Fundamentals of software architecture: Part I

Architecture

Architect

  • Keep current with latest trends and be familiar with a variety of technologies.
  • Continually analyse the architecture, understanding trade-offs on each decision (as everything in software architecture has an advantage and disadvantage, and there are no right or wrong answers)
  • Verify that development teams are following the architecture decisions and design principles defined, because if not, the architecture will not meet the required architectural characteristics, and the application or system will not work as expected.
  • Have a certain level of business domain expertise; if not, he cannot communicate with stakeholders and business users and will quickly lose credibility.
  • Possess interpersonal skills, including teamwork, facilitation, and leadership.
  • Understand the political climate of the enterprise and be able to navigate the politics, applying basic negotiation skills to get most decisions approved. Despite that most decisions made as a developer did not require approval or even a review. almost every decision an architect makes will be challenged, due to increased costs or increased effort (time) involved.

Unlike a developer, who must have a significant amount of technical depth to perform their job, a software architect must have a significant amount of technical breadth.

The architect and developer must be on the same virtual team to make this work, as they are both part of the circle of life within a software project. They must always be kept in synchronisation with each other in order to succeed.

Architects should balance architecture stuff with coding, trying to remain hands-on, delegating the critical path but doing frequent proof-of-concepts or POCs, tackling some of the technical debt stories, architecture stories, bug fixing, leveraging automation by creating simple command-line tools and analysers to help the development team with their day-to-day, doing frequent code reviews to seek out mentoring and coaching opportunities on the team, etc.

Architecture characteristics

An important thing here is that most applications can only support a few of all the architecture characteristics, as each of the supported characteristics requires design effort and often has a negative impact on others. So, never shoot for the best architecture, but rather the least worst architecture; design it to be as iterative as possible and easy to try & change (so you can stress less to find the best on the first attempt).

Some architecture characteristics come from explicit statements in requirements documents, others come from inherent domain knowledge by architects. Have the domain stakeholders select the top three most important characteristics and keep the final list as short as possible.

Operational Characteristics

  • Continuity: the ability to disaster recovery.
  • Performance: includes capacity required, and response times expected.
  • Recoverability: business continuity requirements.
  • Reliability/safety: assess if the system needs to be fail-safe or it’s critical to save lives.
  • Robustness: the ability to handle error and boundary conditions.
  • Scalability: the ability to handle a large number of concurrent users without serious performance degradation
  • Elasticity: the ability to handle bursts of requests.

Structural Characteristics

  • Extensibility: the ability to plug new pieces of functionality in.
  • Installability: ease of system installation on all necessary platforms
  • Leverageability/reuse: ability to leverage common components across multiple products.
  • Localization: support for multiple languages.
  • Maintainability: how easy it is to apply changes and enhance the system.
  • Portability: does the system need to run on more than one platform?
  • Supportability: what level of technical support (logging and other facilities) is needed by the application?
  • Upgradeability: ability to easily/quickly upgrade from a previous version of this application/solution to a newer version on servers and clients.

Cross-Cutting Characteristics

  • Archivability: Will the data need to be archived or deleted after a period of time?
  • Authentication: Security requirements to ensure users are who they say they are.
  • Authorization: Security requirements to ensure users can access only certain functions within the application.
  • Legal: What legislative constraints is the system operating in?
  • Privacy: Ability to hide transactions from internal company employees.
  • Security: Does the data need to be encrypted in the database/network communications?
  • Usability/achievability: Level of training required for users to achieve their goals with the application/solution.

Measuring

Many of the operational characteristics may have obvious measurements, as response time for performance. However, measures for structural characteristics are not so obvious. One metric that helps to detect complex code and therefore, bad maintainability, testability, and so on, is cyclomatic complexity. It is computed by applying graph theory to code, specifically decision points. As a general rule, a value under 10 is acceptable, but it is preferable to fall under 5 (always check if the function is complex because of poor coding or the domain itself!). Another metric that helps to measure some characteristics, as deployability, is code coverage.

Once the architecture characteristics were established, it is important to respect their priorities among the development process; one way to ensure that is using fitness functions, as a mechanism to provide an objective integrity assessment and automatically verify important architecture principles (these functions may overlap other mechanisms as metrics, monitors, testing libraries, chaos engineering, etc.). A fitness function may, for example, check the distance from the main sequence (connascence) and fail if is not acceptable. Another example is Netflix’s Simian Army, that ensures that each service responds usefully to all RESTful verbs.

Modularity

Cohesion

Given the subjectiveness of cohesion, computer scientists have developed a structural metric to determine cohesion (The Chidamber and Kemerer Lack of Cohesion in Methods or LCOM); however all this metric can find is structural lack of cohesion, as it has no way to determine logically if particular pieces are related.

Coupling

  • Instability: the ratio of efferent coupling to the sum of both (efferent and afferent): [Ce/Ci+Ce]. It determines the volatility of a code base; code with higher degrees breaks more easily when changed because of high coupling.
  • Abstractness: the ratio of the sum of abstract artifacts to the sum of concrete ones. It is used to measure the degree of abstraction of the package: code that is too abstract becomes difficult to use and code with too much implementation and not enough abstraction becomes brittle and hard to maintain.
  • Distance from the Main Sequence: This metric is used to measure the balance between stability and abstractness; classes that fall near this line exhibit a healthy mixture of these two competing concerns.

Ideal packages are either completely abstract and stable (I=0, A=1) or completely concrete and unstable (I=1, A=0). On the other hand, packages with high abstractness and high instability (I=1, A=1) are in the zone of uselessness, and packages with low abstractness and low instability (I=0, A=0) are in the zone of pain as they are difficult to change and cannot be extended because they aren’t abstract. This means ideally you should depend on abstractions and not on implementations.

Connascence

  • Strength: the ease with which a developer can refactor that type of coupling.
  • Locality: how proximal the modules are to each other.
  • Degree: relates to the size of its impact — does it impact a few classes or many.

There are the following forms, one weaker than others:

  • Name: when multiple components must agree on the name of an entity.
  • Type: when multiple components must agree on the type of an entity.
  • Meaning: when multiple components must agree on the meaning of specific values.
  • Position: when multiple components must agree on the order of values.
  • Algorithm: when multiple components must agree on a particular algorithm.
  • Execution (order): when the order of execution of multiple components is important.
  • Timing: when the timing of the execution of multiple components is important.
  • Value: when several values must change together.
  • Identity: when multiple components must reference the entity.

The idea is simple: the more remote the connection between two clusters of code (locality), the weaker the connascence between them should be. More detail and some examples on https://connascence.io/.

Components

The simplest component are libraries (that wraps code at a higher level of modularity than classes); they also appear as subsystems or layers, as the deployable unit of work for many event processors, or as services (that tends to run in its own address space and communicates via low-level networking protocols like TCP/IP or higher-level formats like REST or message queues).

Generally the component is the lowest level of the software system an architect interacts directly with. Developers typically take components, jointly designed with the architect role, and further subdivide them into classes, functions, or subcomponents.

The component identification flow may be something like this:

There are some useful techniques for doing it, as:

  • ACTOR/ACTIONS APPROACH: identify actors who perform activities with the application and the actions those actors may perform.
  • EVENT STORMING: the team tries to determine which events occur in the system based on requirements and identified roles, and build components around those event and message handlers (inspired by DDD).
  • WORKFLOW APPROACH: models the components around workflows, much like event storming, but without the explicit constraints of building a message-based system.

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