# depends_on With Conditions — Docker Compose

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

> Wait until a dependency is healthy.

## started, healthy, completed

Plain `depends_on: [db]` only waits until the db **container has started**, not until PostgreSQL accepts connections. The long form adds a **condition**: `service_started`, `service_healthy` (waits for the dependency's health check to pass) or `service_completed_successfully` (waits for a one-off job such as a migration to exit with 0). In the demo, web waits for a healthy api, which waits for a healthy db. Applications should still retry connections, because dependencies can restart later.

## Start in the right order, recover from failure

Health checks and depends_on conditions make dependent services wait; restart policies recover crashes.

![Three ideas: depends_on conditions, health checks, restart policies.](assets/figures/docker-compose/section-4-map.svg) — Figure 4.1 — Dependencies, health and restarts.

## The resolved dependency chain, 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. web depends on api and api depends on db, both with condition service_healthy; Compose fills in required: true by default.

```bash
docker compose config --format json | jq -c ".services.web.depends_on, .services.api.depends_on"
```

Output:

```
{"api":{"condition":"service_healthy","required":true}}
{"db":{"condition":"service_healthy","required":true}}
```

## Run migrations as a completed dependency

Add a migrate service and make the API depend on it with service_completed_successfully.

**Quiz:** What does plain depends_on: [db] wait for?

- [ ] Nothing at all
- [ ] For the database to accept connections
- [ ] For a health check to pass
- [x] Only for the db container to start, not for the database to be ready

*Answer:* Only for the db container to start, not for the database to be ready. Use condition: service_healthy for readiness.