CI is the process of merging all incoming features and patches into a single, well-tested application. It is important to note that this should happen several times a day—you will get a freshly baked version, like from a conveyor belt.

Nowadays, many products are offered to provide you a tool to test, build, and deploy your product unclear. In most cases, CI products work as a server that uses remote build agents to build code pulled from a repository. This process is approximately depicted in the following diagram:

The CI server is responsible for getting updates from a source code management server (such as Git) to pull a fresh version of the code and start building on using agents that are already connected and registered on the server. Some software can use Docker as a runtime for the necessary build agents. In this case, you don't even need to run agents manually. But this is not suitable for every part of an application, because some parts need to be built in an environment that can't be started as a Docker container.

Microservices that have been compiled and tested successfully can be moved to a deployment process that also can be automated using CD.