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Defining Monorepo Boundaries With Tags

Tags are a way to define boundaries in a monorepo. They are configured in the project.json file of each project, and they affect which projects can depend on each other.
The root .eslintrc.json file specifies our rules for each tag. We recommend reading this article to get up to speed.

Our tag convention

Our tags are split in 2 categories. Generic tags and application tags.

Generic tags

Generic tags are based on the frameworks we use and should be used for simple applications and shared libraries.
These are the generic tags ordered from the most generic to more specific.
Tag
Description
Can depend on
Can be depended on by
js
Basic JS library.
js
Any project
node
NodeJS server-side library.
js, node
Any server-side projects.
dom
JS library designed to run in a browser.
js, dom
Any browser-based project.
react
React library.
js, dom, react
Any react project.
nest
NestJS library.
js, node, nest
NestJS projects.
react-next
NextJS library.
js, dom, react, react-next
NextJS projects.
react-spa
React library using react-router.
js, dom, react, react-spa
React projects using react-router.

Application tags

Simple applications can be tagged with the generic tags above, but applications which have one or more "private" library projects should define their own NX tags, extending one of the generic tags.
It can be one tag which is applied to both the application project and all the associated library projects (like "api"). Or there can be multiple application tags with different rules to segment the library projects (like the "judicial-system" tags).
Tag
Description
Can depend on
Can be depended on by
api
API domain project.
js, node, nest, api, client
Other API projects.
application-system
Application system projects
js, dom, react, react-spa, react, application-system, client
Other application system projects
application-system-web
Application system web projects
js, dom, react, react-spa, application-system-web, application-system,
Other application system web projects
application-system-api
Application system api projects
js, node, nest, client, application-system, application-system-api, api,
Other application system api projects
auth-api
Authentication api projects
js, node, nest, client, auth-api
Other auth APIs
client
Client projects.
js, node, nest, client
Other clients and application-system
e2e
Test projects
js, client, application-system
No one
judicial-system
Judicial system projects
js, judicial-system
Other judicial system projects
judicial-system-api
Judicial system web projects
js, node, nest, client, judicial-system, judicial-system-api
Other judicial system api projects
judicial-system-web
Judicial system api projects
js, dom, react, react-spa, judicial-system, judicial-system-web
Other judicial system web projects
portals
Portal project.
js, dom, react, react-spa, portals
Other portal projects.
portals-mypages
My pages portal project.
js, dom, react, react-spa, portals, portals-mypages
Other my pages portal projects.
portals-admin
Admin portal project.
js, dom, react, react-spa, portals, portals-admin
Other admin portal projects.
Here is a chart that displays how these tags are connected. Each tag can depend on other tags with a full line arrow pointing to it. Dotted lines represents connections between application tags where a project can depend on the tag but not necessarily other tags further up.
flowchart BT;
%% Generic tags
node & dom --> js
react --> dom
react-spa & react-next --> react
nest --> node
%% App tags
judicial-system --> js
api & auth-api & client & application-system-api & judicial-system-api --> nest
application-system & application-system-web & portals & portals-admin & portals-mypages & judicial-system-web --> react-spa
classDef appTag fill:#f9f
class api,auth-api,client,application-system,application-system-web,application-system-api,portals-admin,portals-mypages,judicial-system-api,judicial-system-web,judicial-system appTag
%% Shallow dependencies
api & auth-api -.-> client
application-system -.-> client
application-system-api & application-system-web -.-> application-system
judicial-system-api & judicial-system-web -.-> judicial-system
portals-mypages & portals-mypages -.-> portals

Tag prefixes

The above tags must be prefixed by one or both of the following prefixes to opt projects into "Can depend on" and "Can be depended on by" rules.
  • scope: - by applying a scope tag to a project, you are saying that ESLint should enforce project boundaries for that project using the "Can depend on" rules documented above.
  • lib: - lib tags describe what kind of project it is to enforce the "Can be depended on by" rules documented above.
All projects must have a "scope" tag and most libraries should have the same tag as "lib" and "scope".
Later we hope to get rid of the "lib" tags and just have "scope" tags. However, first we need to fix or refactor a few tricky projects.

Examples

Generic NextJS application:
{
"tags": ["scope:react-next"]
}
React library which works for both NextJS and React SPA projects.
{
"tags": ["lib:react", "scope:react"]
}
NestJS library.
{
"tags": ["lib:nest", "scope:nest"]
}

Fixing module boundary errors

A project tagged with "scope:X" can only depend on libs tagged with "lib:Y", "lib:Z", ... - @nrwl/nx/enforce-module-boundaries
Check which file the code is importing and which tags are configured in the importing project and the imported library.
In some cases, your project should not be importing that library, e.g. it's a library belonging to another application, or you're accidentally importing a server side library from a web project. You should look for ways to refactor your code to not depend on that library. For example, you might move the things you need to another shared library which you are allowed to depend on.
If your project should be able to import that library, then there's some issue with the tags which you need to fix.
Either way, feel free to reach out to the core team and ask for guidance.
External checks: Missing nx tags for project boundaries
Our CI checks that all projects have correct tags. If you get this error, it's likely that you forgot to add a "scope:" tag to a new project.json file. You can re-run the CI check locally by running yarn check-tags.