Re-use YAML sections - yaml

Question

I'm putting together a docker-compose file. I'd like to re-use sections that are repetitive. For example, each container re-uses the same deploy config. I tried making a template for it:
...
redis:
image: redis
ports:
- 6379:6379
deploy: deploy_template
volumes:
- /srv/redis/data:/data
deploy_template:
restart_policy:
condition: on-failure
delay: 5s
max_attempts: 10
window: 120s
However this didn't work. Is there any way to do this?

Answer 1

You could use the YAML anchor and alias facility for that, effectively:
version: '2'
dummy: &deploy_template
restart_policy:
condition: on-failure
delay: 5s
max_attempts: 10
window: 120s
services:
redis:
image: redis
ports:
- 6379:6379
deploy: *deploy_template
volumes:
- /srv/redis/data:/data
will be parsed as if you had specified:
version: '2'
dummy:
restart_policy:
condition: on-failure
delay: 5s
max_attempts: 10
window: 120s
services:
redis:
image: redis
ports:
- 6379:6379
deploy:
restart_policy:
condition: on-failure
delay: 5s
max_attempts: 10
window: 120s
volumes:
- /srv/redis/data:/data
You can have multipe *deploy_template values for a single &deploy_template anchor.
The problem however is that the dummy key, and its value, will trip docker-compose, and at least in version 2 there was no place to put this information.
I therefore preprocess my docker-compose file using ruamel.dcw (I am the author of that package), which allows for a user-data top-level key that will not appear in the output and where you can put such anchor information. Starting with:
version: '2'
user-data:
author: dthree <calvin#hobbes.org>
description: redis container
env-defaults:
NAME: redis # default values if not specified in the environment
PORT: 6379
dummy:
- &deploy_template
restart_policy:
condition: on-failure
delay: 5s
max_attempts: 10
window: 120s
- &some_other_template:
x: null
services:
redis:
image: ${NAME}
ports:
- "${PORT}:${PORT}"
deploy: *deploy_template
volumes:
- /srv/${NAME}/data:/data
this will expand to:
version: '2'
services:
redis:
image: ${NAME}
ports:
- ${PORT}:${PORT}
deploy:
restart_policy:
condition: on-failure
delay: 5s
max_attempts: 10
window: 120s
volumes:
- /srv/${NAME}/data:/data
before being handed to docker-compose itself (using the -f option). Any variables in the env-defaults "section" that are not already set in the environment in which you execute the preprocessor, will be given their default value, making it easy to override them.
As an aside: you should be careful with:
- 6379:6379
because if the port number gets below 60, the old YAML parser that docker-compose uses, interprets that scalar as a sexagesimal. I tend to always quote such values, especially when using env. variables.