Exchange | Persistence Layer

The exchange_persistence module (formerly core/data) provides the interface for all durable storage operations required by the exchange.

Overview

BetaTrader uses SQLite for persistent storage of user credentials, order history, and trade executions. Because disk I/O is slow, this module is designed to operate asynchronously, ensuring the Matching Engine is never waiting for a disk flush.

Key Responsibilities

• Manage the DatabaseWorker thread.
• Perform CRUD operations for AuthRepository (Logins).
• Persist every executed trade to the trades table.
• Maintain sequence numbers for ID recovery.

Architecture

graph TD
    CORE(Exchange Core) -->|enqueue| DBW[DatabaseWorker]
    
    subgraph "Persistence Thread"
        DBW --> SQL[SQLiteCpp]
        SQL --> FILE(betatrader.db)
    end

Class Diagram

classDiagram
    class DatabaseWorker {
        +enqueue(Query)
        -m_db: SQLite::Database
        -m_queue: spsc_queue
    }
 
    class AuthRepository {
        <<virtual>>
        +validate(username, password)
        +register(username, password)
        -m_dbWorker: DatabaseWorker*
    }
 
    class TradeIDRepository {
        <<virtual>>
        +getCurrentTradeID()
        +setCurrentTradeID(id)
    }
 
    AuthRepository --> DatabaseWorker : Uses
    TradeIDRepository --> DatabaseWorker : Uses

Component Responsibilities

Component	Description
**DatabaseWorker**	Owns the physical SQLite connection. Drains a dedicated SPSC queue to perform writes.
**AuthRepository**	Domain-specific wrapper for user-related queries.
**TradeIDRepository**	Persists the progress of the TradeIDGenerator.

Critical Design Conventions

• Asynchronous Writes: The main thread "fires and forgets" its query intentions.
• Synchronous Reads (Cold Path): Only at server startup or during login are synchronous reads permitted, as these operations are not latency-critical for the trading path.
• Test Mockability: Repositories use virtual methods, allowing the exchange_app_tests suite to inject in-memory mocks that prevent physical disk access during unit testing.