200-901 Application Deployment and Security Practice Question
In a Docker Compose file, you want to ensure that the 'web' service starts only after the 'db' service is healthy. Which key should you use under the 'web' service?
⚠ Common exam trap
Watch out — candidates often assume `depends_on` alone (without `condition: service_healthy`) guarantees the dependent service is ready, but it only waits for the container to start, not for it to be healthy.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
depends_on
In Docker Compose, the `depends_on` key with the `condition: service_healthy` option ensures that the `web` service starts only after the `db` service has passed its health check. This is defined in the `db` service using a `healthcheck` directive, and Compose waits for the healthy state before starting dependent services.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
networks
Why it's wrong here
The networks key only attaches services to Docker networks for connectivity; it imposes no startup ordering or health gating. It is tempting because service dependencies feel network-related, and it would be correct when services must share an isolated network for name resolution.
- ✗
links
Why it's wrong here
The links key creates network aliases and dependency ordering, but it does not wait for the db container to report healthy. It is tempting because it expresses a dependency, and it would be correct for simple startup ordering without health checks.
- ✓
depends_on
Why this is correct
The depends_on key establishes startup ordering, but adding the condition: service_healthy form makes Compose wait until the db service's healthcheck reports healthy before starting web. This satisfies the stem's requirement that web starts only after db is genuinely healthy, not merely launched.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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