How to document your Software Architecture Decisions

Lukas Mösle
Miragon
Published in
5 min readJan 27, 2025

Throughout my software engineering career, I’ve frequently encountered legacy services that have evolved organically over the years. The original development team is often long gone, leaving behind documentation that is either obsolete or completely absent. This lack of context makes it incredibly challenging for new teams to understand and effectively maintain the existing software. Such situations often push developers into one of two equally problematic approaches: either blindly accepting previous architectural decisions or blindly changing them.

In such scenarios, teams typically begin by creating technical diagrams to map the current software architecture, which helps them understand the system’s present state. However, these diagrams fail to answer a crucial question: Why did the previous team design the system in this specific manner?

This is precisely where Architecture Decision Records (short ADRs) become invaluable. They provide a narrative that explains not just what was built, but why it was built that way.

What are Architecture Decision Records?

Architecture Decision Records (ADRs) are a systematic way to document the critical design choices made by software architects and development teams during the software design and implementation process. ADRs provide crucial contextual information by explaining the rationale behind architectural decisions. By capturing not just the “what” but the “why”, ADRs help stakeholders understand the reasoning, potential impacts, trade-offs, and strategic benefits of architectural choices, ultimately improving communication and long-term system maintainability.

An Architecture Decision Record typically contains five key components: a title, status, context, decision, and consequences. While these are standard, projects may require additional fields for comprehensive documentation.

Title
Choose a concise, numbered title that precisely captures the architectural decision’s essence, enabling quick identification and reference.

Status
Clearly mark the current state of the decision: proposed, accepted, rejected, or superseded. For rejected or superseded records, include a reference to the alternative ADR.

Context
Outline the specific circumstances, constraints, and technical landscape that prompted this architectural decision.

Decision
State the architectural choice definitively and unambiguously. Use clear, affirmative language that leaves no room for misinterpretation about the selected approach.

Consequences
Document the decision’s potential short-term and long-term impacts. Critically analyze and transparently present both the advantages and potential challenges introduced by this architectural choice.

Additional Fields
Personalize your ADRs with extra fields aligned with your project and team preferences. For example add an alternatives section, to showcase alternative solutions considered but not selected.

How an ADR could look like

Let’s consider a real-world example from a typical software project to demonstrate how an ADR works. Imagine you’re working on a growing application that requires an API Gateway to manage client-to-service communication. Without a central gateway, your frontend and backend services are loosely coupled, resulting in inconsistent routing, authentication, and security policies. Your final architecture might look something like this:

To document your decision to use NGINX and configure it according to the diagram, you would create the following ADR:

### ADR-001 Use nginx API Gateway

**Status**

accepted

**Context**

Our application requires an API Gateway to streamline communication between
clients and services, ensuring unified access and routing. This gateway will
centralize routing, enforce consistent security policies, and simplify
frontend-backend communication.

**Decision**

We use nginx as the API Gateway with the following configuration:
- SPA Frontend Delivery: The SPA frontend application will be served under
the `/` endpoint
- Backend Routing: All backend services will be accessible through
the `/api/**` endpoint
- SSO Integration: The SSO service will be available under `/auth` endpoint

**Consequences**

- Unified Access: All client-side requests are routed through the API Gateway
- Improved Security: Direct access to backend services via their internal
domain is eliminated, reducing exposure

NGINX was chosen due to its:

- Proven ability to handle high concurrency and traffic
- Flexible configuration options for routing, and load balancing
- Compatibility with modern authentication and API security patterns

With the ADR-001 Use NGINX as an API Gateway, you successfully document your architecture decision. This helps all project stakeholders better understand this specific part of the architecture.

How to start writing ADRs

The best time to start documenting architecture decisions is now. Begin by creating a reusable ADR template that your team can easily duplicate for future decisions. For existing projects, focus first on documenting new architectural choices rather than capturing every past decision, which can be overwhelming. When encountering previous undocumented decisions during current tasks, take a moment to record them.

The primary goal is ensuring accessibility and visibility for all project stakeholders. Choose a storage approach that aligns with your team’s workflow and organizational documentation standards.

For software projects my recommended storage strategies are:

  • In open-source projects using git repositories, store ADRs as markdown files alongside code and documentation.
  • For teams using wiki platforms, like Confluence create dedicated ADR pages that are easily searchable.
  • Enterprise teams might leverage specialized documentation tools or version-controlled repositories that integrate seamlessly with existing development workflows.

Why is writing Architecture Decision Records important?

Writing Architecture Decision Records (ADRs) is helpful for several reasons:

  • Alignment: Architecture Decision Records (ADRs) ensure all team members have a clear, shared understanding of critical architectural choices. They provide a single source of truth that aligns technical vision across different teams, departments, and even future contributors who may not have been part of the original decision-making process.
  • Exchange of knowledge: ADRs serve as living documentation that captures not just what decisions were made, but the reasoning behind them. This knowledge transfer is crucial for onboarding new team members, helping them quickly understand the system’s design principles, and preventing the loss of institutional knowledge when team members change.
  • Long term maintainability: By documenting architectural decisions, teams create a historical record that helps future developers understand why certain technical approaches were chosen. This context is invaluable when maintaining, refactoring, or scaling software systems, as it provides insights into the trade-offs and constraints that shaped the current architecture.
  • Teamwork: Every team member can add suggestions to the architecture to improve the overall design of the software. ADRs create a transparent, collaborative environment where technical decisions are not made in isolation, but through collective input, review, and consensus. This approach promotes a culture of continuous improvement and shared ownership of the technical strategy.

Conclusion

Architecture Decision Records (ADRs) are a powerful way for documenting and communicating architectural decisions in software development. They provide clarity on the why behind architectural choices, promote alignment among team members, facilitate knowledge sharing, and improve maintainability.

Have you implemented ADRs in your recent software projects? What challenges did you face, and how did you overcome them? Share your experiences in the comments below!

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Miragon
Miragon

Published in Miragon

All about process automation, clean code and architecture.

Lukas Mösle
Lukas Mösle

Written by Lukas Mösle

I develop thoughtful and reliable software solutions, driven by a strong interest in sustainable architecture and open source technologies.

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