Document ways of accessing Session data in GitLabdoc-jej/session-stored-globaly

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diff --git a/doc/development/README.md b/doc/development/README.md
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@@ -44,6 +44,7 @@ description: 'Learn how to contribute to GitLab.'
 - [`Gemfile` guidelines](gemfile.md)
 - [Pry debugging](pry_debugging.md)
 - [Sidekiq debugging](sidekiq_debugging.md)
+- [Accessing session data](session.md)
 - [Gotchas](gotchas.md) to avoid
 - [Avoid modules with instance variables](module_with_instance_variables.md) if possible
 - [How to dump production data to staging](db_dump.md)
diff --git a/doc/development/session.md b/doc/development/session.md
new file mode 100644
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+# Accessing session data
+
+Session data in GitLab is stored in Redis and can be accessed in a variety of ways.
+
+During a web request, for example:
+
+- Rails provides access to the session from within controllers through [`ActionDispatch::Session`](https://guides.rubyonrails.org/action_controller_overview.html#session).
+- Outside of controllers, it is possible to access the session through `Gitlab::Session`.
+
+Outside of a web request it is still possible to access sessions stored in Redis. For example:
+
+- Session IDs and contents can be [looked up directly in Redis](#redis).
+- Data about the UserAgent associated with the session can be accessed through `ActiveSession`.
+
+When storing values in a session it is best to:
+
+- Use simple primitives and avoid storing objects to avoid marshaling complications.
+- Clean up after unneeded variables to keep memory usage in Redis down.
+
+## Gitlab::Session
+
+Sometimes you might want to persist data in the session instead of another store like the database. `Gitlab::Session` lets you access this without passing the session around extensively. For example, you could access it from within a policy without having to pass the session through to each place permissions are checked from.
+
+The session has a hash-like interface, just like when using it from a controller. There is also `NamespacedSessionStore` for storing key-value data in a hash.
+
+```ruby
+# Lookup a value stored in the current session
+Gitlab::Session.current[:my_feature]
+
+# Modify the current session stored in redis
+Gitlab::Session.current[:my_feature] = value
+
+# Store key-value data namespaced under a key
+Gitlab::NamespacedSessionStore.new(:my_feature)[some_key] = value
+
+# Set the session for a block of code, such as for tests
+Gitlab::Session.with_session(my_feature: value) do
+  # Code that uses Session.current[:my_feature]
+end
+```
+
+## Redis
+
+Session data can be accessed directly through Redis. This can let you check up on a browser session when debugging.
+
+```ruby
+# Get a list of sessions
+session_ids = Gitlab::Redis::SharedState.with do |redis|
+  redis.smembers("#{Gitlab::Redis::SharedState::USER_SESSIONS_LOOKUP_NAMESPACE}:#{user.id}")
+end
+
+# Retrieve a specific session
+session_data = Gitlab::Redis::SharedState.with { |redis| redis.get("#{Gitlab::Redis::SharedState::SESSION_NAMESPACE}:#{session_id}") }
+Marshal.load(session_data)
+```
+
+## Getting device information with ActiveSession
+
+The [**Active Sessions** page on a user's profile](../user/profile/active_sessions.md) displays information about the device used to access each session. The methods used there to list sessions can also be useful for development.
+
+```ruby
+# Get list of sessions for a given user
+# Includes session_id and data from the UserAgent
+ActiveSession.list(user)
+```