Depends_on for service ordering in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does depends_on control in Docker Compose?

In a docker-compose.yml file, what is the main purpose of the depends_on key under a service?

A It ensures the dependent service is fully ready before starting the current service.

B It controls the order in which services start but does not wait for them to be ready.

C It automatically restarts the dependent service if it crashes.

D It links the network ports of the dependent service to the current service.

💡 Hint

Think about whether depends_on waits for the service to be fully ready or just starts it first.

The depends_on key only controls the start order of services. It does not wait for the dependent service to be fully ready or healthy before starting the next one.

💻 Command Output

intermediate

Output of docker-compose with depends_on

Given this docker-compose.yml snippet:

version: '3.8'
services:
  db:
    image: postgres
  web:
    image: nginx
    depends_on:
      - db

What will happen when you run docker-compose up?

A Both services start simultaneously without any order.

B The <code>web</code> service waits until <code>db</code> is fully ready before starting.

C The <code>db</code> service starts first, then <code>web</code> starts immediately after, without waiting for <code>db</code> readiness.

D The <code>web</code> service will fail to start because <code>depends_on</code> is invalid syntax.

💡 Hint

Remember what depends_on controls in Docker Compose.

depends_on ensures db starts before web, but web does not wait for db to be ready.

❓ Configuration

advanced

Correct depends_on syntax for multiple dependencies

Which depends_on syntax correctly specifies that a service depends on db and cache services?

A depends_on: [db, cache]

B

depends_on:
  db: true
  cache: true

C

depends_on:
  db
  cache

D

depends_on:
  - db
  - cache

💡 Hint

Check the correct YAML list syntax for depends_on.

The correct syntax uses a YAML list with dashes for each service dependency.

❓ Troubleshoot

advanced

Why does a service start before its dependency is ready?

You have a web service that depends_on a db service. However, web tries to connect to db before it is ready, causing errors. Why does this happen?

A <code>depends_on</code> only controls start order, not readiness or health checks.

B <code>depends_on</code> is deprecated and ignored by Docker Compose.

C <code>db</code> service is not exposed on the network, so <code>web</code> cannot connect.

D <code>web</code> service must be restarted manually after <code>db</code> is ready.

💡 Hint

Think about what depends_on does and does not guarantee.

depends_on only ensures the db service starts before web, but it does not wait for db to be ready or healthy.

✅ Best Practice

expert

How to ensure service readiness with depends_on in Docker Compose v3.8?

Docker Compose's depends_on does not wait for service readiness. What is the best way to ensure a service starts only after its dependency is fully ready?

A Use healthchecks in the dependency service and configure <code>depends_on</code> with condition: service_healthy.

B Add a <code>restart: always</code> policy to the dependent service.

C Manually add <code>sleep</code> commands in the dependent service's startup script.

D Use <code>links</code> instead of <code>depends_on</code> to enforce readiness.

💡 Hint

Check how Docker Compose can use healthchecks with depends_on conditions.

In Docker Compose version 3.4 and above, you can use healthchecks and specify condition: service_healthy in depends_on to wait for readiness.