A Brief Note on Software Architecture Decisions

Architecture decisions are choices made during the design and development of a system that have a significant impact on its structure, behavior, performance, and long-term sustainability. These decisions define how the system will be built, how its components interact, and how it will evolve over time.

When we document architectural decisions, we will mainly focus on the "architecturally significant decisions". These are decisions that have a substantial impact on the architecture of the system. Such decisions often influence the system's structure, behavior, scalability, security, performance, and maintainability. Examples include:

• Choosing between microservices or monolithic architecture.
• Deciding on a specific technology stack.
• Selecting a database type (SQL vs. NoSQL).
• Deciding on cloud vs. on-premise deployment.

For each significant decision, you should not only record the decision itself but also the reasoning behind it. This helps provide context for why a particular direction was chosen over others. Rationales could include factors like cost, time constraints, team expertise, and long-term scalability considerations. It’s important to document both the benefits of the decision and any trade-offs or risks involved.

REMEMBER: Architecture decisions can provide insights for other teams and projects within the organization. If a decision was particularly difficult or unique, documenting it can share valuable knowledge.

When to Document a Architecture Decision

• When it has a long-term impact: For example, choosing a certain technology or architecture pattern that could affect how the system scales in the future.
• When it’s tied to significant trade-offs: If the decision involves a trade-off, such as balancing performance against maintainability or choosing between cost-effective and high-performance solutions, it’s crucial to document these nuances.
• When it's made in the face of uncertainty or risk: For example, choosing an emerging technology or approach that could either provide significant benefits or introduce unknown challenges.
• When it’s not adequately captured elsewhere: If a decision doesn’t fit neatly into a specific architectural view (like the building block view or cross-cutting concepts), or if it’s a one-off decision, it should be documented in a decision record.

Key Characteristics of Architecture Decisions

1. Impact: Architecture decisions usually have a broad impact, affecting not just a single feature but the system as a whole. They influence the architecture, performance, scalability, maintainability, security, and other critical aspects of the system.
2. Long-term consequences: These decisions tend to have long-lasting effects. For example, choosing a database technology or cloud platform could shape the system's architecture for years, and the decision might be hard or expensive to change later.
3. Complexity: Architecture decisions are often made in response to complex requirements and constraints, including technical, business, and user needs. They may involve trade-offs between conflicting objectives, such as performance versus cost, or flexibility versus simplicity.
4. Strategic Nature: Many architecture decisions are made at the strategic level. They may align with an organization's overall goals, such as choosing an architecture that supports scalability or adopting a microservices architecture to enable greater flexibility and agility.

Examples of Architecture Decisions

• Choice of architecture style: Deciding whether the system will be monolithic, microservices-based, or serverless. This decision shapes how the system's components interact and scale.
• Technology selection: Choosing specific technologies, such as which programming language, database, framework, or cloud provider to use. This decision has significant implications for the development process and system performance.
• System scaling approach: How will the system scale? Will it use horizontal scaling (adding more machines) or vertical scaling (increasing the power of individual machines)? This decision impacts cost, performance, and system reliability.
• Security approach: Decisions around encryption, authentication, authorization, and how to protect sensitive data are crucial. For example, choosing between using OAuth2 or custom-built authentication solutions will affect how secure and flexible the system is.

Why Architecture Decisions Matter

1. Shape System Behavior: Architecture decisions directly impact how the system behaves under different conditions—whether it scales efficiently, can handle large traffic volumes, or remains easy to maintain over time.
2. Cost and Efficiency: Some architecture decisions can affect the cost of development and operation. For example, choosing a certain technology or approach might lead to higher infrastructure costs or more complex maintenance efforts.
3. Risk Management: Many architecture decisions come with risks. For example, choosing a cutting-edge technology might offer significant benefits but also exposes the system to the risk of being too early to adopt or not well supported.
4. Maintainability and Flexibility: A well-architected system can be easier to maintain and extend over time. Bad architectural decisions, on the other hand, can lead to complex, difficult-to-change systems that hinder future development.

Architecture Decision Record (ADR)

Using Architecture Decision Records (ADRs) to capture decisions is a methodical yet flexible way to document the architectural choices made during the design and development of a system. The goal is to record key architectural decisions, their rationale, and their consequences in a way that is clear, consistent, and easy to reference.

The purpose of an ADR is not to create a comprehensive, overly complex document, but to record essential information in a straightforward and easily digestible format. The goal is for the record to be simple enough that developers, architects, and stakeholders can quickly understand the decision, its rationale, and implications.

While the ADR should be simple, it should also be organized in a way that allows readers to find key information easily. A typical ADR is structured with a few standard sections to ensure consistency across the entire project or organization. According to Michael Nygard , ADR format has just a few parts as follows:

• Title: The name of the decision (e.g., ADR001: Choose Microservices Architecture Over Monolithic Design)
• Status: The status of the ADR (e.g., Proposed, Approved etc.)
• Context: A description of the situation and the factors influencing the decision.
• Decision: The chosen approach or solution.
• Consequences: The benefits and trade-offs of the decision, including any risks or alternatives considered.

Here are a few examples of ADR for your reference.

Title: ADR001: Choose Microservices Architecture Over Monolithic Design

Status: Approved

Context: The system is expected to scale significantly in the future to handle millions of concurrent users. The current monolithic architecture has been causing difficulties in scaling individual components independently. The team needs a flexible and scalable architecture that allows for independent deployment of services and faster development cycles.

Decision: We will transition to a microservices architecture where each core component (e.g., user management, payment processing, notifications) will be developed, deployed, and scaled independently as separate services.

Consequences:

Benefits: Independent scaling of services based on load. Faster development cycles due to isolated services and teams working independently. Easier to adopt new technologies or make changes in isolated services without impacting the entire system.

Trade-offs:

Complexity: Increased system complexity due to inter-service communication, dependency management, and data consistency across services.

Operational Overhead: Requires infrastructure for service orchestration (e.g., Kubernetes), monitoring, and handling service failures.

Title: ADR002: Implement JWT for Authentication and Authorization

Status: Approved

Context: The system will be accessed by multiple clients (web, mobile, third-party services), requiring a stateless authentication mechanism that can easily scale. The existing session-based authentication model is cumbersome and doesn’t work well in a distributed environment.

Decision: We will adopt JSON Web Tokens (JWT) for user authentication and authorization across the system, allowing for stateless authentication and seamless interaction between multiple services.

Consequences:

Benefits:

Stateless: JWT allows authentication information to be stored in the token itself, making it ideal for distributed systems where servers don’t need to store session data.

Scalable: Since tokens are self-contained, scaling the authentication service becomes easier.

Cross-platform support: JWT is supported by many libraries across web, mobile, and third-party systems.

Trade-offs:

Token Revocation: Revoking a JWT before expiration is not straightforward, which could pose a security risk if a user’s credentials are compromised.

Token Size: JWTs can become large depending on the amount of data stored in them, potentially leading to performance issues.

If you want to explore more ADR formats, you can visit this link.

That's all from my side. I will look forward to your thoughts and suggestions in the comment section.