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Architectural Decision Records (ADRs)

Overview​

An Architectural Decision Record (ADR) is a concise, living document that captures important decisions about both the software and infrastructure components of a system. ADRs provide a structured way to record the context, motivations, alternatives, and consequences of architectural choices—whether selecting a database engine or defining a network topology—ensuring transparency, consistency, and knowledge sharing across teams.

Rationale​

  • Historical Context: Systems evolve over time, encompassing code, cloud resources, network configurations, and physical hardware. Without ADRs, the reasons behind key decisions—like choosing Terraform over CloudFormation or a particular VM instance type—can be lost, leading to confusion or costly rework.
  • Team Alignment: ADRs foster clear communication among stakeholders (developers, operators, architects, product owners) by making the decision-making process visible and accessible across both software and infrastructure domains.
  • Onboarding: New team members gain a holistic understanding of past organizational choices—ranging from microservice frameworks to infrastructure-as-code tooling—reducing the learning curve and preventing redundant research.
  • Governance & Compliance: In regulated industries, ADRs demonstrate due diligence by documenting why certain technologies or patterns were adopted, be they container runtimes or network security standards.

What Should Be Recorded as an ADR​

Create an ADR when a decision has long-term impact on the system’s structure, maintainability, security, or operational cost. Common examples include:

Software Decisions​

  1. Technology Selection: Choosing a database engine (e.g., PostgreSQL vs. MongoDB) or message broker (e.g., RabbitMQ vs. Kafka).
  2. Architectural Patterns: Adopting microservices vs. monolith, event-driven architecture, or serverless functions.
  3. Communication Protocols: Deciding between REST, gRPC, GraphQL, or message queues for inter-service communication.

Infrastructure Decisions​

  1. Cloud Provider & Region Strategy: Selecting AWS vs. Azure vs. GCP, or on‑premises deployments.
  2. Infrastructure-as-Code Tools: Adopting Terraform vs. CloudFormation vs. Pulumi.
  3. Networking & Security: Defining VPC/subnet layouts, load balancer types, firewall rules, VPN or direct-connect options.
  4. Compute Resources: Choosing VM instance types, container orchestration platforms (e.g., Kubernetes vs. ECS), or serverless compute (e.g., AWS Lambda).
  5. Storage & CDN: Selecting object storage, block storage tiers, or CDN providers for content delivery.

What Should NOT Be Recorded as an ADR​

Avoid ADRs for trivial or easily reversible decisions that do not materially affect the long-term architecture:

  • Minor UI Tweaks: Changing color schemes or CSS framework upgrades without broader impact.
  • Routine Refactoring: Small code refactors or quick build-tool updates.
  • Ephemeral Configurations: Short-lived feature flags, temporary scripts, or quick fixes.
  • Ad-hoc Operational Tasks: Day-to-day scaling actions or incident responses without a strategic architecture component.

ADR Structure and Process​

  1. Title & Identifier: Use a sequential number (e.g., ADR-0001) and a descriptive title (e.g., "Select Terraform for IaC").
  2. Status: (Proposed, Accepted, Deprecated, Superseded).
  3. Context: Summarize background, forces, and constraints—covering both software requirements and infrastructure considerations.
  4. Decision: Clearly state what choice was made.
  5. Alternatives Considered: List other options (software frameworks, cloud providers, IaC tools) and reasons for rejection.
  6. Consequences: Detail positive and negative impacts—on performance, cost, maintenance, security—and any follow-up tasks (proof-of-concept, migration plans).
  7. Date & Authors: Record who made the decision and when.
  8. Review & Approval: Capture feedback and sign-off from relevant stakeholders (development, operations, security).
  9. Links & References: Connect to related ADRs, design docs, or external resources (e.g., Terraform modules, network diagrams).

Process Tips:

  • Draft early and share for feedback across both dev and ops teams.
  • Store ADRs in a /adr/ directory alongside code and infrastructure definitions.
  • When changes occur, update status or create a new ADR rather than editing historical records, preserving the evolution of decisions.

Conclusion​

ADRs are a lightweight yet powerful practice for institutionalizing architectural knowledge across software and infrastructure. By judiciously documenting pivotal decisions and their rationales, teams can maintain clarity, reduce technical debt, and streamline future planning and governance.