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Exercise - Configure your application with parameters

Time: Approximately 20 minutes

Introducing parameters

During this exercise we will learn how add parameters and use them for deployment.

docker-app makes the already existing Compose variable substitutions system easy to use. Just replace any part of the compose file with a variable using this form: ${path.to.my-variable} docker-app will seek into the parameters.yml file, which is a simple key/value YAML file, to find the default value.

Here is the corresponding parameters.yml file:

path:
    to:
        my-variable: myvalue
        other-variable: othervalue

NOTE: All the application parameters will be displayed using the inspect command.

Add parameters to an existing docker-compose.yml file

We will re-use the first compose file we wrote, with the hello service.

  • Go back to /workshop and create a single-file hello application using the previous docker-compose.yml
words $ cd /workshop
/workshop $ docker-app init hello --compose-file docker-compose.yml --description "Hello DockerCon application" --single-file

It produces a hello.dockerapp file which should be like the following:

# This section contains your application metadata.
# Version of the application
version: 0.1.0
# Name of the application
name: hello
# A short description of the application
description: Hello DockerCon application
# Namespace to use when pushing to a registry. This is typically your Hub username.
#namespace: myhubusername
# List of application maintainers with name and email for each
#maintainers:
#  - name: John Doe
#    email: [email protected]

---
# This section contains the Compose file that describes your application services.
version: '3.7'
services:
  hello:
    image: hashicorp/http-echo:latest
    command: ["-text", "Hello DockerCon", "-listen",":8080"]
    ports:
     - 8080:8080

---
# This section contains the default values for your application parameters.
{}
  • Edit the hello.dockerapp file and replace the "Hello DockerCon" text with a variable ${hello.text} and add the variable as a parameter in the parameters section
  • Use validate while you edit your application to check everything is ok
  • inspect your application, now the parameters section is displayed
$ docker-app inspect hello
hello 0.1.0

Hello DockerCon application

Service (1) Replicas Ports Image
----------- -------- ----- -----
hello       1        8080  hashicorp/http-echo:latest

Parameter (1) Value
------------- -----
hello.text    Hello DockerCon
  • Replace all the 8080 ports with a ${hello.port} variable, and add it too to the parameters section
  • validate then inspect the application, it displays the hello.port parameter

NOTE: metadata are available as read-only variables under the app prefix. You can use them in your compose file:

  • ${app.name}
  • ${app.version}
  • ${app.description}

NOTE: the parameters section MUST define all the variables with a default value. If any parameter is missing, an error will occur on validate or inspect commands.

  • Comment one of the two parameters, then inspect or validate the application. Don't forget to uncomment the parameter.
$ docker-app validate hello
Error: failed to load Compose file: invalid interpolation format for services.hello.command.[]: "required variable hello.text is missing a value". You may need to escape any $ with another $.

Render an application to a compose file

The render command will produce a compose file, substituting all the variables with the default values.

  • render the hello application
$ docker-app render hello
version: "3.7"
services:
  hello:
    command:
    - -text
    - Hello DockerCon
    - -listen
    - :8080
    image: hashicorp/http-echo:latest
    ports:
    - mode: ingress
      target: 8080
      published: 8080
      protocol: tcp
  • Modify the hello.text parameter in the parameters section and re-render
  • Replace the ${hello.text} variable by ${app.description} and re-render, then revert it
  • Save your rendered compose file using the --output flag
$ docker-app render hello --output hello.yml

Override the parameters

You can also override all the variables using the command line, or create another parameters file, with other values, targeting another environment.

  • Override the port using the --set flag with the render command
$ docker-app render hello --set hello.port=8181
version: "3.7"
services:
  hello:
    command:
    - -text
    - Hello DockerCon
    - -listen
    - :8181
    image: hashicorp/http-echo:latest
    ports:
    - mode: ingress
      target: 8181
      published: 8181
      protocol: tcp
  • Edit a new prod-parameters.yml file and add different values in it for hello.text and hello.port variables
  • render the application using this new settings file and the flag --parameters-files
$ cat prod-parameters.yml
hello:
    text: Hello Workshop
    port: 80 
$ docker-app render hello --parameters-files prod-parameters.yml 
version: "3.7"
services:
  hello:
    command:
    - -text
    - Hello Workshop
    - -listen
    - :80
    image: hashicorp/http-echo:latest
    ports:
    - mode: ingress
      target: 80
      published: 80
      protocol: tcp

NOTE: Parameters can be overridden using a mix of files or command line parameters. The parameters files doesn't need to define all the variables, a subset is enough. The precedence is the following:

  • command line parameters, last one has precedence on the others
  • files
  • default values in the parameters section.
  • Play with render command and mix --set and --parameters-files
$ docker-app render hello -s hello.text="Hello Moby" -f prod-parameters.yml

NOTE: these flags work the same way with inspect, validate or install commands.

  • Try the same mix of parameters with inspect
$ docker-app inspect hello -f prod-parameters.yml
hello 0.1.0

Hello DockerCon application

Service (1) Replicas Ports Image
----------- -------- ----- -----
hello       1        80    hashicorp/http-echo:latest

Parameters (2) Value
-------------- -----
hello.port     80
hello.text     Hello Workshop

Deploy the rendered compose file

The rendered compose file can be directly injected to docker-compose up or docker stack deploy commands to deploy your application.

  • Deploy using docker stack deploy, the - references the standard input
$ docker-app render hello | docker stack deploy my-hello-app -c -
Creating network my-hello-app_default
Creating service my-hello-app_hello
  • Open a browser at the 8080 port, it displays Hello DockerCon
  • Re-deploy changing the text
$ docker-app render hello -s hello.text="Hello Moby" | docker stack deploy my-hello-app -c -
  • Refresh your browser and check the message
  • Deploy another application using the production parameters, it should open it on port 80
$ docker-app render hello -f prod-parameters.yml | docker stack deploy my-prod-app -c -
  • Remove the two stacks using docker stack rm

NOTE: With the parameters, you can now easily scale your application with docker stack.

Bonus Exercise: Add parameterized deploy.replicas to the hello service and use it to scale up and down with the render command and --set hello.replicas=X

Summary

  • With parameters you can use the same compose file and target multiple environments (dev/test/staging/prod/...)
  • Render let's you check what will be the exact compose-file deployed
  • You can pipe the result of a render to your current workflow (docker-compose, docker stack deploy) and still use some of the benefits of docker-app

Here is a generic application package template:

version: 0.1.0
name: base
description: A generic application template which takes an image as a parameter
maintainers:
  - name: garethr
    email: [email protected]

---
version: '3.7'
services:
  app:
    image: ${image}
    ports:
     - ${port}:${port}
    deploy:
      replicas: ${replicas}
      resources:
        limits:
          memory: ${memory}

---
image: 
port: 8080
replicas: 1
memory: 128MB