You can think of Compose as documentation which replaces
docker run - all the options you would put in the run commands are specified in the Compose file which becomes living documentation. That includes setting up the container environment with variables and voume mounts.
The Compose spec also includes higher-level modelling for configuration files, using
secret objects. That helps to make it clear that you're modelling configuration instead of storage.
Compose models can be split across multiple files, which makes it easy to run the same app in different configurations on a single machine.
It's the same Docker Compose CLI:
The help text isn't super clear, but you can use the
-fflag multiple times to build a model from several YAML files.
Application configuration is applied to containers with environment variables and filesystem mounts. Compose supports the same options as the standard
docker run command:
rng/v1.yml - defines the random number app, using environment variables for config settings
rng/v2.yml - uses an environment file for logging configuration, so it can be shared between the components
📋 Run the v1 spec and test the app; then update to the v2 spec - what do you think will happen to the v1 containers?
# deploy v1 docker-compose -f labs/compose-model/rng/v1.yml up -d # try the app at http://localhost:8090 # deploy v2 docker-compose -f labs/compose-model/rng/v2.yml up -d
The upgrade doesn't change anything - environment files are expanded by the CLI and the values are used to set individual environment variables in the containers.
Print the environment variable details from the new web container to see that the env file contents have been expanded:
docker container inspect --format='' rng_rng-web_1
Environment variables are a common way to set application config, but they're not as flexible as config files.
Compose lets you model volume mounts, so you can load local files into the container filesystem. That uses the same approach as
docker run - specifying a source and target path. Compose also has an abstraction called
Platforms like Kubernetes use a similar mechanism for secrets, but the source of the data is an object stored in Kubernetes, not a local file on the server.
📋 Run the v3 spec and look at the volume mount configuration for the web container. How are volumes and secrets treated differently?
# deploy v3 docker-compose -f labs/compose-model/rng/v3.yml up -d docker inspect rng_rng-web_1
The volume and the secret are set as bind mounts. The volume spec defaults to being writeable (
"RW": true), but the secret is read-only.
You'll need to understand the different options for configuring app settings, because applications all tend to have different ideas about configuration.
All the containers and networks for the random number app have been created with the
rng prefix. Compose uses a project name to identify all the objects it creates, and it defaults to the name of the folder the YAML files are in.
You can specify a custom project name, and deploy the same model definition again to create a separate copy of your app.
📋 Deploy the v3 spec with the custom project name
rng-dev - does it start correctly?
# use the -p flag to set a project name: docker-compose -p rng-dev -f labs/compose-model/rng/v3.yml up -d # this will fail because ports cannot be used more than once
Compose creates containers with the new prefix
rng-dev, but they fail to start - they're trying to use ports which are already being used by the
This is where override files come in - you can split your model across multiple Compose files, using different configuration settings for different environments:
rng/core.yml - specifies the core application model, with settings which are the same for all environments. This is not a complete model, so you can't deploy this YAML on its own
rng/dev.yml - sets the ports and configuration file sources for the dev environment
rng/test.yml - sets the ports and configuration file sources for the test environment
📋 Run the app in the dev configuration by merging the Compose files and using the project name
# use -p to set a custom project name, and join files starting # with the core spec and then adding the dev override: docker-compose -p rng-dev -f labs/compose-model/rng/core.yml -f labs/compose-model/rng/dev.yml up -d
You merge Compose files with multiple
-fflags - the files to the right add or override settings from files to the left.
Try the dev app at http://localhost:8190, you'll see random numbers in the range 0-50.
📋 Check the API logs for the dev environment and confirm they're printing at a detailed level
# the project name isn't enough for Compose to find the container: docker-compose -p rng-dev logs rng-api # you need to include the project name and all the override # files in any Compose commands: docker-compose -p rng-dev -f labs/compose-model/rng/core.yml -f labs/compose-model/rng/dev.yml logs rng-api
One drawback of overrides and project names is that you need to supply them in the same order for every Compose command.
You now have two versions of the app running. Containers use very little compute power unless they're under load, so Compose is great for running multiple non-production environments on a single machine.
📋 Deploy the app in the test configuration. Try it out - how is it different from dev?
# make sure to use a new project name and the correct files: docker-compose -p rng-test -f labs/compose-model/rng/core.yml -f labs/compose-model/rng/test.yml up -d # try the app at http://localhost:8290 # print the API logs: docker-compose -p rng-test -f labs/compose-model/rng/core.yml -f labs/compose-model/rng/test.yml logs rng-api
Random numbers are in the range 0-5000 and API logs are reduced to
infolevel in the test configuration.
Docker Compose makes it super easy to join a new project and get up and running - you should be able to clone the repo and run
docker-compose up to start the app.
We can't do that with the latest random number model, because of the overrides and custom project names.
But you can override the defaults with another file in your folder. Write new files to support this workflow:
docker-compose up -dto start the app in the dev configuration
docker logs rng-lab_rng-api_1
Cleanup by removing all containers:
docker rm -f $(docker ps -aq)