# Health Checks — Docker Compose

Source: https://www.skillbyai.com/en/docker-compose/h-check

> Tell Compose when a service is ready.

## test, interval, timeout, retries

A `healthcheck` runs a command inside the container at an `interval`; if it fails `retries` times in a row the container is marked **unhealthy**. `start_period` gives slow services time before failures count. Use a cheap command that proves readiness: `pg_isready` for PostgreSQL, an HTTP request to a health endpoint for APIs (the tool you call, such as wget or curl, must exist in the image). `docker compose ps` shows health status.

## The database health check, run

I ran this with Docker Compose v2.38.1 and jq in the demo "shop" project. docker compose config parses, interpolates, merges and validates the files without starting containers; the Docker daemon was not running, so nothing was started. The db health check uses pg_isready every 5 seconds with 10 retries; CMD-SHELL runs it through a shell.

```bash
docker compose config --format json | jq -c ".services.db.healthcheck"
```

Output:

```
{"test":["CMD-SHELL","pg_isready -U shop -d shop"],"interval":"5s","retries":10}
```

## Check the tool exists in the image

A health check calling curl in an image without curl fails forever; use what the image provides or add it.

**Quiz:** What does a health check enable in depends_on?

- [ ] Automatic scaling
- [x] The service_healthy condition, so dependents wait for readiness
- [ ] Image building
- [ ] Port publishing

*Answer:* The service_healthy condition, so dependents wait for readiness. Health checks define readiness.