Merge branch 'docs-interacting-components-to-reason-about' into 'master'

Document interacting GitLab components and features

See merge request gitlab-org/gitlab-ce!30813

doc/development/README.md

@@ -63,6 +63,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Routing](routing.md)
 - [Repository mirroring](repository_mirroring.md)
 - [Git LFS](lfs.md)
+- [Developing against interacting components or features](interacting_components.md)
 
 ## Performance guides
 

doc/development/interacting_components.md

+# Developing against interacting components or features
+
+It's not uncommon that a single code change can reflect and interact with multiple parts of GitLab
+codebase. Furthermore, an existing feature might have an underlying integration or behavior that
+might go unnoticed even by reviewers and maintainers.
+
+The goal of this section is to briefly list interacting pieces to think about
+when making _backend_ changes that might involve multiple features or [components](architecture.md#components).
+
+## Uploads
+
+GitLab supports uploads to [object storage]. That means every feature and 
+change that affects uploads should also be tested against [object storage], 
+which is _not_ enabled by default in [GDK](https://gitlab.com/gitlab-org/gitlab-development-kit).
+
+When working on a related feature, make sure to enable and test it
+against [Minio](https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/object_storage.md).
+
+See also [File Storage in GitLab](file_storage.md).
+
+## Merge requests
+
+### Forks
+
+GitLab supports a great amount of features for [merge requests](../user/project/merge_requests/index.md). One
+of them is the ability to create merge requests from and to [forks](../gitlab-basics/fork-project.md),
+which should also be highly considered and tested upon development phase.
+
+[object storage]: https://docs.gitlab.com/charts/advanced/external-object-storage/