Depends_on for service ordering in Docker - Step-by-Step Execution

Process Flow - Depends_on for service ordering

Start docker-compose

↓

Check service dependencies

↓

Start services with no dependencies

↓

Start dependent services in order

↓

All services running

Docker Compose uses depends_on to start services in the order defined by dependencies.

Execution Sample

Docker

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

This docker-compose file starts 'db' service first, then 'web' service after 'db' is started.

Process Table

Step	Action	Service Started	Dependency Check	Result
1	Start docker-compose	None	N/A	Waiting to start services
2	Check 'db' dependencies	None	No dependencies	Start 'db' service
3	Start 'db' service	db	N/A	'db' service is running
4	Check 'web' dependencies	db	'db' must be running	'db' is running, start 'web'
5	Start 'web' service	db, web	N/A	'web' service is running
6	All services started	db, web	All dependencies met	Compose up complete

💡 'web' started only after 'db' was running, respecting depends_on order

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
Services Running	None	None	db	db	db, web	db, web

Key Moments - 2 Insights

Why doesn't 'web' start before 'db'?

'web' has depends_on: db, so docker-compose waits until 'db' is started (see execution_table step 4) before starting 'web'.

Does depends_on wait for the service to be fully ready?

No, depends_on only waits for the container to start, not for the service inside to be ready. This is shown in step 4 where 'web' starts after 'db' container is running.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the 'db' service start running?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Check the 'Service Started' column in execution_table row for Step 3.

At which step does docker-compose confirm that 'web' can start because 'db' is running?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Dependency Check' column in execution_table for Step 4.

If 'db' failed to start, what would happen to 'web' service start?

A 'web' would start anyway

B 'web' would start before 'db'

C 'web' would wait indefinitely

D 'web' would start after a timeout

💡 Hint

Depends_on requires the dependency service to be running before starting dependent service (see key_moments answer 1).

Concept Snapshot

depends_on in docker-compose:
- Defines service start order
- Starts dependencies first
- Does NOT wait for full readiness
- Ensures containers start in sequence
- Useful for simple startup ordering

Full Transcript

Docker Compose uses the depends_on key to control the order in which services start. When you run docker-compose up, it first checks which services have no dependencies and starts them. Then it starts services that depend on those, ensuring the order is correct. For example, if a web service depends on a database service, docker-compose starts the database container first, then the web container. However, depends_on only waits for the container to start, not for the service inside to be fully ready. This means the web service might start before the database is fully ready to accept connections. The execution table shows each step: starting docker-compose, starting the db service, then starting the web service after confirming db is running. This helps beginners understand how service ordering works visually and step-by-step.

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