Continuous delivery with Travis and ECS

ECS is a good product. Sadly it’s authored by the same UX designer that authored all other AWS products, so a lot of people couldn’t even succeed in starting a simple hello world container.

Some months ago @fusillicode wrote a two-part tutorial on how to dockerize and deploy on ECS a WordPress app (you can find them here: part 1 and part 2). Of course, given we’re talking of docker, the technology you’re using is not so important.

What’s missing in those posts is how to do a painless deploy.

Normally you would:

• push on your docker repo the new image
• open your task definition
  
• update it without changing anything. This will create a new revision for it
• open the app service in the ECS cluster
  
• update the task definition revision

And ECS would take care of launching a new EC2 instance, starting a container with the updated image, and stopping the old container.

So, this is great news, ECS offers a zero-downtime deployment out of the box.

Nonetheless, a deploy can be tricky as you can see by the above checklist. A good way to avoid all of this is to have a continuous delivery system.

For this post I used a Rails app that is being tested on Travis CI. What I want to obtain is that when I push something in the master branch, if Travis is green, a deploy is automatically made in production.

I already have a running ECS Cluster, with a running service, and Travis is already running.

Continuous docker building & pushing

First thing I need is to let Travis build and push docker images when specs are green. Add the following to your .travis.yml:

sudo: required
services:
- docker
after_success:
- bin/docker_push.sh

We’re telling Travis that:

• we need a privileged container
• we need to have access to the docker exec
• it has to call the script bin/docker_push if tests are green

The content of docker_push.sh is the following:

#! /bin/bash
# Push only if it's not a pull request
if [ -z "$TRAVIS_PULL_REQUEST" ] || [ "$TRAVIS_PULL_REQUEST" == "false" ]; then
# Push only if we're testing the master branch
if [ "$TRAVIS_BRANCH" == "master" ]; then

# This is needed to login on AWS and push the image on ECR
# Change it accordingly to your docker repo
pip install --user awscli
export PATH=$PATH:$HOME/.local/bin
eval $(aws ecr get-login --no-include-email --region $AWS_DEFAULT_REGION)

# Build and push
docker build -t $IMAGE_NAME .
echo "Pushing $IMAGE_NAME:latest"
docker tag $IMAGE_NAME:latest "$REMOTE_IMAGE_URL:latest"
docker push "$REMOTE_IMAGE_URL:latest"
echo "Pushed $IMAGE_NAME:latest"
else
echo "Skipping deploy because branch is not 'master'"
fi
else
echo "Skipping deploy because it's a pull request"
fi

In order to have this script work as expected you need also to add the following ENV variables on Travis:

• AWS_DEFAULT_REGION: default region for your AWS account (e.g. eu-west-1)
• AWS_ACCESS_KEY_ID: Your AWS access key
• AWS_SECRET_ACCESS_KEY: Your AWS secret access key
• IMAGE_NAME: Docker image name for your app (e.g. foo)
• REMOTE_IMAGE_URL: Docker repo image url (e.g. your.repo.url/foo)

Do a push on your master branch, and if everything is ok you will see Travis pushing your image.

Continuous deploy

In order to trigger the deploy on ECS we’ll use the ecs-deploy script.

I downloaded it in my bin folder. In addition, I added a new script, bin/ecs_deploy.sh:

#! /bin/bash
# Deploy only if it's not a pull request
if [ -z "$TRAVIS_PULL_REQUEST" ] || [ "$TRAVIS_PULL_REQUEST" == "false" ]; then
# Deploy only if we're testing the master branch
if [ "$TRAVIS_BRANCH" == "master" ]; then
echo "Deploying $TRAVIS_BRANCH on $TASK_DEFINITION"
./bin/ecs-deploy -c $TASK_DEFINITION -n $SERVICE -i $REMOTE_IMAGE_URL:$TRAVIS_BRANCH
else
echo "Skipping deploy because it's not an allowed branch"
fi
else
echo "Skipping deploy because it's a PR"
fi

We need two more ENV variables on Travis:

• TASK_DEFINITION: the name of your ECS task definition
• SERVICE: the name of your ECS cluster service

Edit the after_success block of your .travis.yml to be like the following:

after_success:
- bin/docker_push.sh
- bin/ecs_deploy.sh

Push it, and voilà! Deploy in progress.