feat(docs): reorganizing content

Author: rafikhan

packages/firestore/devdocs/architecture.md

@@ -2,37 +2,13 @@
 
 This document provides a detailed explanation of the Firestore JavaScript SDK's architecture, its core components, and the flow of data through the system.
 
-## Overview of features
-
-At a high level, all interactions with Firestore can be categorized as either reading or writing data. The SDK provides different mechanisms for these operations, each with distinct guarantees and performance characteristics. There is also a special case of writing data called tansactions detailed below.
-
-
-### Read Operations
-
-There are two fundamental ways to read data from Firestore:
-
-*   **One-Time Reads**: This is for fetching a snapshot of data at a specific moment. It's a simple request-response model. You ask for a document or the results of a query, and the server sends back the data as it exists at that instant.
-
-*   **Real-Time Listeners**: This allows you to subscribe to a document or a query. The server first sends you the initial data and then continues to push updates to your client in real time as the data changes. This is the foundation of Firestore's real-time capabilities.
-
-When a query is executed, the SDK immediately returns data from the local cache, which includes any pending optimistic writes from the **Mutation Queue**. This provides a fast, responsive experience. At the same time, the SDK sends the query to the Firestore backend to fetch the latest version of the documents. When the fresh documents arrive from the backend, the SDK takes these server-authoritative documents and re-applies any pending mutations from the local queue on top of them. It then re-runs the original query against this newly merged data. If the documents still match the query's criteria, they are delivered to the query listener again. This is a common occurrence and means a listener could see an event for the same document twice: first with the cached, optimistic data, and a second time after the backend data is reconciled.
-
-### Write Operations
-
-All data modifications—creates, updates, and deletes—are treated as "writes." The SDK is designed to make writes atomic and resilient.  There are two fundamental ways to write data to Firestore:
-
-*   **One-Time Writes**: When a user performs a write (create, update, or delete), the operation is not sent directly to the backend. Instead, it's treated as a "mutation" and added to the local **Mutation Queue**. The SDK "optimistically" assumes the write will succeed on the backend and immediately reflects the change in the local view of the data, making the change visible to local queries. The SDK then works to synchronize this queue with the backend. This design is crucial for supporting offline functionality, as pending writes can be retried automatically when network connectivity is restored.
-
-*   **Transactions**: This allows you to perform multiple writes as a single atomic operation. Transactions are immediately sent to the server, are **not** guaranteed consistnt with one-time writes, and bypass the other SDK systems.
-
-
 ## Core Components
 
 The SDK is composed of several key components that work together to provide the full range of Firestore features.
 
 ![Architecture Diagram](./architecture.png)
 
-*   **API Layer**: The public-facing API surface that developers use to interact with the SDK.
+*   **API Layer**: The public-facing API surface that developers use to interact with the SDK. This layer is responsible for translating the public API calls into the internal data models and passing them to the appropriate core components.
 *   **Core**:
     *   **Event Manager**: Acts as a central hub for all eventing in the SDK. It is responsible for routing events between the API Layer and Sync Engine. It manages query listeners and is responsible for raising snapshot events, as well as handling connectivity changes and some query failures.
     *   **Sync Engine**: The central controller of the SDK. It acts as the glue between the Event Manager, Local Store, and Remote Store. Its responsibilities include:
@@ -43,8 +19,8 @@ The SDK is composed of several key components that work together to provide the
     *   **Remote Table**: A cache of the most recent version of documents as known by the Firestore backend.
     *   **Mutation Queue**: A queue of all the user-initiated writes (set, update, delete) that have not yet been acknowledged by the Firestore backend.
     *   **Local View**: A cache that represents the user's current view of the data, combining the Remote Table with the Mutation Queue.
-*   **Remote Store**: The component responsible for all network communication with the Firestore backend.
-*   **Persistence Layer**: The underlying storage mechanism (i.e. IndexedDB) used by the Local Store.
+*   **Remote Store**: The component responsible for all network communication with the Firestore backend. It manages the gRPC streams for reading and writing data, and it abstracts away the complexities of the network protocol from the rest of the SDK.
+*   **Persistence Layer**: The underlying storage mechanism used by the Local Store to persist data on the client. In the browser, this is implemented using IndexedDB.
 
 The architecture and systems within the SDK map closely to the directory structure, which helps developers navigate the codebase. Here is a mapping of the core components to their corresponding directories.
 
@@ -57,6 +33,29 @@ The architecture and systems within the SDK map closely to the directory structu
 
 For a more detailed explanation of the contents of each directory, see the [Code Layout](./code-layout.md) documentation.
 
+## Overview of features
+
+At a high level, all interactions with Firestore can be categorized as either reading or writing data. The SDK provides different mechanisms for these operations, each with distinct guarantees and performance characteristics. There is also a special case of writing data called tansactions detailed below.
+
+
+### Read Operations
+
+There are two fundamental ways to read data from Firestore:
+
+*   **One-Time Reads**: This is for fetching a snapshot of data at a specific moment. It's a simple request-response model. You ask for a document or the results of a query, and the server sends back the data as it exists at that instant.
+
+*   **Real-Time Listeners**: This allows you to subscribe to a document or a query. The server first sends you the initial data and then continues to push updates to your client in real time as the data changes. This is the foundation of Firestore's real-time capabilities.
+
+When a query is executed, the SDK immediately returns data from the local cache, which includes any pending optimistic writes from the **Mutation Queue**. This provides a fast, responsive experience. At the same time, the SDK sends the query to the Firestore backend to fetch the latest version of the documents. When the fresh documents arrive from the backend, the SDK takes these server-authoritative documents and re-applies any pending mutations from the local queue on top of them. It then re-runs the original query against this newly merged data. If the documents still match the query's criteria, they are delivered to the query listener again. This is a common occurrence and means a listener could see an event for the same document twice: first with the cached, optimistic data, and a second time after the backend data is reconciled.
+
+### Write Operations
+
+All data modifications—creates, updates, and deletes—are treated as "writes." The SDK is designed to make writes atomic and resilient.  There are two fundamental ways to write data to Firestore:
+
+*   **One-Time Writes**: When a user performs a write (create, update, or delete), the operation is not sent directly to the backend. Instead, it's treated as a "mutation" and added to the local **Mutation Queue**. The SDK "optimistically" assumes the write will succeed on the backend and immediately reflects the change in the local view of the data, making the change visible to local queries. The SDK then works to synchronize this queue with the backend. This design is crucial for supporting offline functionality, as pending writes can be retried automatically when network connectivity is restored.
+
+*   **Transactions**: For grouping multiple write operations into a single atomic unit, the SDK provides `runTransaction`. Unlike standard writes, transactions do not use the optimistic, offline-capable write pipeline. Instead, they are sent directly to the backend, which requires an active internet connection. This ensures atomicity but means transactions do not benefit from the offline capabilities of the standard write pipeline.
+
 
 # Data Flow