200-901 Application Deployment and Security Practice Question
In a Docker Compose file, a service 'web' depends on 'db'. The 'db' service uses a volume to persist data. Which compose key ensures that the database starts before the web service?
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
depends_on creates a startup order: Docker Compose starts 'db' before 'web'. It does not wait for 'db' to be ready; that requires healthchecks.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
volumes
Why it's wrong here
The volumes key declares persistent storage mounts so database data survives container recreation; it does not influence start order. It is correct when persisting state across restarts, but the stem asks which key sequences the db container ahead of web, which depends_on provides.
- ✓
depends_on
Why this is correct
The depends_on key expresses a startup dependency, so Docker Compose starts the db service before the web service. It controls start order only; the volume persists data independently, and healthchecks would add readiness gating beyond this requirement.
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links
Why it's wrong here
The links key creates legacy network aliases between containers for name resolution, and is deprecated in Compose v2; it does not enforce startup ordering. It was used for container discovery, whereas depends_on expresses the dependency that starts db before web.
- ✗
networks
Why it's wrong here
The networks key attaches services to custom Docker networks for inter-container connectivity and DNS resolution; it has no bearing on startup ordering. It is the right key when isolating or segmenting service traffic, whereas depends_on governs the sequence in which Compose starts containers.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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