@absolutejs/health

Liveness + readiness probes for the AbsoluteJS substrate. One Elysia plugin, two endpoints, a standard JSON envelope that load balancers and Kubernetes-style orchestrators understand out of the box.

import { Elysia } from 'elysia';
import {
  createHealthChecker,
  healthPlugin,
  metricsCheck,
  probeCheck,
  httpCheck,
} from '@absolutejs/health';

const checker = createHealthChecker({
  checks: [
    // Synthesis from a substrate package's metrics() snapshot.
    metricsCheck('queue', () => worker.metrics(), (m) => ({
      status: m.failed > 100 ? 'warn' : 'pass',
      observed: { runs: m.runs, failed: m.failed },
    })),

    // Wrap an arbitrary probe.
    probeCheck('postgres', () => pg.query('SELECT 1')),

    // Downstream HTTP dependency.
    httpCheck('otlp-collector', 'http://collector:4318/healthz', {
      kind: 'readiness',  // run only under /readyz
    }),
  ],
});

const app = new Elysia().use(await healthPlugin({ checker }));
// GET /healthz → liveness:  200 / 503 with { status, checks, at }
// GET /readyz  → readiness: same shape, filtered to readiness + both

Two endpoints, two semantics

The distinction matters: a draining instance returns readyz: fail

• healthz: pass. The LB stops sending new traffic while in-flight requests finish.

Body shape

{
  "status": "pass",
  "at": 1717161600000,
  "checks": {
    "queue": {
      "status": "pass",
      "latencyMs": 1,
      "observed": { "runs": 1057, "failed": 0 }
    },
    "postgres": { "status": "pass", "latencyMs": 12 }
  }
}

Compatible with the IETF health-check JSON draft and the Kubernetes livez / readyz conventions. content-type: application/health+json.

Status codes

• pass → 200 OK
• warn → 200 OK (don't reroute traffic; surface in your dashboard)
• fail → 503 Service Unavailable

LBs route on status code; humans + dashboards read the body.

Aggregation

Status is the WORST of any check: fail > warn > pass. A single failing dependency fails the whole envelope — same as Kubernetes /healthz rollup behavior.

Check factories

• probeCheck(name, () => promise) — wrap any () => Promise<unknown>. Resolves → pass; throws → fail with the error message.
• metricsCheck(name, () => metrics(), evaluator) — read a substrate package's metrics() and decide pass/warn/fail. Snapshot values can land in observed for dashboards.
• httpCheck(name, url, options?) — GET url and grade by HTTP status. 2xx → pass, anything else → fail.
• Roll your own — any { name, check: () => CheckResult | Promise<CheckResult> } works.

Kind filtering

{ name: 'shutting-down', kind: 'readiness', check: () => ({
  status: draining ? 'fail' : 'pass'
})}

A check's kind ('liveness' / 'readiness' / 'both', default 'both') controls which endpoint runs it. Run heavy downstream checks under readiness only; keep liveness limited to "the JS process is responsive."

License

BSL-1.1 with named carveout against hosted uptime / synthetic- monitoring services. Change date: 2030-05-31 (Apache 2.0).