In an Agile work environment, architecture is described and handled a bit differently meaning that not all decisions will be made upfront, when the projects start nor at once (actually, one the benefits of having and architecture is the possibility of deferring decisions).
As Agile methods are not opposed to documentation (only to meaningless one), we need a way of keeping track of architecture decisions taken during a project or across the whole organisation and documenting them. As nobody likes large documents (and large documents are never kept up to date), a small, modular, “agile” document format would be desired.
In a world of evolutionary architecture it is important to record certain design decisions. Whenever you are going back to a certain project to review the architecture or somebody new joins the team, there is a strong possibility the rationales for past decisions are lost, forgotten or worst, propagated by the “word of mouth”, “we always did it this way” and in this case one can only blindly accept the decisions or change it. To avoid such a behaviour, a repository of past architecture decisions and rationales behind them must be kept in an orderly manner, either linking those decisions to past project or to organisational best practices.
An architecture decision record (ADR) is a document that captures an important architectural decision made, along with it’s context, rationale and consequences.
An architecture decision (AD) is a software design choice (or project design choice) that address a significant requirement.
An architecture decision log (ADL) is the collections of all ADRs created and maintained for a project or organisation.
We keep a collection of records for architectural decisions; those that affects the structure, the composition, non-functional characteristics, interfaces, deployment strategies, etc.
An ADR is a short text file in a format similar to RFCs. Each record describes a set of constraints and assumptions and a single decision in response.
In my particular case, we keep the ADRs in a Confluence repository, with a naming convention like ADR-NNN-Subject, where NNN is a unique number. All ADRs are numbered sequentially and numbers will not be reused. Where an ADR is deprecated, then we mark this in the file name as ADR-NNN-Subject-Deprecated, at least to have a history of those decisions. In addition, when and ADR becomes deprecated because it was superseded by another ADR, we mention this in the file content.
The ADR contains, at minimum, the following sections:
- A meaningful name for ADR
- What is the status of ADR, such as proposed, accepted, deprecated, superseded
- What is the problem we encounter that is motivating the change or the proposal. This section describes the forces at play, technological, projects, constraints.
- What is the change or action we’re proposing
- This section describes the resulting context after applying the decision. All consequences should be listed, both negative and positive as each decision will have not only positive consequences but also negative and neutral that will affect the technological landscape from now on and fore coming projects.
- All involved parties (developers, stakeholders, PMs) can see ADRs, even if the teams composition change over time and the motivation for previous decisions is visible for everyone.
Of course, you can include also other sections in the ADR, as it is fit for your practice. In my case, I preferred to keep them as simple as possible, until we need further sections.
Examples of ADRs, simple and complex, you can find here.
We use ADRs at the organisation level, to record important decisions for our architecture, like “ADR-002-SplitDNS” which states the problem we encountered, the rationale behind the decision of implementing SplitDNS and all the consequences we can foresee. Organisational level ADRs ar related to stuff like how do we publish an API, what is the naming convention for IT, what system is Master Data System for client data, what the client identity management architecture, and so on.
At the same time, I encourage development teams to keep their own internal ADRs for each project (especially in an Agile setup), where we can have a log of all design and architecture decisions for a software project made at team level and the rationale behind them.
Same important is to also document decisions that have not been taken. In my ADRs I usually write also what other options were on the table when the decision was made and the pros and cons for each one and why they weren’t chosen. Acting like this, allow us to later review a decision and if the circumstances have changed significantly, we can take a look at what other options we had, maybe something will fit now. Or maybe is time to come out with some new options.
When to write an ADR
An ADR should be written whenever a decision with a significant impact is made; it’s up to the organisation or team to decide what “significant impact” means. An example of such a decision was that databases for non critic applications will be MariaDB.
Sometimes a decision was made in the past, or an implicit decision was mutually taken but never documented (“we always did it this way”) and it’s not clear what the decision was and what was the rationale behind it. In such a situation, writing an ADR, formalizing an unofficial decision or standard it’s a must.
Whenever we are doing a change to a system (well, not for every system, for the moment only for critical ones) we are making also an impact assessment and document the change and impact in an ADR because it has to be clear for everyone how the system evolved during various changed, how the system was extended and more.
For short, whenever a technical decision is made, which has some impact, document it in an ADR.