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.