# Software Design Description (SDD) for `apollo-rust-client`
This document conforms to the **IEEE Std 1016™-2009 Standard for Information Technology—Systems Design—Software Design Descriptions**. It provides a comprehensive technical blueprint of the `apollo-rust-client` crate, outlining its architecture, module structures, interface specifications, and data design.
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## 1. Introduction
### 1.1 Purpose
The purpose of this Software Design Description (SDD) is to detail the design specifications of `apollo-rust-client`. This library acts as a robust, cross-platform client for the **Apollo Configuration Center**, serving native Rust backend services and WebAssembly (WASM) targets (such as modern browsers or Node.js applications).
### 1.2 System Scope
The `apollo-rust-client` is designed to:
- Establish secure authentication and fetch real-time and static configurations from Apollo Configuration servers.
- Handle multiple configuration formats including `Properties`, `JSON`, `YAML`, and plain `Text`.
- Provide multi-level caching strategies, optimizing native targets with disk-based persistence and WebAssembly targets with browser localStorage caching and serverless memory-only fallbacks.
- Support real-time updates via background polling worker threads and an asynchronous observer (event-listener) mechanism.
- Expose direct bindings to JS/TS runtimes through WebAssembly bindings managed by `wasm-bindgen`.
### 1.3 Definitions, Acronyms, and Abbreviations
| **Apollo** | Distributed Configuration Center developed by CTrip. |
| **Namespace** | Logical grouping of configuration items (e.g., database configs, app configs). |
| **Cluster** | Deployment environment configuration grouping (e.g., `default`, `production`). |
| **Grayscale Release** | Canary deployment pattern allowing targeting configs to specific IPs or labels. |
| **WASM Heap** | Linear memory space allocated by browsers/Node.js for WebAssembly modules. |
| **Observer Pattern** | Design pattern where observers (event listeners) register to receive state change notices. |
| **Thundering Herd** | Multi-thread race condition where numerous threads simultaneously fetch missing cache values. |
| **SDD** | Software Design Description (IEEE 1016 standard). |
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## 2. Document Map
The SDD documentation is organized into modular design viewpoints for accessibility and maintainability:
1. **[Architectural Design](architectural_design.md)**
- Context, structures, global coordination pattern, and platform execution environments (Native vs. WASM).
2. **[Decomposition Description](decomposition_description.md)**
- Details the structural elements, properties, and roles of the core structures: `ClientConfig`, `Client`, `Cache`, and `Namespace`.
3. **[Interface Design](interface_design.md)**
- External API specifications for Rust and WebAssembly/JS, plus network contract bindings with the Apollo HTTP REST endpoints.
4. **[Data & Concurrency Design](data_design.md)**
- In-memory representations, disk-cache schemas, caching hierarchies, and multi-threaded synchronization models.
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## 3. References
1. **IEEE Std 1016™-2009**: *IEEE Standard for Information Technology — Systems Design — Software Design Descriptions*.
2. **Apollo Configuration Center API Documentation**: CTrip Apollo Open API and Client REST Protocols.
3. **Rust Language Design & Guidelines**: Safe concurrency, memory model, and crate design best practices.
4. **wasm-bindgen Reference Guide**: High-level JS-Rust interop bindings specifications.