Split into sections, port `downgrade_ee_to_ce` doc from EEdoc-updating-gitlab

[ci skip]

1 files changed, 82 insertions, 0 deletions

diff --git a/doc/downgrade_ee_to_ce/README.md b/doc/downgrade_ee_to_ce/README.md
new file mode 100644
index 00000000000..3625c4191b8
--- /dev/null
+++ b/doc/downgrade_ee_to_ce/README.md
@@ -0,0 +1,82 @@
+# Downgrading from EE to CE
+
+If you ever decide to downgrade your Enterprise Edition back to the Community
+Edition, there are a few steps you need take before installing the CE package
+on top of the current EE package, or, if you are in an installation from source,
+before you change remotes and fetch the latest CE code.
+
+## Disable Enterprise-only features
+
+First thing to do is to disable the following features.
+
+### Authentication mechanisms
+
+Kerberos and Atlassian Crowd are only available on the Enterprise Edition, so
+you should disable these mechanisms before downgrading and you should provide
+alternative authentication methods to your users.
+
+### Git Annex
+
+Git Annex is also only available on the Enterprise Edition. This means that if
+you have repositories that use Git Annex to store large files, these files will
+no longer be easily available via Git. You should consider migrating these
+repositories to use Git LFS before downgrading to the Community Edition.
+
+### Remove Jenkins CI Service entries from the database
+
+The `JenkinsService` class is only available on the Enterprise Edition codebase,
+so if you downgrade to the Community Edition, you'll come across the following
+error:
+
+```
+Completed 500 Internal Server Error in 497ms (ActiveRecord: 32.2ms)
+
+ActionView::Template::Error (The single-table inheritance mechanism failed to locate the subclass: 'JenkinsService'. This
+error is raised because the column 'type' is reserved for storing the class in case of inheritance. Please rename this
+column if you didn't intend it to be used for storing the inheritance class or overwrite Service.inheritance_column to
+use another column for that information.)
+```
+
+All services are created automatically for every project you have, so in order
+to avoid getting this error, you need to remove all instances of the
+`JenkinsService` from your database:
+
+**Omnibus Installation**
+
+```
+$ sudo gitlab-rails runner "Service.where(type: 'JenkinsService').delete_all"
+```
+
+**Source Installation**
+
+```
+$ bundle exec rails runner "Service.where(type: 'JenkinsService').delete_all" production
+```
+
+## Downgrade to CE
+
+After performing the above mentioned steps, you are now ready to downgrade your
+GitLab installation to the Community Edition.
+
+**Omnibus Installation**
+
+To downgrade an Omnibus installation, it is sufficient to install the Community
+Edition package on top of the currently installed one. You can do this manually,
+by directly [downloading the package](https://packages.gitlab.com/gitlab/gitlab-ce)
+you need, or by adding our CE package repository and following the
+[CE installation instructions](https://about.gitlab.com/downloads/).
+
+**Source Installation**
+
+To downgrade a source installation, you need to replace the current remote of
+your GitLab installation with the Community Edition's remote, fetch the latest
+changes, and checkout the latest stable branch:
+
+```
+$ git remote set-url origin git@gitlab.com:gitlab-org/gitlab-ce.git
+$ git fetch --all
+$ git checkout 8-x-stable
+```
+
+Remember to follow the correct [update guides](../update/README.md) to make
+sure all dependencies are up to date.