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Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

02-selfhosting/docker/docker-healthchecks.md

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+---
+title: "Docker Healthchecks"
+domain: selfhosting
+category: docker
+tags: [docker, healthcheck, monitoring, uptime-kuma, compose]
+status: published
+created: 2026-03-23
+updated: 2026-03-23
+---
+
+# Docker Healthchecks
+
+A Docker healthcheck tells the daemon (and any monitoring tool) whether a container is actually working — not just running. Without one, a container shows as `Up` even if the app inside is crashed, deadlocked, or waiting on a dependency.
+
+## Why It Matters
+
+Tools like Uptime Kuma report containers without healthchecks as:
+
+> Container has not reported health and is currently running. As it is running, it is considered UP. Consider adding a health check for better service visibility.
+
+A healthcheck upgrades that to a real `(healthy)` or `(unhealthy)` status, making monitoring meaningful.
+
+## Basic Syntax (docker-compose)
+
+```yaml
+healthcheck:
+  test: ["CMD", "wget", "-q", "--spider", "http://localhost:8080/health"]
+  interval: 30s
+  timeout: 10s
+  retries: 3
+  start_period: 30s
+```
+
+| Field | Description |
+|---|---|
+| `test` | Command to run. Exit 0 = healthy, non-zero = unhealthy. |
+| `interval` | How often to run the check. |
+| `timeout` | How long to wait before marking as failed. |
+| `retries` | Failures before marking `unhealthy`. |
+| `start_period` | Grace period on startup before failures count. |
+
+## Common Patterns
+
+### HTTP service (wget — available in Alpine)
+```yaml
+healthcheck:
+  test: ["CMD", "wget", "-q", "--spider", "http://localhost:2368/"]
+  interval: 30s
+  timeout: 10s
+  retries: 3
+  start_period: 30s
+```
+
+### HTTP service (curl)
+```yaml
+healthcheck:
+  test: ["CMD", "curl", "-f", "http://localhost:8080/health"]
+  interval: 30s
+  timeout: 10s
+  retries: 3
+  start_period: 30s
+```
+
+### MySQL / MariaDB
+```yaml
+healthcheck:
+  test: ["CMD", "mysqladmin", "ping", "-h", "localhost", "-u", "root", "-psecret"]
+  interval: 10s
+  timeout: 5s
+  retries: 3
+  start_period: 20s
+```
+
+### PostgreSQL
+```yaml
+healthcheck:
+  test: ["CMD-SHELL", "pg_isready -U postgres"]
+  interval: 10s
+  timeout: 5s
+  retries: 5
+```
+
+### Redis
+```yaml
+healthcheck:
+  test: ["CMD", "redis-cli", "ping"]
+  interval: 10s
+  timeout: 5s
+  retries: 3
+```
+
+### TCP port check (no curl/wget available)
+```yaml
+healthcheck:
+  test: ["CMD-SHELL", "nc -z localhost 8080 || exit 1"]
+  interval: 30s
+  timeout: 5s
+  retries: 3
+```
+
+## Using Healthchecks with `depends_on`
+
+Healthchecks enable proper startup ordering. Instead of a fixed sleep, a dependent container waits until its dependency is actually ready:
+
+```yaml
+services:
+  app:
+    depends_on:
+      db:
+        condition: service_healthy
+
+  db:
+    image: mysql:8.0
+    healthcheck:
+      test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
+      interval: 10s
+      timeout: 5s
+      retries: 3
+      start_period: 20s
+```
+
+This prevents the classic race condition where the app starts before the database is ready to accept connections.
+
+## Checking Health Status
+
+```bash
+# See health status in container list
+docker ps
+
+# Get detailed health info including last check output
+docker inspect --format='{{json .State.Health}}' <container> | jq
+```
+
+## Ghost Example
+
+Ghost (Alpine-based) uses `wget` rather than `curl`:
+
+```yaml
+healthcheck:
+  test: ["CMD", "wget", "-q", "--spider", "http://localhost:2368/ghost/api/v4/admin/site/"]
+  interval: 30s
+  timeout: 10s
+  retries: 3
+  start_period: 30s
+```
+
+## Gotchas & Notes
+
+- **Alpine images** don't have `curl` by default — use `wget` or install curl in the image.
+- **`start_period`** is critical for slow-starting apps (databases, JVM services). Failures during this window don't count toward `retries`.
+- **`CMD` vs `CMD-SHELL`** — use `CMD` for direct exec (no shell needed), `CMD-SHELL` when you need pipes, `&&`, or shell builtins.
+- **Uptime Kuma** will pick up Docker healthcheck status automatically when monitoring via the Docker socket — no extra config needed.
+
+## See Also
+
+- [[debugging-broken-docker-containers]]
+- [[netdata-docker-health-alarm-tuning]]