Merge branch 'gitaly-docs' into 'master'

Added some Gitaly docs to `docs/development`

[ci skip]

See merge request gitlab-org/gitlab-ce!14473

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diff --git a/doc/development/README.md b/doc/development/README.md
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@@ -44,6 +44,7 @@
 - [Building a package for testing purposes](build_test_package.md)
 - [Manage feature flags](feature_flags.md)
 - [View sent emails or preview mailers](emails.md)
+- [Working with Gitaly](gitaly.md)
 
 ## Databases
 
diff --git a/doc/development/gitaly.md b/doc/development/gitaly.md
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+# GitLab Developers Guide to Working with Gitaly
+
+[Gitaly](https://gitlab.com/gitlab-org/gitaly) is a high-level Git RPC service used by GitLab CE/EE,
+Workhorse and GitLab-Shell. All Rugged operations in GitLab CE/EE are currently being phased out to
+be replaced by Gitaly API calls.
+
+Visit the [Gitaly Migration Board](https://gitlab.com/gitlab-org/gitaly/boards/331341) for current
+status of the migration.
+
+## Feature Flags
+
+Gitaly makes heavy use of [feature flags](feature_flags.md).
+
+Each Rugged-to-Gitaly migration goes through a [series of phases](https://gitlab.com/gitlab-org/gitaly/blob/master/doc/MIGRATION_PROCESS.md):
+* **Opt-In**: by default the Rugged implementation is used.
+  * Production instances can choose to enable the Gitaly endpoint by enabling the feature flag.
+  * For testing purposes, you may wish to enable all feature flags by default. This can be done by exporting the following
+    environment variable: `GITALY_FEATURE_DEFAULT_ON=1`.
+  * On developer instances (ie, when `Rails.env.development?` is true), the Gitaly endpoint
+    is enabled by default, but can be _disabled_ using feature flags.
+* **Opt-Out**: by default, the Gitaly endpoint is used, but the feature can be explicitly disabled using the feature flag.
+* **Madatory**: The migration is complete and cannot be disabled. The old codepath is removed.
+
+### Enabling and Disabling Feature
+
+In the Rails console, type:
+
+```ruby
+Feature.enable(:gitaly_feature_name)
+Feature.disable(:gitaly_feature_name)
+```
+
+Where `gitaly_feature_name` is the name of the Gitaly feature. This can be determined by finding the appropriate
+`gitaly_migrate` code block, for example:
+
+```ruby
+gitaly_migrate(:tag_names) do
+...
+end
+```
+
+Since Gitaly features are always prefixed with `gitaly_`, the name of the feature flag in this case would be `gitaly_tag_names`.
+
+## Gitaly-Related Test Failures
+
+If your test-suite is failing with Gitaly issues, as a first step, try running:
+
+```shell
+rm -rf tmp/tests/gitaly
+```
+
+---
+
+[Return to Development documentation](README.md)