Combining CD and microservices
We live in the world of microservices. Nowadays, every system is either microservice-based or in the process of becoming microservice-based. After the first publication of the bestseller book by Sam Newman, Building Microservices, the software world has shifted into the fine-grained modular systems in which all communication happens over the network. Some companies have gone one step further and realized that they need to consolidate some of the microservices as they created too many of them. Some other companies even take a step back and consolidate microservices into a monolith.
While the topic of microservices is broad on its own and outside the scope of this book, it is important to understand how the microservice architecture affects the CD pipeline. Should we create a separate pipeline for each service? If yes, then how do we test the interaction between the services and the system as a whole?
Before answering these questions, let's look at the following diagram, which represents a small microservice-based system:
Figure 1.7 – Sample microservice system
There are three services in our system, each with a database. A user only interacts with Service 1. As a more concrete example, this system could represent an online store, where Service 1 could represent the checkout service, Service 2 could represent the product catalog service, and Service 3 could represent customer service.
We could either implement one CD pipeline for the entire system or a separate CD pipeline for each microservice. Which approach is the right one? Let's consider both options. If we create one CD pipeline, this means that the automated acceptance testing phase runs against the entire system from the end user's perspective, which seems correct. However, one CD pipeline also means that we deploy all the services at the same time, which is completely against the microservice principles. Remember that in every microservice-based system, services are loosely coupled and should always be independently deployable.
So, we need to take the second approach and create a separate CD pipeline for each service. However, in such a case, the automated acceptance testing phase never runs against the entire system. So, how can we be sure that everything works correctly from the end user's perspective? To answer this question, we need a little more context about the microservice architecture.
In the microservice architecture, each service is a separate unit that's usually developed and maintained by a separate team. Services are loosely coupled, and they communicate over a well-defined API, which should always be kept backward compatible. In that context, each internal microservice does not differ much from an external service. That's why we should always be able to deploy a new service without testing other services. Note that it does not exclude the possibility of having a separate acceptance test for the entire system. All it explains is that the acceptance test of the entire system should not be a gatekeeper for deploying a single service.
Information
The CD process is suitable for both monolith and microservice-based systems. In the former case, we should always create a separate CD pipeline for each microservice.
For the sake of simplicity, all the examples in this book present a system that consists of a single service.