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Exercise 5 - Selectively Disabling Services

Time: Approximately 15 minutes

Difficulty: Medium

Table of Contents

  1. Why disable services?
  2. How to disable services
  3. Updating the Vote and Results services
  4. Using our Application Stack for local development

Exercise Objectives

By the end of this exercise, you will have:

  • Learned about a few use cases for disabling of services
  • Updated the vote and results services to allow them to be disabled
  • Deployed the application with the vote app disabled and started a dev-enabled container for the vote app that hooks into the application stack

Why disable services?

In most deployments, you may not want to disable a service. However, there are a few valid use cases:

  • Disabling a database service because you want to connect to a managed database service (like Amazon's RDS)
  • Disabling a web application when deploying locally because you want to make changes to one of the services (run that service in a dev-enabled container)
  • Disabling a proxy service because you already have a proxy in your cluster (yes, we don't have a reverse proxy, but you can add one in Exercise 7)

For this exercise, we are going to instrument only the vote and results services for disabling.

How to disable services

As we learned in the previous exercise, we can add parameters to modify our compose file and allow overrides at deploy time. Since the compose file has to be rendered, the capability to disable a service was added.

Starting with compose schema version 3.4, any compose file can include extension fields, which all start with x-. If a service contains a x-enabled field, Docker App will only include that service if it's value is "truthy." For example...

services:
  app:
    x-enabled: false
    image: nginx

In this case, the app service will always be disabled, as the value is false. However, we probably don't want to always disable a service (why include it in the first place?). Therefore, it sounds like a great candidate for a parameter!

Updating the Vote and Results services

To update the vote and results services, let's do the following:

  1. In the vote service, add a x-enabled field that has a value of ${vote.enabled}.

    Solution 
    services:
  vote:
    x-enabled: ${vote.enabled}

  2. Update the parameters.yml to set a default value for vote.enabled to true.

    Solution 
    vote:
  enabled: true

  3. Do the same thing for the results service, but use results.enabled as the parameter name.

  4. Do not forget to build the application image.

$ docker app build voting-app -t username/voting-app:0.1.0

  1. Use the docker app inspect command. You'll see the vote.enabled and results.enabled parameters are set to the default value true.

    Sample Output 
    $ docker app inspect username/voting-app:0.1.0 --pretty
version: 0.1.0
name: voting-app
description: ""
maintainers:
- name: root
  email: ""

Services (4) Replicas Ports Image
------------ -------- ----- -----
results      1        5001  mikesir87/examplevotingapp_result
vote         2        5000  mikesir87/examplevotingapp_vote
redis        1              redis:alpine
db           1              postgres:9.4
worker       1              dockersamples/examplevotingapp_worker

Networks (2)
------------
backend
frontend

Volume (1)
----------
db-data

Parameters (4)  Value
--------------  -----
optionA         Cats
optionB         Dogs
results.enabled true
vote.enabled    true

Using our Application Stack for development

In this section we will see how we can use the x-enabled flag to develop and test a service of our application.

  1. First of all we need to add a attachable: true to the frontend network in the docker-compose.yml. This option is required to allow a standalone container to attach to an overlay network created by a stack.

    Solution 
    networks:
  frontend:
    name: front-tier
    attachable: true

  2. Deploy the application without enabling the vote service. Ensure with the docker service ls that the vote service has not been deployed

    Solution 
    $ docker app run username/voting-app:0.1.0 --name voting-app -s vote.enabled=false  --target-context=swarm
Creating network front-tier
Creating network back-tier
Creating service voting-app_db
Creating service voting-app_results
Creating service voting-app_redis
Creating service voting-app_worker

$ docker service ls
ID                  NAME                 MODE                REPLICAS            IMAGE                                          PORTS
89zwowq45acp        voting-app_db        replicated          1/1                 postgres:9.4                                   
nza74rbbrwde        voting-app_redis     replicated          1/1                 redis:alpine                                   
dlhpp09kfhee        voting-app_results   replicated          1/1                 mikesir87/examplevotingapp_result:latest       
j24jm0lyp8de        voting-app_worker    replicated          1/1                 dockersamples/examplevotingapp_worker:latest

  3. Deploy the vote service as a standalone container, attached to the frontend network with OPTION_A and OPTION_B environment variable set to Cats and Dogs. You should see the port badge for 5000 appear on the master node. Once it appears, click on it to validate the vote service has started.

    Solution 
    $ docker --context swarm container run --name voting-app-vote-svc-dev -d --rm -e OPTION_A=Cats -e OPTION_B:Dogs -p 5000:80 --network front-tier mikesir87/examplevotingapp_vote
    Details

  4. Clone the voting app repo (found at github.com/mikesir87/example-voting-app).

    $ git clone https://github.com/mikesir87/example-voting-app.git

  5. Now, let's make a change to the application! In the vote/templates/index.html file, let's make a change. You can change anything, but one idea might be to adjust the "Tip: you can change your vote" text to say "Tip: try changing your vote!". After saving the change, refresh the page displaying the vote app and you should see the change.

  6. Now that we've made the change we want to save in our app, let's stop the dev container.

    $ docker container stop $(docker container ls --filter name=voting-app-vote-svc-dev -q)

  7. Let's build our new image and push it to Docker Hub. Tag the image using [your-docker-hub-username]/updated-vote-app.

    $ cd vote
$ docker build -t myuser/updated-vote-app .
…
$ docker push myuser/updated-vote-app

  8. Back in our docker app, let's update the vote service to use our new image.

    services:
  vote:
    image: myuser/updated-vote-app

  9. Build your updated app and then deploy it to your cluster. Validate that your vote service has the updated image!