":"ok"}
curl -s --http2-prior-knowledge http://localhost:3800/ready
# => 503 {"status":"waiting_for_bundle"} until the first bundle upload, then 200 {"status":"ready"}

h2c: Why httpGet Probes Do NOT Work​

The renderer listens with cleartext HTTP/2 (h2c). Kubernetes httpGet probes, ALB target-group health checks, Control Plane HTTP probes, and other HTTP/1.1-only checkers cannot speak h2c and cannot reach these endpoints directly. Do not configure an httpGet probe against the renderer port — it will always fail.

Use these probe shapes instead:

  • exec probe with an h2c-aware client packaged in your image, e.g. curl -sf --http2-prior-knowledge http://localhost:3800/ready. This is the only shape that checks application-level readiness. Verify your image's curl has HTTP/2 support: curl --version | grep -i http2.
  • tcpSocket probe as a shallow fallback: it proves the port is bound, not that the renderer can serve.

exec probes run inside the container, so the default localhost host binding works. tcpSocket probes connect to the pod/workload IP, so they require the renderer host set to 0.0.0.0. See Configuring Startup, Readiness, and Liveness Probes for the full probe-style discussion and timing guidance.

Kubernetes Probes​

A working probe set for a renderer container with enableHealthEndpoints: true and curl (with HTTP/2 support) in the image:

containers:
  - name: node-renderer
    image: my-registry/my-app:latest
    command: ['node', 'renderer/node-renderer.js']
    ports:
      - containerPort: 3800
    env:
      - name: RENDERER_HOST
        value: '0.0.0.0' # required by the tcpSocket probes below
      - name: RENDERER_PORT
        value: '3800'
      - name: RENDERER_ENABLE_HEALTH_ENDPOINTS
        value: 'true'
    # Startup: shield liveness while the renderer boots. TCP is enough here
    # because readiness below gates traffic.
    startupProbe:
      tcpSocket:
        port: 3800
      initialDelaySeconds: 10
      periodSeconds: 5
      failureThreshold: 12 # tune to your cold-start time; 10 + (5 * 12) = 70 s total
      timeoutSeconds: 1
    # Readiness: shallow TCP check. Do NOT gate readiness on /ready unless you
    # also pre-warm the renderer — see "Gating traffic on /ready" below.
    # (httpGet cannot be used in any case: the renderer listener is h2c-only.)
    readinessProbe:
      tcpSocket:
        port: 3800
      periodSeconds: 5
      failureThreshold: 3
      timeoutSeconds: 1
    # Liveness: shallow by default so CPU/GC pauses don't restart the pod.
    livenessProbe:
      tcpSocket:
        port: 3800
      periodSeconds: 10
      failureThreshold: 3
      timeoutSeconds: 1

Gating traffic on /ready​

/ready reports 503 until the answering worker has compiled at least one bundle, and each worker compiles its first bundle when it serves its first render request. If the readiness probe is the only thing standing between a fresh pod and its first render — a standalone renderer behind a Service, or a sidecar (pod readiness requires all containers to be ready, so an unready renderer container blocks traffic to the Rails container too) — then gating readiness on /ready deadlocks the rollout: no traffic → no first render → never ready.

Only use /ready as the readiness gate when something other than probe-gated traffic delivers the first render, for example a deployment pipeline step or Rails initializer that POSTs a warm-up render to each new replica directly (bypassing the Service), or a container postStart hook that does the same. That warm-up must reach every renderer worker that can answer probes; with workersCount > 1, one render per replica is not enough unless you intentionally run a single worker or fan out warm-up renders to each worker. With a warm-up path in place:

readinessProbe:
  exec:
    command:
      - curl
      - -sf
      - --max-time
      - '3'
      - --http2-prior-knowledge
      - http://localhost:3800/ready
  periodSeconds: 5
  failureThreshold: 3
  timeoutSeconds: 5

Without a warm-up path, keep the shallow tcpSocket readiness probe and use /ready for monitoring, dashboards, and post-deploy verification instead.

For stricter hung-process detection, replace the tcpSocket liveness probe with an exec probe against /health (same curl command as the /ready example above, with the path changed). A fully blocked event loop still accepts TCP connections, so only the exec form catches it. Use the stricter form deliberately — it restarts the container on slow event loops, not just dead ones.

Cold-start note: Each worker compiles its first bundle when it serves its first render request, so /ready stays 503 until then — pre-seeding the bundle cache on disk does not by itself flip /ready. This is harmless wherever the check does not gate the traffic that would deliver that first render or replace the container (monitoring, dashboards, post-deploy checks). Wherever it does gate that traffic or container lifetime — a Kubernetes Service routing only to ready replicas, a sidecar whose unready state blocks pod readiness, an ECS container health check, an ALB target group — see "Gating traffic on /ready" above before using it as the gate. A 503 from /ready during the cold-start window is correct behavior, not a failure.

ECS Health Check​

ECS container health checks run inside the container (like a Kubernetes exec probe), so they work against the h2c listener with curl and the default localhost binding. Use /health by default so a normal cold start cannot fail the task before the first render compiles a bundle:

{
  "containerDefinitions": [
    {
      "name": "node-renderer",
      "command": ["node", "renderer/node-renderer.js"],
      "portMappings": [{ "containerPort": 3800 }],
      "environment": [{ "name": "RENDERER_ENABLE_HEALTH_ENDPOINTS", "value": "true" }],
      "healthCheck": {
        "command": [
          "CMD-SHELL",
          "curl -sf --max-time 3 --http2-prior-knowledge http://localhost:3800/health || exit 1"
        ],
        "interval": 10,
        "timeout": 5,
        "retries": 3,
        "startPeriod": 30
      }
    }
  ]
}

Tune startPeriod to match the observed image pull, boot, and first-render latency for your app. Larger bundles or slower registries may need 60 seconds or more.

If you intentionally warm the renderer before startPeriod expires and want ECS to replace a task that cannot serve compiled bundles, change the path to /ready.

ALB note: ALB target-group health checks are HTTP/1.1 and cannot probe the renderer's h2c port. If the renderer sits behind a load balancer, prefer the ECS container health check above for renderer health, or use an NLB with the TCP health-check protocol for a shallow port check.

Docker Compose​

services:
  renderer:
    build: .
    command: node renderer/node-renderer.js
    environment:
      RENDERER_ENABLE_HEALTH_ENDPOINTS: 'true'
    healthcheck:
      test:
        ['CMD', 'curl', '-sf', '--max-time', '3', '--http2-prior-knowledge', 'http://localhost:3800/health']
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 10s

Semantics and Caveats​

  • Per-worker checks. With workersCount > 1, the Node.js cluster module distributes incoming connections across worker processes, and each worker has its own VM pool. A probe therefore checks the one worker that answers it. Workers load bundles independently (each compiles the bundle on its first render request), so a freshly restarted worker can briefly report 503 on /ready while its siblings serve traffic. This is the intended per-process readiness signal for orchestrators probing one container. During a rollout or cold start, raw probe logs may show a short mix of 503 and 200 responses; Kubernetes smooths that with failureThreshold / successThreshold, so a single unloaded-worker 503 should not flap pod readiness.
  • No license check. License validation happens on the Rails side; /ready does not (and cannot) report license state.
  • Liveness checks nothing but the event loop. Do not point /health at dependency monitoring; that is what readiness and your APM are for.
  • Custom routes still work. If you need richer checks (warm-up gates, dependency checks, custom payloads), the configureFastify health-check recipe still applies and can coexist with the built-in endpoints as long as your custom routes use different paths. Remove or rename any existing custom /health or /ready route before enabling enableHealthEndpoints; Fastify raises a duplicate-route startup error when built-in and custom routes share the same path. If an async Fastify plugin registers the duplicate route during app.register() boot, you will see Fastify's raw FST_ERR_DUPLICATED_ROUTE error instead of the enableHealthEndpoints migration hint.

Rails-Side Readiness​

To gate a Rails readiness endpoint on the renderer, keep using the TCP-check recipe in Container Deployment, or upgrade it to an HTTP/2 client call against /ready if your Ruby HTTP client supports h2c.