Introduction

Microservices: definition#

Unfortunately, there is no universally acknowledged definition for the term microservice. In the context of this course the following definition will be used:

Microservices are independently deployable modules.

For example, an e-commerce system can be divided into modules for:

• ordering
• registration
• product search

Normally, all of these modules would be implemented together in one application. In this case, a change in one of the modules can only be brought into production by bringing a new version of the entire application with all its modules into production. However, when the modules are implemented as microservices, the ordering process cannot only be changed independently of the other modules, but it can even be brought into production independently.

This speeds up deployment and reduces the number of necessary tests since only a single module needs to be deployed. Due to this greater level of decoupling, a large project can turn into a number of smaller projects. Each project is in charge of an individual microservice.

To achieve this at the technical level, every microservice has to be an independent process. A better solution for decoupling microservices is to provide an independent virtual machine or Docker container for each microservice.

In that case, a deployment will replace the Docker container of an individual microservice with a new Docker container, which starts the new version and its direct requests. The other microservices will not be affected if such an approach is used.

Advantages of this microservice definition#

The definition of microservices as independently deployable modules has several advantages:

• It is very compact.

• It is very general and covers all kinds of systems which are commonly denoted as microservices.

• The definition is based on modules and is thus a well-understood concept. This allows us to adopt many ideas concerning modularization. This definition also highlights that microservices are part of a larger system and cannot function entirely on their own. Microservices have to be integrated with other microservices.

• The independent deployment is a feature that creates numerous advantages and is therefore very important. Thus, the definition, in spite of its brevity, explains what the most essential feature of a microservice really is.

Deployment monolith#

A system that is not made up of microservices can only be deployed in its entirety. Therefore, it is called a deployment monolith. Of course, a deployment monolith can be divided into modules. The term deployment monolith does not make a statement about the internal structure of the system.

Size of a microservice#

The above definition of microservices does not say anything about the size of a microservice. Of course, the term microservice suggests that especially small services are meant. However, in practice, microservices can vary hugely in size. Some microservices keep an entire team busy, while others comprise only a few hundred lines of code. Thus, the size of microservices is ill-suited to be part of the definition.

Chapter walkthrough#

This chapter introduces microservices and discusses:

• Advantages and disadvantages of microservices to enable the reader to evaluate the applicability and usefulness of this architecture for a specific project.

• The discussion of benefits explains which problems microservices can solve and how this architecture can be adapted for different scenarios.

• The discussion of disadvantages illustrates where technical challenges and risks lie and how these can be addressed.

• Recognizing advantages and disadvantages is critical for technology and architecture decisions since those have to be aimed at maximizing benefits and reducing disadvantages.

                                                 Q U I Z  

In the next lesson, let’s discuss the advantages of using microservices.