Dependencies and troubleshooting in Docker Compose

Intermediate Docker

Mike Metzger

Data Engineering Consultant

What are dependencies?

Dependencies define the order of resource startup
Resources (containers) may require other resources
Example web application
- Database container postgresql must start first

Container Dependency Example - postgres

What are dependencies?

Dependencies define the order of resources
Resources (containers) may require other resources
Example web application
- Database container postgresql must start first
- Then the python_app

Container Dependency Example - python_app

What are dependencies?

Dependencies define the order of resources
Resources (containers) may require other resources
Example web application
- Database container postgresql must start first
- Then the python_app
- Finally, the nginx web server

Container Dependency Example - Full

depends_on

Dependencies defined using the depends_on attribute
Can chain dependencies as per example
Or, can have multiple dependencies per resource if required
Order of the compose.yaml file does not matter

services:
  postgresql:
    image: postgresql:latest


  python_app:
    image: custom_app
    depends_on:
      - postgresql


  nginx:
    image: nginx/latest
    depends_on:
      - python_app

Shutting down applications

Shutting down an application occurs in reverse order
Stops nginx resource

Container Dependency Example - Full

Shutting down applications

Shutting down an application occurs in reverse order
Stops nginx resource
Then stops the python_app resource

Container Dependency Example - python_app

Shutting down applications

Shutting down an application occurs in reverse order
Stops nginx resource
Then stops the python_app resource
And finally the postgresql resource

Container Dependency Example - postgres

Other options

Docker Compose provides other options for dependencies
condition: defines how to decide when resource is ready.
- service_started - Resource has started normally
  - Default behavior
- service_completed_successfully - Resource ran to completion, such as a initial configuration / etc
- service_healthy - Resource meets a criteria defined by healthcheck

services:
  nginx:
    image: nginx/latest
    depends_on:
      python_app:
        condition: service_started


python_app:
    image: custom_app
    depends_on:
      postgresql:
        condition: service_healthy

Docker Compose troubleshooting tools

Docker Compose has additional troubleshooting tools

docker compose logs - Gathers output from all resources in application

redis-1  | * oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo
redis-1  | * Running mode=standalone, port=6379.
redis-1  | * Server initialized
redis-1  | * Ready to accept connections tcp
web-1    | * Serving Flask app 'app.py'
web-1    |  * Running on all addresses (0.0.0.0)
web-1    |  * Running on http://172.20.0.2:5000
web-1    | Press CTRL+C to quit

docker compose logs <resourcename>

docker compose top

docker compose top shows status of resources within an application

composetest-redis-1
UID   PID    PPID   C    STIME   TTY   TIME       CMD
999   2767   2726   0    01:16   ?     00:03:27   redis-server *:6379

composetest-web-1
UID    PID    PPID   C    STIME   TTY   TIME       CMD
root   2768   2740   0    01:16   ?     00:00:23   /usr/local/bin/python /usr/local/bin/flask run

Let's practice!

Intermediate Docker

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