ES: doc about multi-indices archtecture

Include diagram

doc/development/elasticsearch.md

@@ -148,26 +148,41 @@ Uses an [Edge NGram token filter](https://www.elastic.co/guide/en/elasticsearch/
 - Searches can have their own analyzers. Remember to check when editing analyzers
 - `Character` filters (as opposed to token filters) always replace the original character, so they're not a good choice as they can hinder exact searches
 
-## Architecture
+## Zero downtime reindexing with multiple indices
 
-GitLab uses `elasticsearch-rails` for handling communication with Elasticsearch server. However, in order to achieve zero-downtime deployment during schema changes, an extra abstraction layer is built to allow:
+Currently GitLab can only handle a single version of setting. Any setting/schema changes would require reindexing everything from scratch. Since reindexing can take a long time, this can cause search functionality downtime.
 
-* Indexing (writes) to multiple indexes, with different mappings
-* Switching to different index for searches (reads) on the fly
+To avoid downtime, GitLab is working on to allow multiple indices to function at the same time. Whenever the schema changes, the admin will be able to create a new index and reindex to it, while users still searches using the older stable index. Any data updates would be forwarded to both indices. Once the new index is ready, admin can mark it as the read node where search takes place, and remove the old index.
 
-Currently we are on the process of migrating models to this new design (e.g. `Snippet`), and it is hardwired to work with a single version for now.
+This is also helpful for migrating self-hosted elasticsearch server onto AWS.
 
-Traditionally, `elasticsearch-rails` provides class and instance level `__elasticsearch__` proxy methods. If you call `Issue.__elasticsearch__`, you will get an instance of `Elasticsearch::Model::Proxy::ClassMethodsProxy`, and if you call `Issue.first.__elasticsearch__`, you will get an instance of `Elasticsearch::Model::Proxy::InstanceMethodsProxy`. These proxy objects would talk to Elasticsearch server directly.
+Currently we are on the process of migrating to this new design. Everything is hardwired to work with one single version for now.
 
-In the new design, `__elasticsearch__` instead represents one extra layer of proxy. It would keep multiple versions of the actual proxy objects, and it would forward read and write calls to the proxy of the intended version.
+### Architecture
 
-The `elasticsearch-rails`'s way of specifying each model's mappings and other settings is to create a module for the model to include. However in the new design, each model would have its own corresponding subclassed proxy object, where the settings reside in. For example, snippet related setting in the past reside in `SnippetsSearch` module, but in the new design would reside in `SnippetClassProxy` (which is a subclass of `Elasticsearch::Model::Proxy::ClassMethodsProxy`). This reduces namespace pollution in model classes.
+The traditional setup, provided by `elasticsearch-rails`, is to communicate through its internal proxy classes. Developers would write model-specific logic in a module for the model to include in (e.g. `SnippetsSearch`). The `__elasticsearch__` methods would return a proxy object, e.g.:
+
+- `Issue.__elasticsearch__` returns an instance of `Elasticsearch::Model::Proxy::ClassMethodsProxy`
+- `Issue.first.__elasticsearch__` returns an instance of `Elasticsearch::Model::Proxy::InstanceMethodsProxy`.
+
+These proxy objects would talk to Elasticsearch server directly (see top half of the diagram).
+
+![Elasticsearch Architecture](img/elasticsearch_architecture.svg)
+
+In the planned new design, each model would have a pair of corresponding subclassed proxy objects, where model-specific logic are located at. For example, `Snippet` would have `SnippetClassProxy` and `SnippetInstanceProxy` (being subclass of `Elasticsearch::Model::Proxy::ClassMethodsProxy` and `Elasticsearch::Model::Proxy::InstanceMethodsProxy` respectively).
+
+`__elasticsearch__` would represent another layer of proxy object, keeping track of multiple actual proxy objects. It would forward method calls to the intended proxy. For example, `model.__elasticsearch__.foo` would:
+
+- forward to one stable index, if `foo` is a read method
+- forward to all indices, if `foo` is for document indexing/updating/deleting.
 
 The global configurations per version are now in the `Elastic::(Version)::Config` class. You can change mappings there.
 
 ### Creating new version of schema
 
-Currently GitLab would still work with a single version of setting. Once it is implemented, multiple versions of setting can exists in different folders (e.g. `ee/lib/elastic/v12p1` and `ee/lib/elastic/v12p3`). To keep a continuous git history, the latest version lives under the `/latest` folder, but is aliased as the latest version.
+NOTE: **Note:** this is not applicable yet as multiple indices functionality is not fully implemented.
+
+Since multiple versions of setting can exist in different folders (e.g. `ee/lib/elastic/v12p1` and `ee/lib/elastic/v12p3`). To keep a continuous git history, the latest version lives under the `/latest` folder, but is aliased as the latest version.
 
 If the current version is `v12p1`, and we need to create a new version for `v12p3`, the steps are as follows: