Add developer doc for multi-version awareness

Multiple versions of our application can run in production at the same
time and developers should be aware of this

doc/development/README.md

@@ -218,6 +218,7 @@ Complementary reads:
 
 - [Defining relations between files using projections](projections.md)
 - [Reference processing](./reference_processing.md)
+- [Compatibility with multiple versions of the application running at the same time](multi_version_compatibility.md)
 
 ## Other GitLab Development Kit (GDK) guides
 

doc/development/multi_version_compatibility.md

+# Compatibility with multiple versions of the application running at the same time
+
+When adding or changing features, we must be aware that there may be multiple versions of the application running
+at the same time and connected to the same PostgreSQL and Redis databases. This could happen during a rolling deploy
+when the servers are updated one by one.
+
+During a rolling deploy, post-deployment DB migrations are run after all the servers have been updated. This means the
+servers could be in these intermediate states:
+
+1. Old application code running with new DB migrations already executed
+1. New application code running with new DB migrations but without new post-deployment DB migrations
+
+We must make sure that the application works properly in these states.
+
+For GitLab.com, we also run a set of canary servers which run a more recent version of the application. Users with
+the canary cookie set would be handled by these servers. Some URL patterns may also be forced to the canary servers,
+even without the cookie being set. This also means that some pages may match the pattern and get handled by canary servers,
+but AJAX requests to URLs (like the GraphQL endpoint) won't match the pattern.
+
+With this canary setup, we'd be in this mixed-versions state for an extended period of time until canary is promoted to
+production and post-deployment migrations run.
+
+## Examples of previous incidents
+
+### Some links to issues and MRs were broken
+
+When we moved MR routes, users on the new servers were redirected to the new URLs. When these users shared these new URLs in
+Markdown (or anywhere else), they were broken links for users on the old servers.
+
+For more information, see [the relevant issue](https://gitlab.com/gitlab-org/gitlab/-/issues/118840).
+
+### Stale cache in issue or merge request descriptions and comments
+
+We bumped the Markdown cache version and found a bug when a user edited a description or comment which was generated from a different Markdown
+cache version. The cached HTML wasn't generated properly after saving. In most cases, this wouldn't have happened because users would have
+viewed the Markdown before clicking **Edit** and that would mean the Markdown cache is refreshed. But because we run mixed versions, this is
+more likely to happen. Another user on a different version could view the same page and refresh the cache to the other version behind the scenes.
+
+For more information, see [the relevant issue](https://gitlab.com/gitlab-org/gitlab/-/issues/208255).
+
+### Project service templates incorrectly copied
+
+We changed the column which indicates whether a service is a template. When we create services, we copy attributes from the template
+and set this column to `false`. The old servers were still updating the old column, but that was fine because we had a DB trigger
+that updated the new column from the old one. For the new servers though, they were only updating the new column and that same trigger
+was now working against us and setting it back to the wrong value.
+
+For more information, see [the relevant issue](https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9176).
+
+### Sidebar wasn't loading for some users
+
+We changed the data type of one GraphQL field. When a user opened an issue page from the new servers and the GraphQL AJAX request went
+to the old servers, a type mismatch happened, which resulted in a JavaScript error that prevented the sidebar from loading.
+
+For more information, see [the relevant issue](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/1772).
+
+### CI artifact uploads were failing
+
+We added a `NOT NULL` constraint to a column and marked it as a `NOT VALID` constraint so that it is not enforced on existing rows.
+But even with that, this was still a problem because the old servers were still inserting new rows with null values.
+
+For more information, see [the relevant issue](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/1944).