In the previous section, we learned how to use Docker Compose to manage the Notes application stack. Looking ahead, we can see the potential need to use multiple instances of the Notes container when we deploy to Docker Swarm on AWS EC2. In this section, we will make a small modification to the Docker Compose file for an ad hoc test with multiple Notes containers. This test will show us a couple of problems. Among the available solutions are two packages that fix both problems by installing a Redis instance.
A common tactic for handling high traffic loads is to deploy multiple service instances as needed. This is called horizontal scaling, where we deploy multiple instances of a service to multiple servers. What we'll do in this section is learn a little about horizontal scaling in Docker by starting two Notes instances to see how it behaves.
As it currently exists, Notes stores some data—the session data—on the local...
