SpacetimeDB C++ Module Quickstart

This guide will walk you through creating your first SpacetimeDB module in C++. We'll build a simple chat server to demonstrate the core concepts.

What is a SpacetimeDB Module?

A SpacetimeDB module is C++ code that gets compiled to WebAssembly and runs inside the database. Instead of the traditional architecture (database → app server → clients), SpacetimeDB lets you write your entire backend logic that runs inside the database itself, giving you microsecond latency and automatic real-time sync to clients.

Modules consist of four main components:

Tables: Database tables defined as C++ structs
Reducers: Functions that modify data and can be called by clients
Views: Read-only query functions that return data (std::vector or std::optional) to clients
Procedures: Pure functions that return values and can optionally access the database via transactions

Prerequisites

Before we begin, make sure you have:

SpacetimeDB CLI installed
Emscripten SDK (emsdk) for WebAssembly compilation
CMake 3.16+
A C++20 compatible compiler

Creating Your First Module

Step 1: Initialize the Project

Create a new C++ module using the SpacetimeDB CLI:

spacetime init --lang cpp my-chat-module
cd my-chat-module

This creates a project with the following structure:

my-chat-module/spacetimedb/
├── CMakeLists.txt
├── src/
    └── lib.cpp
└── .gitignore

Step 2: Define Your Data Structures

Open lib.cpp and replace the generated code with our chat server implementation:

#include <spacetimedb.h>

using namespace SpacetimeDB;

// Define a User table to store connected users
struct User {
    Identity identity;      // SpacetimeDB's built-in user identity type
    std::optional<std::string> name;  // User's display name (optional)
    bool online;           // Whether the user is currently connected
};

// Register the struct for BSATN serialization
SPACETIMEDB_STRUCT(User, identity, name, online)

// Register as a public table with identity as primary key
SPACETIMEDB_TABLE(User, user, Public)
FIELD_PrimaryKey(user, identity);

// Define a Message table to store chat messages
struct Message {
    Identity sender;       // Who sent the message
    Timestamp sent;        // When the message was sent
    std::string text;      // Message content
};

SPACETIMEDB_STRUCT(Message, sender, sent, text)
SPACETIMEDB_TABLE(Message, message, Public)

Step 3: Add Helper Functions and Reducers

First, add validation helper functions:

// Validate that a name is not empty, return an Outcome which houses a error as std::string
Outcome<std::string> validate_name(const std::string& name) {
    if (name.empty()) {
        return Err<std::string>("Names must not be empty");
    }
    return Ok(name);
}

// Validate that a message is not empty, return an Outcome which houses a error as std::string
Outcome<std::string> validate_message(const std::string& text) {
    if (text.empty()) {
        return Err<std::string>("Messages must not be empty");
    }
    return Ok(text);
}

Now add the reducers (functions that clients can call to modify the database):

// Called when a user sets their name
SPACETIMEDB_REDUCER(set_name, ReducerContext ctx, std::string name) {
    auto validated = validate_name(name);
    if (validated.is_err()) {
        return Err(validated.error());
    }
    
    // Find and update the user by identity (primary key)
    auto user_row = ctx.db[user_identity].find(ctx.sender);
    if (user_row.has_value()) {
        auto user = user_row.value();
        user.name = validated.value();
        ctx.db[user_identity].update(user);
        return Ok();
    }
    
    return Err("Cannot set name for unknown user");
}

// Called when a user sends a message
SPACETIMEDB_REDUCER(send_message, ReducerContext ctx, std::string text) {
    auto validated = validate_message(text);
    if (validated.is_err()) {
        return Err(validated.error());
    }
    
    Message msg{ctx.sender, ctx.timestamp, validated.value()};
    ctx.db[message].insert(msg);
    return Ok();
}

Step 4: Add Lifecycle Reducers

Lifecycle reducers are special functions called automatically by SpacetimeDB:

// Called when a client connects
SPACETIMEDB_CLIENT_CONNECTED(client_connected, ReducerContext ctx) {
    auto user_row = ctx.db[user_identity].find(ctx.sender);
    if (user_row.has_value()) {
        auto user = user_row.value();
        user.online = true;
        ctx.db[user_identity].update(user);
    } else {
        User new_user{ctx.sender, std::nullopt, true};
        ctx.db[user].insert(new_user);
    }
    return Ok();
}

// Called when a client disconnects  
SPACETIMEDB_CLIENT_DISCONNECTED(client_disconnected, ReducerContext ctx) {
    auto user_row = ctx.db[user_identity].find(ctx.sender);
    if (user_row.has_value()) {
        auto user = user_row.value();
        user.online = false;
        ctx.db[user_identity].update(user);
    } else {
        LOG_WARN("Disconnect event for unknown user");
    }
    return Ok();
}

Step 5: Build Your Module

Build the module using the provided CMake configuration:

spacetime build -p ./spacetimedb

This compiles your C++ code to WebAssembly, producing build/lib.wasm.

Step 6: Publish to SpacetimeDB

Start your local SpacetimeDB instance:

spacetime start

Publish your module:

spacetime publish . my-chat-db

Step 7: Test Your Module

You can test your reducers using the CLI:

# Set a user's name
spacetime call my-chat-db set_name "Alice"

# Send a message
spacetime call my-chat-db send_message "Hello, world!"

# View all users
spacetime sql my-chat-db "SELECT * FROM user"

# View all messages
spacetime sql my-chat-db "SELECT * FROM message"

Key Concepts Explained

Tables vs. Structs

Structs are just data types - they need SPACETIMEDB_STRUCT for serialization
Tables are database tables created with SPACETIMEDB_TABLE and store data persistently
The same struct can be used for multiple tables or just as a data type

Database Access Pattern

SpacetimeDB C++ uses a unique accessor pattern:

ctx.db[tableName] - Access table for iteration and basic operations, eg. ctx.db[user]
ctx.db[tableName_fieldName] - Access indexed fields for optimized operations, eg. ctx.db[user_id]

// Table access
ctx.db[user].insert(new_user);

// Field accessor (for indexed fields, e.g., user_identity = table 'user' + field 'identity')
ctx.db[user_identity].delete_by_key(identity);

Constraints and Indexes

Constraints are applied after table registration using FIELD_ macros:

SPACETIMEDB_TABLE(User, user, Public)
FIELD_PrimaryKey(user, identity);       // Primary key
FIELD_Unique(user, email);              // Unique constraint
FIELD_Index(user, age);                 // Index for fast queries

Public vs. Private Tables

Public tables: Automatically synced to subscribed clients
Private tables: Only accessible by reducers, not synced to clients

Next Steps

Now that you have a basic chat server:

Add more features: User roles, message editing, channels
Add constraints: Unique usernames, message length limits
Explore indexing: For fast queries on large datasets
Try scheduled reducers: For periodic cleanup or notifications
Generate client code: Use spacetime generate to create TypeScript, C#, or Rust clients

Advanced Example: Adding Indexes

You can add an index to make querying messages by sender more efficient:

SPACETIMEDB_TABLE(Message, message, Public)
FIELD_Index(message, sender);  // Add index on sender for faster queries

// Now you can efficiently query by sender using the field accessor:
SPACETIMEDB_REDUCER(get_user_messages, ReducerContext ctx, Identity user_identity) {
    for (const auto& msg : ctx.db[message_sender].filter(user_identity)) {
        LOG_INFO("Message: " + msg.text);
    }
    return Ok();
}

Troubleshooting

Build errors: Ensure you have the latest Emscripten SDK and are using emcmake cmake

Module not found: Check that SpacetimeDB is running

Type errors: Remember that C++ types need exact matches - use uint32_t, not int

Constraint violations: Constraints are enforced by the database - duplicate primary keys will cause reducers to fail

Example Projects

For more complex examples, see:

modules/sdk-test-cpp/src/lib.cpp - Comprehensive type and operation testing
modules/module-test-cpp/src/lib.cpp - Advanced indexing and enum examples

Ready to learn more? Check out the C++ Reference Documentation for detailed API information.

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SpacetimeDB C++ Module Quickstart

What is a SpacetimeDB Module?

Prerequisites

Creating Your First Module

Step 1: Initialize the Project

Step 2: Define Your Data Structures

Step 3: Add Helper Functions and Reducers

Step 4: Add Lifecycle Reducers

Step 5: Build Your Module

Step 6: Publish to SpacetimeDB

Step 7: Test Your Module

Key Concepts Explained

Tables vs. Structs

Database Access Pattern

Constraints and Indexes

Public vs. Private Tables

Next Steps

Advanced Example: Adding Indexes

Troubleshooting

Example Projects

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SpacetimeDB C++ Module Quickstart

What is a SpacetimeDB Module?

Prerequisites

Creating Your First Module

Step 1: Initialize the Project

Step 2: Define Your Data Structures

Step 3: Add Helper Functions and Reducers

Step 4: Add Lifecycle Reducers

Step 5: Build Your Module

Step 6: Publish to SpacetimeDB

Step 7: Test Your Module

Key Concepts Explained

Tables vs. Structs

Database Access Pattern

Constraints and Indexes

Public vs. Private Tables

Next Steps

Advanced Example: Adding Indexes

Troubleshooting

Example Projects