SpacetimeDB C++ Module Library

The SpacetimeDB C++ Module Library provides a modern C++20 API for building SpacetimeDB modules that run inside the database as WebAssembly.

Current State

This library provides a production-ready C++ bindings for SpacetimeDB with complete type system support:

✅ Features

• Module compilation and publishing to SpacetimeDB
• All lifecycle reducers (init, client_connected, client_disconnected)
• User-defined reducers with unlimited parameters
• Table registration with constraints (PrimaryKey, Unique, AutoInc)
• Insert, update and delete operations
• All primitive types (u8-u256, i8-i256, bool, f32, f64, string)
• All special types (Identity, ConnectionId, Timestamp, TimeDuration, Uuid, Result<>)
• Vector types for all primitives and special types
• Optional types (std::optional)
• Custom struct serialization via BSATN
• Complex enum support with proper variant names
• Enhanced logging system with file/line info

🏗️ Architecture

• Hybrid Compile-Time/Runtime System: C++20 concepts for compile-time validation with preinit runtime registration
• V9 Type Registration System: Unified type registration with comprehensive error detection and circular reference prevention
• Nominal Type System: Types identified by their declared names with explicit registration via SPACETIMEDB_STRUCT macros
• Multi-Layer Validation: Static assertions, runtime constraint checking, and error module replacement strategy

See ARCHITECTURE.md for detailed technical documentation.

✅ Advanced Features Available

• Btree indexes: Full support with FIELD_Index macros and optimized queries
• Range queries: Complete range query system with range_from(), range_to(), range_inclusive(), etc.
• Client visibility filters: Row-level security with SPACETIMEDB_CLIENT_VISIBILITY_FILTER macro
• Scheduled reducers: SPACETIMEDB_SCHEDULE macro for time-based execution
• Procedures: Pure functions with return values using SPACETIMEDB_PROCEDURE macro
• Views: Read-only query functions with SPACETIMEDB_VIEW macro
• Field accessor patterns: Efficient indexed operations with ctx.db[table_field]

See the working examples in modules/*-cpp/src/lib.cpp for comprehensive feature usage.

Features

• Modern C++20 API: Uses concepts, structured bindings, and other C++20 features
• BSATN Serialization: Binary Serialization And Type Notation for efficient data transfer
• Automatic Field Registration: Tables register their fields using SPACETIMEDB_STRUCT macro
• Unified Reducer System: Single macro for all reducer types with automatic lifecycle detection
• Type-Safe Database Access: Template-based table accessors with compile-time type checking
• Memory Safety: WASI shims for safe memory operations in WebAssembly environment
• Enhanced Logging: Multiple log levels with file/line information
• Namespace Support: Clean namespace qualification for enums with just 2 lines of code

Prerequisites

• Emscripten SDK (emsdk)
• CMake 3.16+
• C++20 compatible compiler

Quick Start

Option 1: Using spacetime init (Recommended)

# Create a new C++ project
spacetime init --lang cpp my-project
cd my-project

# Build and publish
spacetime build -p ./spacetimedb
spacetime publish -p ./spacetimedb my-database

Option 2: Manual Setup

For existing projects, add the following to your C++ module:

#include <spacetimedb.h>

using namespace SpacetimeDB;

// Define a table structure
struct User {
    Identity identity;
    std::string name;
    std::string email;
};

// Register BSATN serialization
SPACETIMEDB_STRUCT(User, identity, name, email)

// Register as a table
SPACETIMEDB_TABLE(User, users, Public)

// Add constraints using FIELD_ macros
FIELD_PrimaryKey(users, identity);
FIELD_Unique(users, email);

// Define an enum with namespace qualification
SPACETIMEDB_ENUM(UserRole, Admin, Moderator, Member)
SPACETIMEDB_NAMESPACE(UserRole, "Auth")  // Will be "Auth.UserRole" in client code

// User-defined reducer
SPACETIMEDB_REDUCER(add_user, ReducerContext ctx, std::string name, std::string email) {
    User user{ctx.sender, name, email}; // id will be auto-generated
    ctx.db[users].insert(user);
    LOG_INFO("Added user: " + name);
    return Ok();
}

// Delete user by id (using primary key)
SPACETIMEDB_REDUCER(delete_user, ReducerContext ctx) {
    ctx.db[users_identity].delete_by_key(ctx.sender);
    return Ok();
}

// Lifecycle reducers (optional)
SPACETIMEDB_INIT(init, ReducerContext ctx) {
    LOG_INFO("Module initialized");
    return Ok();
}

SPACETIMEDB_CLIENT_CONNECTED(on_connect, ReducerContext ctx) {
    LOG_INFO("Client connected: " + ctx.sender.to_hex_string());
    return Ok();
}

SPACETIMEDB_CLIENT_DISCONNECTED(on_disconnect, ReducerContext ctx) {
    LOG_INFO("Client disconnected: " + ctx.sender.to_hex_string());
    return Ok();
}

// Define a view for querying data (finds the calling user)
SPACETIMEDB_VIEW(std::optional<User>, find_my_user, Public, ViewContext ctx) {
    // Use indexed field to find user by their identity
    return ctx.db[users_identity].find(ctx.sender);
}

// Define a procedure (pure function with return value)
SPACETIMEDB_PROCEDURE(uint32_t, add_numbers, ProcedureContext ctx, uint32_t a, uint32_t b) {
    return a + b;
}

Building Modules

Build Steps

# Navigate to your module directory
cd modules/your-module

# Build the project
spacetime build -p ./spacetimedb

# Publish to SpacetimeDB
spacetime publish --bin-path ./spacetimedb/build/lib.wasm your-database-name
# Or use the directory (auto-detects build/lib.wasm)
spacetime publish ./spacetimedb your-database-name

Custom Module Source

To build a different source file:

# Build a specific test module
emcmake cmake -B build -DMODULE_SOURCE=src/test_module.cpp -DOUTPUT_NAME=test_module .
cmake --build build
# This creates build/test_module.wasm

API Reference

Macros

Table Definition

• SPACETIMEDB_TABLE(Type, table_name, Public/Private) - Register a table
• SPACETIMEDB_STRUCT(Type, field1, field2, ...) - Register type for BSATN serialization

Enum Definition

• SPACETIMEDB_ENUM(EnumName, Value1, Value2, ...) - Define a simple enum
• SPACETIMEDB_ENUM(EnumName, (Variant1, Type1), (Variant2, Type2), ...) - Define an enum with payloads
• SPACETIMEDB_NAMESPACE(EnumName, "Namespace") - Add namespace qualification to an enum

Reducers

• SPACETIMEDB_REDUCER(name, ReducerContext ctx, ...) - User-defined reducer
  • Returns ReducerResult (alias for Outcome<void>)
  • Use return Ok(); for success or return Err("message"); for errors
  • Failed reducers (Err) trigger transaction rollback
• SPACETIMEDB_INIT(name, ReducerContext ctx) - Module initialization reducer (optional)
• SPACETIMEDB_CLIENT_CONNECTED(name, ReducerContext ctx) - Client connection reducer (optional)
• SPACETIMEDB_CLIENT_DISCONNECTED(name, ReducerContext ctx) - Client disconnection reducer (optional)

Views

• SPACETIMEDB_VIEW(return_type, name, Public/Private, ViewContext ctx) - Read-only query function
• SPACETIMEDB_VIEW(return_type, name, Public/Private, AnonymousViewContext ctx) - Anonymous view (no sender identity)
• Note: Views currently only support the context parameter (no additional parameters yet)

Procedures

• SPACETIMEDB_PROCEDURE(return_type, name, ProcedureContext ctx, ...) - Pure function that returns a value
  • Returns the type directly (not wrapped in Outcome)
  • Can return any SpacetimeType (primitives, structs, enums, Unit, etc.)
  • Database access requires explicit transactions (use ctx.WithTx() or ctx.TryWithTx())
  • Always public (no access control)

Field Constraints (applied after table registration)

• FIELD_PrimaryKey(table_name, field) - Primary key constraint
• FIELD_PrimaryKeyAutoInc(table_name, field) - Auto-incrementing primary key
• FIELD_Unique(table_name, field) - Unique constraint
• FIELD_UniqueAutoInc(table_name, field) - Auto-incrementing unique field
• FIELD_Index(table_name, field) - Index for faster queries
• FIELD_IndexAutoInc(table_name, field) - Auto-incrementing indexed field
• FIELD_AutoInc(table_name, field) - Auto-increment without other constraints

Logging

LOG_DEBUG("Debug message");
LOG_INFO("Info message");
LOG_WARN("Warning message");
LOG_ERROR("Error message");
LOG_PANIC("Fatal error message");

// With timing
{
    LogStopwatch timer("Operation name");
    // ... code to time ...
} // Automatically logs duration

Architecture

The library uses a sophisticated hybrid compile-time/runtime architecture:

• Compile-Time Validation (table_with_constraints.h): C++20 concepts and static assertions for constraint validation
• V9 Type Registration System (internal/v9_type_registration.h): Unified type registration with error detection and circular reference prevention
• Priority-Ordered Initialization (internal/Module.cpp): preinit functions with numbered priorities ensure correct registration order
• Error Detection System (internal/Module.cpp): Multi-layer validation with error module replacement for clear diagnostics
• BSATN Serialization (bsatn/): Binary serialization system with algebraic type support for all data types
• Database Interface (database.h, table_with_constraints.h): Type-safe table access with optimized field accessors
• Reducer System (reducer_macros.h): Unified macro system for all reducer types with parameter type capture
• Logging (logger.h): Comprehensive logging with source location tracking

For detailed technical documentation, see ARCHITECTURE.md.

Note on Architecture Documentation: ARCHITECTURE.md contains references to some legacy implementation details. The current implementation is streamlined and production-ready.

Limitations

1. Type System

   • Very large type combinations may exceed WASM memory limits
   • Complex recursive type references require careful ordering

2. Database Operations

   • Index-based operations use field accessors: ctx.db[table_field].delete_by_key(value)
   • Table constraints are declared and enforced by server
   • Supports insert, delete, and update operations through field accessors

3. Advanced Features

   • Btree indexes: FIELD_Index creates btree indexes for efficient range queries
   • Range queries: Full support for range_from(), range_to(), range_inclusive(), etc.
   • Client visibility filters: Row-level security via SPACETIMEDB_CLIENT_VISIBILITY_FILTER
   • Limited migrations: Only adding tables supported automatically
   • SQL execution: Available via CLI (spacetime sql) but not within modules

Examples

See the modules/*-cpp/src/ directory for example modules:

• lib.cpp - Comprehensive working module with all primitive types, tables, and reducers
• Full equivalence with Rust and C# SDK test modules
• Examples of all constraint types and database operations

Contributing

This library is part of the SpacetimeDB project. Please see the main repository for contribution guidelines.