Documenting architecture decisions
You should record decisions that affect the architecture of your service, in order to preserve the context of your choices.
As agile projects age, it is sometimes hard to keep track of the reasoning behind the decisions made. This is especially true as new people join the projects when those involved in the early stages are no longer around.
It is important to preserve the reasoning so the current team can include it as context when making their own decisions about changes they need to make. For example, understanding whether a particular choice was made for the sake of expediency and can therefore be changed with little impact, or whether there were external reasons behind that decision that need to be factored in.
How to document decisions
Architecture decisions should be stored in version control so there is a record of what was changed, who by, and when. Decisions that affect a specific application should be in that application’s code repository. You may also want to store larger-scale decisions in a central documentation repository.
A suggested format is the Architecture Decision Record (ADR), proposed by Michael Nygard in a blog post and since adopted widely. That post describes the format in full, but as a summary it consists of the following sections:
- Title: a description of the decision (not the problem)
- Status: see lifecycle
- Context: the facts behind the need to make the decision
- Decision: what the team has decided to do
- Consequences: both positive and negative consequences of the decision
Amazon Web Services (AWS) also provide useful advice about how and why the ADR process should be adopted.
Lifecycle of an ADR
In a new project or one with a lot of decisions to be made, consider setting up regular ADR discussion and review meetings. This will give the team time to work through these decisions.
Each decision goes through the following phases.
“Proposed” status
Assuming you’re using GitHub for version control, there is no need to record a ‘status’ for an ADR when it is first proposed. The status of your ADR’s pull request (PR) reflects the status of the decision until it has been accepted.
Consider using GitHub’s “draft PR” feature especially if you have lots of decisions at varying levels of readiness.
It’s important to find the right group of reviewers for your ADR. Ensure that you have input from all relevant technical disciplines (such as site reliability engineers, developers, security architects, etc) and from any teams that might be affected.
Some programmes use technical writers to ensure their ADRs meet our quality standards and are as accessible to the widest audience possible. Future team members that lack the full context you have for a decision will appreciate your efforts!
“Accepted” status
Once an ADR has been accepted then teams may need to know about its existence long into the future. Think about how to make your ADRs discoverable, especially if they require more than just a one-off set of code or infrastructure changes.
Again, the “accepted” status doesn’t need to be explicit in your document - it can be assumed that an
ADR in the main branch of your GitHub repository is accepted.
Otherwise, teams can get confused about the real status of a decision, as the ADR goes through
the pull request process!
Implementation
In some cases there will be an implementation phase during which you may discover that the decision was wrong, or needs to be amended.
If there are clarifications, then updating your existing ADR may be appropriate.
You might find additional consequences to your decision, so consider updating the Consequences section to reflect your evolving understanding of the problem space.
If a decision was never implemented at all, then as long as your reviewers and other stakeholders agree then it may be appropriate to update your existing ADR with a new decision.
However, if some implementation of the original decision has occurred, you should write a new ADR to explain the new decision.
Any ADRs that have not been fully implemented across all the relevant teams should be a topic for your regular discussion and review meeting.
Consider using your issue tracker to make sure that decisions you’ve made are followed through and put into effect. The work isn’t done until the decision is implemented!
“Superseded” status
If a decision has been superseded by another decision, the old ADR must be clearly marked as “superseded”.
A “Status: superseded” line should appear directly under the heading. Add a link to the new ADR as soon as it’s been accepted.
What an ADR is not
An ADR isn’t a complete description of a software system or architecture.
Some teams use requests for comment (RFCs), wikis, team manuals, or other collaboration tools to document their architecture, and these should be updated whenever a new ADR is accepted.