feat: add types to show API shape and design

- Once we have consensus on the API and types, I'll move on to
implementation details and tests
- Have to allow dead code and unused right now until we fully
implement and test the API
- Have to ignore doc tests temporarily for the same reason

Author: pmorris-dev

Cargo.lock

@@ -8,4 +8,5 @@ rust-version = "1.89"
 
 [dependencies]
 sqlx = { version = "0.8.6", features = ["runtime-tokio", "sqlite"] }
+thiserror = "2.0.17"
 tokio = { version = "1.48.0", features = ["full"] }

crates/sqlx-sqlite-conn-mgr/Cargo.toml

@@ -8,19 +8,20 @@ management.
 
 ## Features
 
-   * **Single connection pool per database**: Prevents violation of access policies
-     and/or a glut of open file handles and (mostly) idle threads
+   * **Maintains one read connection pool and one write connection per database**:
+     Prevents violation of access policies and/or a glut of open file handles and
+     (mostly) idle threads
    * **Connection pooling**:
       * Read-only pool for concurrent reads (up to 6 connections)
-   * **Lazy write pool**: Single write connection pool (max=1) initialized on
+   * **Lazy write pool**: Single write connection pool (max_connections=1) initialized on
      first use
    * **Exclusive write access**: WriteGuard ensures serialized writes
-     (enforced by max_connections=1)
    * **WAL mode**: Automatically enabled on first `acquire_writer()` call
      (idempotent)
+      * See [WAL documentation](https://www.sqlite.org/wal.html) for details
    * **30-second idle timeout**: Both read and write connections close after
      30 seconds of inactivity
-   * **No perpetual caching**: Zero minimum connections (min_connections=0) to
+   * **No perpetual connection caching**: Zero minimum connections (min_connections=0) to
      avoid idle thread overhead
 
 ## Design Philosophy
@@ -110,9 +111,11 @@ queries.
    is released via `WriteGuard` drop.
 
 5. **Connection Management**:
-   * Read pool: max 6 concurrent connections, 0 cached
+   * Read pool: 6 concurrent connections by default, 0 cached
+      * Can be configured via `SqliteDatabaseConfig`
    * Write pool: max 1 connection, 0 cached
    * Idle timeout: 30 seconds for both pools
+      * Can be configured via `SqliteDatabaseConfig`
    * No perpetual caching to minimize idle thread overhead
 
 ## Error Handling

crates/sqlx-sqlite-conn-mgr/README.md

@@ -0,0 +1,54 @@
+//! Configuration for SQLite database connection pools
+
+use std::time::Duration;
+
+/// Configuration for SqliteDatabase connection pools
+///
+/// # Examples
+///
+/// ```
+/// use sqlx_sqlite_conn_mgr::SqliteDatabaseConfig;
+/// use std::time::Duration;
+///
+/// // Use defaults
+/// let config = SqliteDatabaseConfig::default();
+///
+/// // Customize specific fields
+/// let config = SqliteDatabaseConfig {
+///     max_read_connections: 3,
+///     idle_timeout: Duration::from_secs(60),
+/// };
+///
+/// // Override just one field
+/// let config = SqliteDatabaseConfig {
+///     max_read_connections: 3,
+///     ..Default::default()
+/// };
+/// ```
+#[derive(Debug, Clone)]
+pub struct SqliteDatabaseConfig {
+   /// Maximum number of concurrent read connections
+   ///
+   /// This controls the size of the read-only connection pool.
+   /// Higher values allow more concurrent read queries but consume more resources.
+   ///
+   /// Default: 6
+   pub max_read_connections: u32,
+
+   /// Idle timeout for both read and write connections
+   ///
+   /// Connections that remain idle for this duration will be closed automatically.
+   /// This helps prevent resource exhaustion from idle threads.
+   ///
+   /// Default: 30 seconds
+   pub idle_timeout: Duration,
+}
+
+impl Default for SqliteDatabaseConfig {
+   fn default() -> Self {
+      Self {
+         max_read_connections: 6,
+         idle_timeout: Duration::from_secs(30),
+      }
+   }
+}

crates/sqlx-sqlite-conn-mgr/src/config.rs

@@ -0,0 +1,45 @@
+//! SQLite database with connection pooling and optional write access
+
+use sqlx::{Pool, Sqlite};
+use std::path::PathBuf;
+use std::sync::atomic::AtomicBool;
+
+/// SQLite database with connection pooling for concurrent reads and optional exclusive writes.
+///
+/// ## Architecture
+///
+/// The database maintains two connection pools:
+/// - **`read_pool`**: Pool of read-only connections for concurrent reads
+/// - **`write_conn`**: Single-connection pool for exclusive write access (enforced by max_connections=1)
+///
+/// ## State Management
+///
+/// - **`wal_initialized`**: Tracks whether WAL journal mode has been enabled (lazy initialization)
+/// - **`closed`**: Prevents use after the database has been closed
+/// - **`path`**: Database file path for cleanup operations
+///
+/// ## Usage Pattern
+///
+/// ```text
+/// 1. Connect to database (creates/reuses connection pools)
+/// 2. Read operations: Access read_pool for concurrent reads
+/// 3. Write operations: Acquire writer (lazily enables WAL on first call)
+/// 4. Close database when done
+/// ```
+#[derive(Debug)]
+pub struct SqliteDatabase {
+   /// Pool of read-only connections (defaults to max_connections=6) for concurrent reads
+   read_pool: Pool<Sqlite>,
+
+   /// Single read-write connection pool (max_connections=1) for serialized writes
+   write_conn: Pool<Sqlite>,
+
+   /// Tracks if WAL mode has been initialized (set on first write)
+   wal_initialized: AtomicBool,
+
+   /// Marks database as closed to prevent further operations
+   closed: AtomicBool,
+
+   /// Path to database file (used for cleanup and registry lookups)
+   path: PathBuf,
+}

crates/sqlx-sqlite-conn-mgr/src/database.rs

@@ -0,0 +1,23 @@
+//! Error types for sqlx-sqlite-conn-mgr
+
+use thiserror::Error;
+
+/// Errors that may occur when working with sqlx-sqlite-conn-mgr
+#[derive(Error, Debug)]
+pub enum Error {
+   /// IO error when accessing database files. Standard library IO errors
+   /// are converted to this variant.
+   #[error("IO error: {0}")]
+   Io(#[from] std::io::Error),
+
+   /// Error from the sqlx library. Standard sqlx errors are converted to this variant
+   #[error("Sqlx error: {0}")]
+   Sqlx(#[from] sqlx::Error),
+
+   /// Database has been closed and cannot be used
+   #[error("Database has been closed")]
+   DatabaseClosed,
+}
+
+/// A type alias for Results with our Error type
+pub type Result<T> = std::result::Result<T, Error>;

crates/sqlx-sqlite-conn-mgr/src/error.rs

@@ -1 +1,33 @@
-//! # SQLx Connection Pool Manager
+//! # sqlx-sqlite-conn-mgr
+//!
+//! A minimal wrapper around SQLx that enforces pragmatic SQLite connection policies
+//! for mobile and desktop applications.
+//!
+//! ## Core Types
+//!
+//! - **[`SqliteDatabase`]**: Main database type with separate read and write connection pools
+//! - **[`SqliteDatabaseConfig`]**: Configuration for connection pool settings
+//! - **[`WriteGuard`]**: RAII guard ensuring exclusive write access
+//! - **[`Error`]**: Error type for database operations
+//!
+//! ## Architecture
+//!
+//! - **Dual pools**: Separate read-only pool (max 6 connections) and write pool (max 1 connection)
+//! - **Lazy WAL mode**: Write-Ahead Logging enabled automatically on first write
+//! - **Exclusive writes**: Single-connection write pool enforces serialized write access
+//! - **Concurrent reads**: Multiple readers can query simultaneously via the read pool
+
+// TODO: Remove these allows once implementation is complete
+#![allow(dead_code)]
+#![allow(unused)]
+
+mod config;
+mod database;
+mod error;
+mod write_guard;
+
+// Re-export public types
+pub use config::SqliteDatabaseConfig;
+pub use database::SqliteDatabase;
+pub use error::{Error, Result};
+pub use write_guard::WriteGuard;

crates/sqlx-sqlite-conn-mgr/src/lib.rs

@@ -0,0 +1,62 @@
+//! WriteGuard for exclusive write access to the database
+
+use sqlx::Sqlite;
+use sqlx::pool::PoolConnection;
+use sqlx::sqlite::SqliteConnection;
+use std::ops::{Deref, DerefMut};
+
+/// RAII guard for exclusive write access to a database connection
+///
+/// This guard wraps a pool connection and returns it to the pool on drop.
+/// Only one `WriteGuard` can exist at a time (enforced by max_connections=1),
+/// ensuring serialized write access.
+///
+/// The guard derefs to `SqliteConnection` allowing direct use with sqlx queries.
+///
+/// # Example
+/// TODO: Remove ignore once implementation is complete
+/// ```ignore
+/// use sqlx_sqlite_conn_mgr::SqliteDatabase;
+/// use sqlx::query;
+///
+/// # async fn example() -> Result<(), sqlx_sqlite_conn_mgr::Error> {
+/// let db = SqliteDatabase::connect("test.db").await?;
+/// let mut writer = db.acquire_writer().await?;
+/// // Use &mut *writer for write queries (e.g. INSERT/UPDATE/DELETE)
+/// query("INSERT INTO users (name) VALUES (?)")
+///     .bind("Alice")
+///     .execute(&mut *writer)
+///     .await?;
+/// // Writer is automatically returned when dropped
+/// # Ok(())
+/// # }
+/// ```
+#[derive(Debug)]
+pub struct WriteGuard {
+   conn: PoolConnection<Sqlite>,
+}
+
+impl WriteGuard {
+   /// Create a new WriteGuard by taking ownership of a pool connection
+   pub(crate) fn new(conn: PoolConnection<Sqlite>) -> Self {
+      Self { conn }
+   }
+}
+
+impl Deref for WriteGuard {
+   type Target = SqliteConnection;
+
+   fn deref(&self) -> &Self::Target {
+      &*self.conn
+   }
+}
+
+impl DerefMut for WriteGuard {
+   fn deref_mut(&mut self) -> &mut Self::Target {
+      &mut *self.conn
+   }
+}
+
+// Drop is automatically implemented - PoolConnection returns itself to the pool
+
+// WriteGuard is automatically Send because PoolConnection<Sqlite> is Send