Bug 1894349 - Document for the contentrelevancy component r=bdk,ttran DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D209094

toolkit/components/contentrelevancy/docs/index.md

@@ -1,6 +1,7 @@
 # Content Relevancy
 
-This is the home for the project: Interest-based Content Relevance Ranking & Personalization for Firefox, a client-based privacy preserving approach to enhancing content experience of Firefox.
+This is the home for the project: Interest-based Content Relevance Ranking & Personalization for Firefox,
+a client-based privacy preserving approach to enhancing content experience of Firefox.
 
 ```{toctree}
 :titlesonly:
@@ -9,3 +10,111 @@ This is the home for the project: Interest-based Content Relevance Ranking & Per
 
 telemetry.md
 ```
+
+## Overview
+
+The following diagram illustrates the system overview of the component.
+The system consists of three main parts: the relevancy manager, the relevancy store,
+and the internal & external data sources.
+The cross-platform component [`relevancy`][relevancy-component] serves as the backbone
+that is responsible for interest classification, persistence, and ranking.
+
+```{mermaid}
+graph TB
+subgraph Firefox
+  direction LR
+  subgraph rmgr[Relevancy Manager]
+    subgraph rcmp[Rust Component]
+      classify[Perform Interest Classification]
+      manage[Manage]
+      query[Query & Rank]
+    end
+    iic[Initiate Interest Classification]
+    rtuc[Respond to User Commands]
+    irt[Input Retriever]
+  end
+
+  subgraph places[Places]
+    direction TB
+    tfv[(Top Frecent Visits)]
+    mrv[(Most Recent Visits)]
+    others[(Other Inputs)]
+  end
+
+  subgraph rstore[Relevancy Store]
+    direction TB
+    iui[Inferred User Interests]
+    icm[Ingested Classifier Metadata]
+  end
+end
+
+subgraph settings[Remote Settings]
+  rs[("content-relevancy")]
+end
+
+iic --> |call| classify
+classify --> |write| rstore
+query --> |"query (read-only)"| rstore
+manage --> |update| rstore
+places --> |input| irt --> iic
+rs --- |fetch<br/> classification<br/> data| classify
+rtuc --> |call| manage
+```
+
+### Relevancy Manager
+
+The relevancy manager manages the following tasks for the relevancy component:
+- Start a browser update timer to periodically perform interest classifications
+- Listen and respond to user commands such as enable / disable the component,
+  update / reset the relevancy store upon Places changes, etc.
+- Nimbus integration
+- Telemetry
+
+The interest classification is fulfilled by the `relevancy` Rust component.
+The `relevancy` component downloads & ingests interest data (e.g. "small classifiers")
+from Remote Settings and then applies those classifiers to the chosen inputs.
+Classification results are persisted in the relevancy store for future uses.
+
+### Relevancy Store
+
+The relevancy store, an SQLite database, serves as the storage for both
+the user interests and the ingested classifier metadata. It's managed by the
+`relevancy` Rust component, which provides consumers (e.g. the relevancy manager)
+with a series of APIs for ingestion, querying, ranking, and updating.
+
+### Data Sources
+
+Interest classification relies on various internal & external data sources to function.
+- Interest classifiers and metadata are prepared offline and served through
+  Remote Settings. The download and ingestion are handled by the Rust component
+- Classification inputs (e.g. top frecent visits and most recent visits) are retrieved
+  from Places. An input utility module is provided to facilitate the input retrieval
+
+## Working on the Code
+
+### Component Structure
+
+- `ContentRelevancyManager.sys.mjs` implements the relevancy manager. A singleton
+  relevancy manager is initialized as a background task in `BrowserGlue.sys.mjs`
+- Modules defined in `private` should be treated as internal artifacts for the
+  component and should not be consumed by other browser components.
+  `InputUtils.sys.mjs` defines several utility functions for input retrieval
+
+### Telemetry
+
+Please reference the [telemetry](./telemetry.md) section.
+
+### Testing
+
+Since the component is relatively small and does not have any user facing interfaces,
+XPCShell tests are recommended for general testing. Mochitests can be used if you're
+testing functionalities that would require a more complete browser environment. Only
+the Nimbus tests are written as Mochitests at the moment.
+
+Notes:
+- The component requires a browser profile for both Places and the relevancy store,
+  `do_get_profile()` is called in `head.js`
+- Most XPCShell tests are skipped for Android as some test dependencies (`PlacesTestUtils`)
+  are not available on Android
+
+[relevancy-component]: https://github.com/mozilla/application-services/tree/main/components/relevancy