Software design guides
Use ubiquitous language instead of CRUD terminology
The code should use the same ubiquitous language as used in the product and user documentation. Failure to use ubiquitous language correctly can be a major cause of confusion for contributors and customers when there is constant translation or use of multiple terms. This also goes against our communication strategy.
In the example below, CRUD
terminology introduces ambiguity. The name says we are creating an
association record, but we are adding an existing issue to an epic. The name
used from Rails convention, leaks to higher abstractions such as service objects.
The code speaks the framework jargon rather than ubiquitous language.
# Bad EpicIssues::CreateService
Using ubiquitous language makes the code clear and doesn’t introduce any cognitive load to a reader trying to translate the framework jargon.
# Good Epic::AddExistingIssueService
You can use CRUD when representing simple concepts that are not ambiguous, like creating a project, and when matching the existing ubiquitous language.
# OK: Matches the product language. Projects::CreateService
New classes and database tables should use ubiquitous language. In this case the model name and table name follow the Rails convention.
Existing classes that don’t follow ubiquitous language should be renamed, when possible.
Some low level abstractions such as the database tables don’t need to be renamed.
For example, use
self.table_name= when the model name diverges from the table name.
We can allow exceptions only when renaming is challenging. For example, when the naming is used for STI, exposed to the user, or if it would be a breaking change.
Use namespaces to define bounded contexts
A healthy application is divided into macro and sub components that represent the contexts at play, whether they are related to business domain or infrastructure code.
As GitLab code has so many features and components it’s hard to see what contexts are involved. We should expect any class to be defined inside a module/namespace that represents the contexts where it operates.
When we namespace classes inside their domain:
- Similar terminology becomes unambiguous as the domain clarifies the meaning:
- Top-level namespaces could be associated to one or more groups identified as domain experts.
- We can better identify the interactions and coupling between components.
For example, several classes inside
MergeRequests::domain interact more with
Ci::domain and less with
A good guideline for naming a top-level namespace (bounded context) is to use the related
Continuous Integration feature category maps to
# bad class JobArtifact end # good module Ci class JobArtifact end end
Projects and Groups are generally container concepts because they identify tenants.
They allow features to exist at the project or group level, like repositories or runners,
but do not nest such features under
Groups:: namespaces should be used only for concepts that are strictly related to them:
For controllers we allow
app/controllers/groups to be exceptions.
We use this convention to indicate the scope of a given web endpoint.
Do not use the stage or group name because a feature category could be reassigned to a different group in the future.
# bad module Create class Commit end end # good module Repositories class Commit end end
On the other hand, a feature category may sometimes be too granular. Features tend to be treated differently according to Product and Marketing, while they may share a lot of domain models and behavior under the hood. In this case, having too many bounded contexts could make them shallow and more coupled with other contexts.
Bounded contexts (or top-level namespaces) can be seen as macro-components in the overall app.
Good bounded contexts should be deep
so consider having nested namespaces to further break down complex parts of the domain.
For example, instead of having separate and granular bounded contexts like:
ContainerNetworkSecurity::, we could have:
module ContainerSecurity module HostSecurity end module NetworkSecurity end module Scanning end end
If classes that are defined into a namespace have a lot in common with classes in other namespaces, chances are that these two namespaces are part of the same bounded context.