000. Record Architecture Decisions
Date: 2021-10-01
Status
Accepted
Context
During our work on the Monitor Space Hazards Beta, we will have to make architectural decisions about the service.
When making decisions, we should record them somewhere for future reference, to help us remember why we made them, and to help teams working in related areas understand why we made them.
We should make our decisions public so that other teams can find them more easily, and because making things open makes things better.
Decision
We will use Architecture Decision Records, as described by Michael Nygard in this article
An architecture decision record is a short text file describing a single decision.
We will keep ADRs in this technical documentation site under Decisions
We should use a lightweight text formatting language like Markdown.
ADRs will be numbered sequentially and monotonically. Numbers will not be reused.
If a decision is reversed, we will keep the old one around, but mark it as superseded. (It’s still relevant to know that it was the decision, but is no longer the decision.)
We will use a format with just a few parts, so each document is easy to digest:
Title These documents have names that are short noun phrases. For example, “ADR 1: Record architectural decisions” or “ADR 9: Use Docker for deployment”
Status A decision may be “proposed” if it’s still under discussion, or “accepted” once it is agreed. If a later ADR changes or reverses a decision, it may be marked as “deprecated” or “superseded” with a reference to its replacement.
Context This section describes the forces at play, including technological, political, social, and local to the service. These forces are probably in tension, and should be called out as such. The language in this section is value-neutral. It is simply describing facts.
Decision This section describes our response to these forces. It is stated in full sentences, with active voice. “We will …”
Consequences This section describes the resulting context, after applying the decision. All consequences should be listed here, not just the “positive” ones. A particular decision may have positive, negative, and neutral consequences, but all of them affect the team and service in the future.
The whole document should be one or two pages long. We will write each ADR as if it is a conversation with a future person joining the team. This requires good writing style, with full sentences organised into paragraphs. Bullets are acceptable only for visual style, not as an excuse for writing sentence fragments.
We will link to these ADRs from other documentation where relevant.
Consequences
One ADR describes one significant decision for the service. It should be something that has an effect on how the rest of the service will run.