The Software Architecture definition is something that, for a long time, the industry as a whole has not been able to agree or to find a consensual definition. In some cases, it is defined as the blueprint of a system and, in other, it is the roadmap for developing a system, including all the options in the middle.

The truth is that it is both things and, probably, much more than that. To try to figure out what it is, I think we are still far from a formal definition, we can focus on what it is analysed when we take a look at concrete architectures.

• Structure
• Architecture characteristics
• Architecture decisions
• Design principles

Structure

When we talk about the structure we are referring to the type or types of architecture styles selected to implement a system such as microservices, layered, or a microkernel. These styles do not describe and architecture but its structure.

Architecture characteristics

The architecture characteristics define the quality attributes of a system, the “-ilities” the system must support. These characteristics are not related to the business functionality of the system but with its proper function. They are sometimes known as non-functional requirements. Some of them are:

Availability	Reliability	Testability
Scalability	Security	Agility
Fault Tolerance	Elasticity	Recoverability
Performance	Deployability	Learnability

A long list of them, maybe too long, can be found on one of the articles on the Wikipedia: List of system quality attributes.

Architecture decisions

Architecture decisions define the rules of how a system should be built. Architecture decisions form the constraints of a system and inform the development teams of what it is allowed and what it is not when building the system.

An example, it is the decision of who should have access to the databases on the system, deciding that only business and service layers can access them and excluding the presentation layer.

When some of these decisions need to be broken due to constraints at one part of the system, this can be done using a variance.

Design principles

Design principles are guidelines rather than strong rules to follow. Things like synchronous versus asynchronous communications within a microservices architecture. It is some kind of a preferred way to do it but this does not mean developers cannot take different approaches on concrete situations.

Reference: “Fundamentals of Software Architecture by Mark Richards and Neal Ford (O’Reilly). Copyright 2020 Mark Richards, Neal Ford, 978-1-492-04345-4″