Add testing best practices for Elasticsearch

doc/development/testing_guide/best_practices.md

@@ -851,6 +851,46 @@ using expectations, or dependency injection along with stubs, to avoid the need
 for modifications. If you have no other choice, an `around` block like the global
 variables example can be used, but avoid this if at all possible.
 
+#### Elasticsearch specs
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61171) in GitLab 14.0.
+
+Specs that require Elasticsearch must be marked with the `:elastic` trait. This
+creates and deletes indices between examples to ensure a clean index, so that there is no room
+for polluting the tests with nonessential data.
+Most tests for Elasticsearch logic relate to:
+
+- Creating data in Postgres and waiting for it to be indexed in Elasticsearch.
+- Searching for that data.
+- Ensuring that the test gives the expected result.
+
+There are some exceptions, such as checking for structural changes rather than individual records in an index. 
+
+The `:elastic_with_delete_by_query` trait was added to reduce run time for pipelines by creating and deleting indices
+at the start and end of each context only. The [Elasticsearch DeleteByQuery API](https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-delete-by-query.html) 
+is used to delete data in all indices in between examples to ensure a clean index.
+
+Note that Elasticsearch indexing uses [`Gitlab::Redis::SharedState`](../../../ee/development/redis.md#gitlabrediscachesharedstatequeues).
+Therefore, it is recommended to use `:clean_gitlab_redis_shared_state` in conjunction with the Elasticsearch traits.
+
+Specs using Elasticsearch require that you:
+
+- Create data in Postgres and then index it into Elasticsearch.
+- Enable Application Settings for Elasticsearch (which is disabled by default).
+
+To do so, use:
+
+```ruby
+before do
+  stub_ee_application_setting(elasticsearch_search: true, elasticsearch_indexing: true)
+end
+```
+
+Additionally, you can use the `ensure_elasticsearch_index!` method to overcome the asynchronous nature of Elasticsearch.
+It uses the [Elasticsearch Refresh API](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-refresh.html#refresh-api-desc)
+to make sure all operations performed on an index since the last refresh are available for search. This method is typically
+called after loading data into Postgres to ensure the data is indexed and searchable.
+
 #### Test Snowplow events
 
 WARNING: