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Exercise - Discover Docker Application Package

Time: Approximately 20 minutes

During this exercise we will introduce Docker Application Package and its dedicated tool docker-app. Application Package is a construction above Compose file to improve the application lifecycle and workflow, from development to test to production.

An application package is a set of 3 documents, plus custom files we name attachments:

  • a metadata.yml file describing the application metadata (name, version, description, ...)
  • a docker-compose.yml file describing the application structure
  • a parameters.yml file with key/value parameters (we will see this part in the next exercise)

First things first, let's initialize our first application package, using the previous words exercise.

Initialize the Application Package

$ docker-app init --help

Usage:  docker-app init <app-name> [-c <compose-file>] [-d <description>] [-m name:email ...] [flags]

Start building a Docker application. Will automatically detect a docker-compose.yml file in the current directory.

Options:
  -c, --compose-file string      Initial Compose file (optional)
  -d, --description string       Initial description (optional)
  -m, --maintainer stringArray   Maintainer (name:email) (optional)
  -s, --single-file              Create a single-file application
  • Use docker-app init command to create the application package from the compose file
workshop $ cd words
words $ docker-app init myapp -d "My word application" -m dapworkshop:[email protected]
words $ tree .
.
|-- docker-compose.yml
`-- myapp.dockerapp
    |-- docker-compose.yml
    |-- metadata.yml
    `-- parameters.yml

1 directory, 4 files

This command creates a new directory with the name of your app + .dockerapp suffix. It copies the initial compose file and generates metadata.yml with the given information, plus an parameters.yml file.

An application package can also be a single file, using the multi-yaml document feature. It's very handy for sharing an app!

  • Merge the application package to the one-file version
words $ docker-app merge myapp -o myapp-merged.dockerapp
words $ cat myapp-merged.dockerapp
# Version of the application
version: 0.1.0
# Name of the application
name: myapp
# A short description of the application
description: My word 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: dapworkshop
    email: [email protected]

---
version: "3.7"
services:
    web:
     image: dockerdemos/lab-web
     ports:
      - "33000:80"

    words:
     image: dockerdemos/lab-words
     deploy:
       replicas: 5
       endpoint_mode: dnsrr

    db:
     image: dockerdemos/lab-db

---
{}

NOTE: If you don't specify the -o, the merge will transform your application to a one-file version in place, removing the 3 files. The same behavior applies to the split command.

  • Split the one-file application package to a directory
words $ docker-app split myapp-merged.dockerapp -o myapp-split
words $ tree.
.
|-- docker-compose.yml
|-- myapp-merged.dockerapp
|-- myapp-split
|   |-- docker-compose.yml
|   |-- metadata.yml
|   `-- parameters.yml
`-- myapp.dockerapp
    |-- docker-compose.yml
    |-- metadata.yml
    `-- parameters.yml

2 directories, 8 files
  • Let's clean our workspace and remove the split and merged versions, keep only myapp.dockerapp directory
words $ rm -rf myapp-split myapp-merged.dockerapp

Inspect your application

docker-app comes with another very handy command: inspect. With it you can display all important informations without having to read the YAML yourself.

  • Inspect your application package
words $ docker-app inspect myapp
myapp 0.1.0

Maintained by: dapworkshop <[email protected]>

My word application

Services (3) Replicas Ports Image
------------ -------- ----- -----
db           1              dockerdemos/lab-db
web          1        33000 dockerdemos/lab-web
words        3              dockerdemos/lab-words

inspect command will

  • pretty print the metadata
  • list the services
  • but also volumes
  • networks
  • secrets
  • attachments
  • parameters

NOTE: All docker-app commands have multiple ways to take an application as an argument:

# You can omit it if current directory is a `*.dockerapp`
myapp.dockerapp $ docker-app inspect
# or if there is only one `*.dockerapp` (file or directory) in the current directory
words $ docker-app inspect
# you can reference it by its name only `path/to/myapp`
workshop $ docker-app inspect words/myapp
# or by its full path `path/to/myapp.dockerapp`
workshop $ docker-app inspect words/myapp.dockerapp
  • Try to inspect the application from different working directories.

Validate your application

The validate command checks everything is ok with your application:

  • Compose file is valid

  • Required Metadata are filled (name, version) and have the good type

  • Default parameters

  • Validate your application

words $ docker-app validate myapp
words $ echo $?
0

Let's modify a bit this app:

  • Comment the version field in myapp.dockerapp/metadata.yml using #
  • Add some garbage characters to the name: name: myapp$%%^
  • Validate the application
words $ docker-app validate myapp
Error: failed to validate metadata:
- name: Does not match format 'hostname'
- version: version is required
words $ echo $?
1

  • Fix myapp.dockerapp/metadata.yml

  • Comment the services: line in myapp.dockerapp/docker-compose.yml

  • Validate the application

words $ docker-app validate myapp
Error: failed to load composefiles: failed to parse Compose file version: "3.7"
#services:
    web:
     image: dockerdemos/lab-web
     ports:
      - "33000:80"

    words:
     image: dockerdemos/lab-words
     deploy:
       replicas: 3
       endpoint_mode: dnsrr

    db:
     image: dockerdemos/lab-db

: yaml: line 2: did not find expected key

words $ echo $?
1
  • Fix myapp.dockerapp/docker-compose.yml

Summary

  • docker-app let's you define metadata and parameters on top of a compose file
  • inspect displays all important informations in a compact way
  • validate checks everything is ok in the application. You can use it as a CI step.
  • split/merge transforms your application to a multiple-files/one-file. The one-file version is easy to share.