Architectural Decision Record (ADR)
An Architectural Decision Record (ADR) is a document that captures key design and architectural decisions made during the development of a software system. It serves as a reference for understanding the rationale behind design choices, trade-offs considered, and the impact of those decisions on the system's architecture. This helps ensure consistency, clarity, and informed decision-making over time.
Why ADRs Are Important
- Historical Context: Over time, systems evolve, and decisions are made based on the context available at that time. ADRs serve as a historical record, making it easier to understand why certain decisions were made, even if the context changes later on.
- Consistency: By recording decisions and the rationale behind them, ADRs help maintain architectural consistency across different parts of the system, especially as new team members come onboard.
- Transparency: Documenting decisions encourages transparency, ensuring that all stakeholders, including developers, architects, and product teams, are on the same page.
- Onboarding and Knowledge Transfer: New team members can quickly get up to speed by reviewing the ADRs and understanding the architectural reasoning behind past decisions.
- Avoiding Repeated Discussions: When decisions are documented, teams are less likely to revisit the same decisions repeatedly, leading to more efficient development processes.
Key Components of an ADR
A typical ADR will include the following sections:
- Title: A clear, descriptive title for the decision.
- Status: Indicates whether the decision is "proposed," "approved," or "deprecated."
- Context: This section explains the background of the decision, such as the problem or opportunity it addresses. It provides the necessary context to understand why the decision was needed.
- Decision: A clear statement of the decision that was made.
- Consequences: Describes the outcomes of the decision, including both positive and negative effects, and how it may impact future work.
- Date: The date when the decision was made.
- Related Decisions: Links to other ADRs that are related to the current decision, providing a broader picture of the system's evolution.
Example 1: Choosing a Database
Title: Database Choice for User Management Service
Status: Approved
Date: March 7, 2025
Context:
We need to store user data for a new user management service. The system needs to support high availability, scalability, and the ability to handle complex queries efficiently. We are considering several options, including SQL and NoSQL databases.
Decision:
We will use PostgreSQL as the primary database for the user management service due to its robustness, support for complex queries, and transactional capabilities. We will use Redis as a caching layer to handle high-traffic operations like session management.
Consequences:
- Positive: PostgreSQL provides ACID compliance and is suitable for handling complex queries related to user data (e.g., user search, login history). Redis will speed up performance for frequently accessed data, reducing the load on the primary database. - Negative: Using both PostgreSQL and Redis introduces additional complexity, as we now need to maintain two systems. There is also potential for data inconsistency if caching is not properly handled.
Related Decisions:
- ADR-001: "Choosing the Cache Layer" - ADR-002: "Deciding on Database Transaction Model"
Example 2: Adopting Microservices
Title: Adopting Microservices for the Product Catalog Service
Status: Approved
Date: June 15, 2025
Context:
The current monolithic approach to the product catalog service has resulted in scaling issues as the product catalog grows. Each update to the catalog impacts the whole system, which leads to long deployment times and challenges with scaling specific components. We need a solution that allows for more granular scaling and faster deployments.
Decision:
We will migrate to a microservices architecture for the product catalog service. This will involve breaking the monolithic catalog into smaller, independent services, each responsible for a specific set of catalog-related tasks (e.g., product details, inventory management, pricing).
Consequences:
- Positive: Microservices allow for independent scaling of different components, improving overall system performance. Deployments can be more frequent and isolated, reducing the impact on other services. - Negative: Microservices introduce complexity in terms of inter-service communication, management, and deployment. We will need to invest in infrastructure for service discovery, API gateways, and monitoring. Also, the risk of data duplication increases, as each service might require its own database.
Related Decisions:
- ADR-003: "API Gateway for Microservices" - ADR-004: "Choosing a Service Discovery Mechanism"
Example 3: Frontend Framework Decision
Title: Choosing Frontend Framework for New Web Application
Status: Approved
Date: February 10, 2025
Context:
We are building a new web application, and need to choose a frontend framework. We need a framework that supports fast development, is maintainable in the long run, and integrates well with the backend services. The options considered include React, Angular, and Vue.
Decision:
We will use React for the frontend. React has a large community, robust ecosystem, and allows for flexible, component-based architecture. It also integrates well with our existing backend APIs.
Consequences:
- Positive: React's component-based structure will allow for reusable components, making the application easier to maintain and scale. Its large ecosystem provides a wealth of libraries, and it is widely supported by the developer community. - Negative: React's learning curve may be steeper for team members unfamiliar with it, especially when it comes to managing state with tools like Redux. Additionally, as the application grows, the need for proper architecture (e.g., managing side effects, code splitting) becomes more critical.
Related Decisions:
- ADR-005: "State Management for React Application" - ADR-006: "Using TypeScript with React"
Benefits of Using ADRs
- Clarity and Alignment: ADRs help clarify architectural decisions, ensuring that all stakeholders understand why certain technologies, patterns, or approaches were chosen.
- Traceability: They create a traceable history of decisions, which is particularly useful in complex projects or for onboarding new team members.
- Avoiding Regret: By explicitly stating trade-offs and consequences, ADRs help avoid regret later when unforeseen issues arise. If problems occur, it's easier to trace them back to the decision that led to the current situation.
- Encouraging Reflection: ADRs promote a culture of reflection, where decisions are carefully considered and documented, which leads to more deliberate and informed choices over time.
How ADRs Can Evolve
As the system evolves, decisions may need to be revisited. For example, if a previously chosen technology no longer fits the system's needs, an ADR may be created to document the change and explain why a different technology or approach is being adopted. This ensures that the system’s evolution is well-documented, providing context for future decision-makers.