Before we continue exploring the Continuous Delivery steps, we should discuss a deployment change introduced with Docker Swarm. We thought that each release means a new deployment. That is not true with Docker Swarm. Instead of deploying each release, we are now updating services. After building Docker images, all we have to do is update the service that is already running. In most cases, all there is to do is to run the docker service update --image <IMAGE> <SERVICE_NAME> command. The service already has all the information it needs and all we have to do is to change the image to the new release.
For service update to work, we need to have a service. We need to create it and make sure that it has all the information it needs. In other words, we create a service once and update it with each release. That greatly simplifies the release process.
Since a service is created only once, the Return On Investment (ROI) is too low for us to automate this step. Remember...