Depends_on for service ordering in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of depends_on in a Docker Compose file?

depends_on defines the order in which services start. It ensures that a service starts only after the services it depends on have started.

intermediate

Does depends_on guarantee that a service is ready to use before starting the dependent service?

No. depends_on only controls start order, not readiness. Services might start but not be fully ready.

beginner

How do you specify that web service depends on db service in Docker Compose?

services: web: depends_on: - db

intermediate

What is a limitation of depends_on in Docker Compose?

By default, depends_on does not wait for services to be "ready". It only waits for containers to start. (In v2.1+, use condition: service_healthy to wait for healthchecks.)

intermediate

What can you use to ensure a service is ready before starting another, since depends_on does not check readiness?

You can use healthchecks and scripts that wait for a service to be ready before starting the dependent service.

What does depends_on control in Docker Compose?

A The environment variables

B The network ports used

C The container image versions

D The order services start

depends_on controls the order in which services start.

Does depends_on wait for a service to be fully ready before starting another service?

A No, it only waits for the container to start

B Yes, always

C Only in Docker Compose version 2

D Only if healthchecks are defined

depends_on only waits for containers to start, not for readiness.

How do you declare that service app depends on service db?

A app: depends_on: - db

B app: depends_on: db

C app: depends: db

D app: depends: - db

The correct syntax is depends_on: followed by a list with - db.

Which Docker Compose version introduced support for depends_on conditions (like service_healthy) regarding service readiness?

A Version 1

B Version 2

C Version 4

D Version 3

Version 2 (specifically 2.1) introduced condition: service_healthy to wait for readiness via healthchecks.

What is a common method to ensure a service is ready before starting another service?

A Use <code>depends_on</code> only

B Restart the Docker daemon

C Use healthchecks and wait scripts

D Use environment variables

Healthchecks and wait scripts help ensure readiness before starting dependent services.

Explain how depends_on works in Docker Compose and its limitations.

Think about what <code>depends_on</code> guarantees and what it does not.

Describe how you would ensure a database service is ready before starting a web service in Docker Compose.

Consider combining Docker Compose features and scripts.

Practice

1. What is the main purpose of depends_on in a Docker Compose file?

easy

A. To specify the image version for services

B. To set environment variables for services

C. To define network settings for services

D. To control the order in which services start

Solution

1. Step 1: Understand the role of depends_on

   depends_on is used to specify which services must start before others in Docker Compose.

2. Step 2: Differentiate from other settings

   It does not set environment variables, network settings, or image versions; it only controls start order.

3. Final Answer:

   To control the order in which services start -> Option D

4. Quick Check:

   Service start order = depends_on [OK]

Hint: Depends_on controls start order, not readiness or config [OK]

Common Mistakes:

• Thinking depends_on waits for full service readiness
• Confusing depends_on with environment or network settings
• Assuming depends_on sets image versions

2. Which of the following is the correct syntax to make service web depend on service db in a Docker Compose file?

easy

A. web:
depends:
db

B. web:
depends:
- db

C. web:
depends_on:
- db

D. web:
depends_on: db

Solution

1. Step 1: Recall correct depends_on syntax

   The correct syntax uses depends_on as a key with a list of service names under it.

2. Step 2: Check each option

   web: depends_on: - db correctly uses depends_on: followed by a list with - db. Others have wrong keys or missing list format.

3. Final Answer:

   web: depends_on: - db -> Option C

4. Quick Check:

   depends_on needs a list under it [OK]

Hint: depends_on requires a list of services under it [OK]

Common Mistakes:

• Using 'depends' instead of 'depends_on'
• Not using a list for dependent services
• Writing depends_on as a single string

3. Given this Docker Compose snippet:

services:
  db:
    image: postgres
  web:
    image: nginx
    depends_on:
      - db

What happens when you run docker-compose up?

medium

A. The web service starts only after the db service has fully initialized

B. The db service starts before the web service, but web may start before db is ready

C. Both services start simultaneously without any order

D. The web service starts first, then the db service

Solution

1. Step 1: Understand depends_on behavior

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

2. Step 2: Analyze the startup sequence

   db container starts first, then web starts immediately after, even if db is not fully ready.

3. Final Answer:

   The db service starts before the web service, but web may start before db is ready -> Option B

4. Quick Check:

   depends_on controls start order, not readiness [OK]

Hint: depends_on orders start but doesn't wait for readiness [OK]

Common Mistakes:

• Assuming depends_on waits for full service readiness
• Thinking services start simultaneously
• Believing web starts before db

4. You wrote this Docker Compose snippet:

services:
  web:
    image: nginx
    depends_on: db
  db:
    image: postgres

But it fails to start with an error. What is the problem?

medium

A. depends_on should be a list, not a single string

B. The db service must be defined before web

C. Image names are incorrect

D. depends_on cannot be used with nginx

Solution

1. Step 1: Check depends_on syntax

   depends_on expects a list of services, but here it is given as a single string.

2. Step 2: Identify correct format

   It should be:

   depends_on:
     - db

   not depends_on: db.

3. Final Answer:

   depends_on should be a list, not a single string -> Option A

4. Quick Check:

   depends_on requires a list [OK]

Hint: depends_on must list services with dash (-) [OK]

Common Mistakes:

• Using a string instead of a list for depends_on
• Assuming service order depends on definition order
• Blaming image names for depends_on errors

5. You have three services: db, cache, and web. The web service needs both db and cache to start first. Also, cache must start after db. How should you write the depends_on section for web and cache to enforce this order?

hard

A. web:
depends_on:
- db
- cache
cache:
depends_on:
- db

B. web:
depends_on:
- cache
cache:
depends_on:
- web
- db

C. web:
depends_on:
- db
cache:
depends_on:
- web

D. web:
depends_on:
- db
cache:
depends_on:
- web
- db

Solution

1. Step 1: Define cache dependency on db

   cache must start after db, so cache depends_on db.

2. Step 2: Define web dependency on both db and cache

   web needs both db and cache started first, so web depends_on both db and cache.

3. Final Answer:

   web: depends_on: - db - cache cache: depends_on: - db -> Option A

4. Quick Check:

   Order: db -> cache -> web [OK]

Hint: Chain depends_on to enforce multi-step start order [OK]

Common Mistakes:

• Making cache depend on web incorrectly
• Not listing both dependencies for web
• Assuming depends_on waits for readiness