Reducer Context

Every reducer receives a special context parameter as its first argument. This context provides read-write access to the database, information about the caller, and additional utilities like random number generation.

The reducer context is required for accessing tables, executing database operations, and retrieving metadata about the current reducer invocation.

Accessing the Database

The primary purpose of the reducer context is to provide access to the module's database tables.

TypeScript

import { schema, table, t } from 'spacetimedb/server';

const user = table(
  { name: 'user', public: true },
  {
    id: t.u64().primaryKey().autoInc(),
    name: t.string(),
  }
);

const spacetimedb = schema({ user });
export default spacetimedb;

export const create_user = spacetimedb.reducer({ name: t.string() }, (ctx, { name }) => {
  ctx.db.user.insert({ id: 0n, name });
});

C#

using SpacetimeDB;

public static partial class Module
{
    [SpacetimeDB.Table]
    public partial struct User
    {
        [SpacetimeDB.PrimaryKey]
        [SpacetimeDB.AutoInc]
        public ulong id;
        public string name;
    }

    [SpacetimeDB.Reducer]
    public static void CreateUser(ReducerContext ctx, string name)
    {
        ctx.Db.User.Insert(new User { id = 0, name = name });
    }
}

Rust

use spacetimedb::{table, reducer, ReducerContext};

#[table(accessor = user)]
pub struct User {
    #[primary_key]
    #[auto_inc]
    id: u64,
    name: String,
}

#[reducer]
fn create_user(ctx: &ReducerContext, name: String) {
    ctx.db.user().insert(User { id: 0, name });
}

C++

C++ Modules and SpacetimeDB 2.0

C++ support is currently in beta and subject to change. SpacetimeDB C++ 2.0 is coming soon, but C++ server modules are currently pinned to v1.12.0. If you are following the C++ tab in this guide, use the v1.12.0 release track for now.

#include <spacetimedb.h>
using namespace SpacetimeDB;

struct User {
    uint64_t id;
    std::string name;
};
SPACETIMEDB_STRUCT(User, id, name);
SPACETIMEDB_TABLE(User, user, Public);
FIELD_PrimaryKeyAutoInc(user, id);

SPACETIMEDB_REDUCER(create_user, ReducerContext ctx, std::string name) {
    ctx.db[user].insert(User{0, name});
    return Ok();
}

Caller Information

The context provides information about who invoked the reducer and when.

Sender Identity

Every reducer invocation has an associated caller identity.

TypeScript

import { schema, table, t, type Identity } from 'spacetimedb/server';

const player = table(
  { name: 'player', public: true },
  {
    identity: t.identity().primaryKey(),
    name: t.string(),
    score: t.u32(),
  }
);

const spacetimedb = schema({ player });
export default spacetimedb;

export const update_score = spacetimedb.reducer({ newScore: t.u32() }, (ctx, { newScore }) => {
  // Get the caller's identity
  const caller = ctx.sender;
  
  // Find and update their player record
  const existingPlayer = ctx.db.player.identity.find(caller);
  if (existingPlayer) {
    ctx.db.player.identity.update({
      ...existingPlayer,
      score: newScore,
    });
  }
});

C#

using SpacetimeDB;

public static partial class Module
{
    [SpacetimeDB.Table]
    public partial struct Player
    {
        [SpacetimeDB.PrimaryKey]
        public Identity Identity;
        public string Name;
        public uint Score;
    }

    [SpacetimeDB.Reducer]
    public static void UpdateScore(ReducerContext ctx, uint newScore)
    {
        // Get the caller's identity
        Identity caller = ctx.Sender;
        
        // Find and update their player record
        if (ctx.Db.Player.Identity.Find(caller) is Player player)
        {
            player.Score = newScore;
            ctx.Db.Player.Identity.Update(player);
        }
    }
}

Rust

use spacetimedb::{table, reducer, ReducerContext, Identity};

#[table(accessor = player)]
pub struct Player {
    #[primary_key]
    identity: Identity,
    name: String,
    score: u32,
}

#[reducer]
fn update_score(ctx: &ReducerContext, new_score: u32) {
    // Get the caller's identity
    let caller = ctx.sender();
    
    // Find and update their player record
    if let Some(mut player) = ctx.db.player().identity().find(caller) {
        player.score = new_score;
        ctx.db.player().identity().update(player);
    }
}

C++

#include <spacetimedb.h>
using namespace SpacetimeDB;

struct Player {
    Identity identity;
    std::string name;
    uint32_t score;
};
SPACETIMEDB_STRUCT(Player, identity, name, score);
SPACETIMEDB_TABLE(Player, player, Public);
FIELD_PrimaryKey(player, identity);

SPACETIMEDB_REDUCER(update_score, ReducerContext ctx, uint32_t new_score) {
    // Get the caller's identity
    auto caller = ctx.sender;
    
    // Find and update their player record
    if (auto player = ctx.db[player_identity].find(caller)) {
        player->score = new_score;
        ctx.db[player_identity].update(*player);
    }
    return Ok();
}

Connection ID

The connection ID identifies the specific client connection that invoked the reducer. This is useful for tracking sessions or implementing per-connection state.

note

The connection ID may be None/null/undefined for reducers invoked by the system (such as scheduled reducers or lifecycle reducers) or when called via the CLI without specifying a connection.

Timestamp

The timestamp indicates when the reducer was invoked. This value is consistent throughout the reducer execution and is useful for timestamping events or implementing time-based logic.

Random Number Generation

The context provides access to a random number generator that is deterministic and reproducible. This ensures that reducer execution is consistent across all nodes in a distributed system.

warning

Never use external random number generators (like Math.random() in TypeScript or Random in C# without using the context). These are non-deterministic and will cause different nodes to produce different results, breaking consensus.

Module Identity

The context provides access to the module's own identity, which is useful for distinguishing between user-initiated and system-initiated reducer calls.

This is particularly important for scheduled reducers that should only be invoked by the system, not by external clients.

TypeScript

import { schema, table, t, SenderError } from 'spacetimedb/server';

const scheduledTask = table(
  { name: 'scheduled_task', scheduled: (): any => send_reminder },
  {
    taskId: t.u64().primaryKey().autoInc(),
    scheduledAt: t.scheduleAt(),
    message: t.string(),
  }
);

const spacetimedb = schema({ scheduledTask });
export default spacetimedb;

export const send_reminder = spacetimedb.reducer({ arg: scheduledTask.rowType }, (ctx, { arg }) => {
  // Only allow the scheduler (module identity) to call this
  if (ctx.sender != ctx.identity) {
    throw new SenderError('This reducer can only be called by the scheduler');
  }
  
  console.log(`Reminder: ${arg.message}`);
});

C#

using SpacetimeDB;

public static partial class Module
{
    [SpacetimeDB.Table(Accessor = "ScheduledTask", Scheduled = nameof(SendReminder))]
    public partial struct ScheduledTask
    {
        [SpacetimeDB.PrimaryKey]
        [SpacetimeDB.AutoInc]
        public ulong taskId;
        public ScheduleAt scheduledAt;
        public string message;
    }

    [SpacetimeDB.Reducer]
    public static void SendReminder(ReducerContext ctx, ScheduledTask task)
    {
        // Only allow the scheduler (module identity) to call this
        if (ctx.Sender != ctx.Identity)
        {
            throw new Exception("This reducer can only be called by the scheduler");
        }
        
        Log.Info($"Reminder: {task.message}");
    }
}

Rust

use spacetimedb::{table, reducer, ReducerContext, ScheduleAt};

#[table(accessor = scheduled_task, scheduled(send_reminder))]
pub struct ScheduledTask {
    #[primary_key]
    #[auto_inc]
    task_id: u64,
    scheduled_at: ScheduleAt,
    message: String,
}

#[reducer]
fn send_reminder(ctx: &ReducerContext, task: ScheduledTask) {
    // Only allow the scheduler (module identity) to call this
    if ctx.sender() != ctx.identity() {
        panic!("This reducer can only be called by the scheduler");
    }
    
    spacetimedb::log::info!("Reminder: {}", task.message);
}

C++

#include <spacetimedb.h>
using namespace SpacetimeDB;

struct ScheduledTask {
    uint64_t task_id;
    ScheduleAt scheduled_at;
    std::string message;
};
SPACETIMEDB_STRUCT(ScheduledTask, task_id, scheduled_at, message);
SPACETIMEDB_TABLE(ScheduledTask, scheduled_task, Private);
FIELD_PrimaryKeyAutoInc(scheduled_task, task_id);

// Register the table for scheduling (column 1 = scheduled_at field, 0-based index)
SPACETIMEDB_SCHEDULE(scheduled_task, 1, send_reminder);

SPACETIMEDB_REDUCER(send_reminder, ReducerContext ctx, ScheduledTask task) {
    // Only allow the scheduler (module identity) to call this
    if (ctx.sender != ctx.identity()) {
        return Err("This reducer can only be called by the scheduler");
    }
    
    LOG_INFO("Reminder: " + task.message);
    return Ok();
}

Context Properties Reference

TypeScript

Property	Type	Description
db	DbView	Access to the module's database tables
sender	Identity	Identity of the caller
senderAuth	AuthCtx	Authorization context for the caller (includes JWT claims and internal call detection)
connectionId	ConnectionId | undefined	Connection ID of the caller, if available
timestamp	Timestamp	Time when the reducer was invoked

note

TypeScript uses Math.random() for random number generation, which is automatically seeded deterministically by SpacetimeDB.

C#

Property	Type	Description
Db	DbView	Access to the module's database tables
Sender	Identity	Identity of the caller
SenderAuth	AuthCtx	Authorization context for the caller (includes JWT claims and internal call detection)
ConnectionId	ConnectionId?	Connection ID of the caller, if available
Timestamp	Timestamp	Time when the reducer was invoked
Rng	Random	Random number generator
Identity	Identity	The module's identity

Rust

Property	Type	Description
db	Local	Access to the module's database tables
sender	Identity	Identity of the caller
connection_id	Option<ConnectionId>	Connection ID of the caller, if available
timestamp	Timestamp	Time when the reducer was invoked

Methods:

• identity() -> Identity - Get the module's identity
• rng() -> &StdbRng - Get the random number generator
• random<T>() -> T - Generate a single random value
• sender_auth() -> &AuthCtx - Get authorization context for the caller (includes JWT claims and internal call detection)

C++

Property	Type	Description
db[table]	Table<T>	Access to a specific table's operations
sender	Identity	Identity of the caller
timestamp	Timestamp	Time when the reducer was invoked
connection_id	std::optional<ConnectionId>	Connection ID of the caller, if available

Methods:

• identity() -> Identity - Get the module's identity
• rng() -> StdbRng& - Get the random number generator (deterministic and reproducible)
• sender_auth() -> const AuthCtx& - Get authorization context for the caller (includes JWT claims and internal call detection)

note

C++ uses the std::optional type for the connection_id to represent values that may not be present. The rng() method returns a deterministic random number generator that is seeded consistently across all nodes.