Architectural Decision Records (ADRs): A Practical Guide for Engineering Teams

GitLab publicly documents architectural and design decisions in its handbook, using Architectural Decision Records (ADRs) stored in /docs/adr/ and committed in the same PR as the related change, according to a practical guide for engineering teams. ADRs are lightweight 1-2 page Markdown documents that capture a significant architectural decision, its context, alternatives considered, reasoning, and consequences, helping teams preserve decision context, improve onboarding, and enable better future decisions. The guide recommends creating an ADR for decisions that are hard to reverse, involve trade-offs, are cross-cutting, security-related, performance-critical, infrastructure-related, affect external APIs, or introduce new frameworks, and provides a simple template with fields for status, date, context, decision, alternatives, and consequences.

What ADRs are, when to write one, and how to start using Architectural Decision Records 1. Background Recently, I’ve been looking into how large engineering organizations manage and document architectural decisions over time. Across most teams I’ve worked with, there’s rarely a clear record explaining why a given approach was chosen. The implementation lives in code, but the reasoning is lost. This makes it harder to: Understand historical decisions Onboard new team members Revisit or improve past solutions Avoid repeating previous discussions To address this, many companies use Architectural Decision Records ADRs . 2. What Are ADRs? An Architectural Decision Record ADR is a short document that captures: A significant architectural decision The context in which it was made The alternatives considered The reasoning behind the chosen solution The consequences of that decision ADRs are: Lightweight usually 1–2 pages Stored in the Git repository Written in Markdown Version-controlled alongside the code They are not long design documents — just focused records of important decisions. 3. Why Use ADRs? Preserve Decision Context Instead of asking: “Why did we implement it this way?” We can look at the ADR and see: What constraints existed What trade-offs were considered Why other options were rejected Improve Onboarding New team members can: Read ADRs to understand major system choices Learn system philosophy faster Avoid repeating already-discussed ideas Enable Better Future Decisions When revisiting architecture, ADRs help us: Evaluate whether assumptions still hold Understand consequences of past trade-offs Make changes intentionally instead of reactively Align Team Thinking Writing ADRs forces us to: Clarify trade-offs Explicitly evaluate alternatives Make decisions more intentional 4. What Kind of Decisions Should Have an ADR? If you’re unsure, ask in your team’s design review or ping whoever owns that part of the system. Not every small change needs one. ADRs are useful for decisions that are: Hard to reverse When there are multiple viable options When the decision involves trade-offs — You’re balancing things like: speed vs scalability, simplicity vs flexibility, cost vs performance Cross-cutting affects multiple systems/services Security-related Performance-critical Infrastructure-related Affecting external APIs or contracts Introducing new frameworks or major dependencies If a decision is important, create an ADR even if there are no clear trade-offs or alternatives. Documenting the context and reasoning is still valuable. Examples: Choosing between REST and gRPC for a new internal service Adopting a message queue instead of synchronous calls Picking a multi-tenancy strategy shared DB vs schema-per-tenant Introducing a caching layer and its invalidation strategy 5. A Simple ADR Template We can use a simple template like this: ADR-0001: Picking a multi-tenancy strategy shared DB vs schema-per-tenant StatusProposed | Accepted | Rejected | Superseded DateYYYY-MM-DD ContextDescribe the problem, background, and why this decision is needed. - What is happening?- What constraints exist? technical, business, timeline - What problem are we solving? DecisionDescribe the chosen solution. Alternatives Considered- Option A- Option B- Option C Consequences Positive- ... Negative- ... Each ADR would: Have a number ADR-0001, ADR-0002, etc. Be stored in /docs/adr/ Be committed in the same PR as the related change when possible 6. Example From Industry Many large engineering organizations use ADRs. For example, GitLab documents architectural and design decisions publicly in their handbook: When making a significant architectural decision: Create a new ADR file in /docs/adr/ Share it in PR for review Once agreed, mark the status as Accepted If the decision changes later: Create a new ADR Reference the previous one Mark the old one as superseded. Keep it simple. No heavy governance. 8. What You Get Out of It If we adopt ADRs, we can expect: Better long-term maintainability Clear historical record of architectural evolution Faster onboarding More structured technical discussions Reduced knowledge silos 9. How to Start You don’t need everyone to agree before you start. You can do this on your own: Make a /docs/adr/ folder and put a simple template in it Write ADR-0001 about a decision your team already made. Writing it after the fact is fine Add a link to it in your next PR Wait and see if people find it useful before you add any rules around it How long an ADR takes depends on the decision. A big one may need days of investigation and discussion. But that work happens anyway before you decide. The ADR just captures what you already worked out, so the writing part is usually short. 10. Take the Template The full template is here, free to copy and change however you want: Save it as /docs/adr/0000-template.md in your repo and copy it every time you write a new ADR. If you change something to fit your team better, I'd like to hear about it. Leave a comment on the gist.