Crash Course

Features

• Long-Lived, Stateful Compute: Each unit of compute is like a tiny server that remembers things between requests – no need to re-fetch data from a database or worry about timeouts. Like AWS Lambda, but with memory and no timeouts.
• Blazing-Fast Reads & Writes: State is stored on the same machine as your compute, so reads and writes are ultra-fast. No database round trips, no latency spikes. State is persisted to Rivet for long term storage, so it survives server restarts.
• Realtime: Update state and broadcast changes in realtime with WebSockets. No external pub/sub systems, no polling – just built-in low-latency events.
• Infinitely Scalable: Automatically scale from zero to millions of concurrent actors. Pay only for what you use with instant scaling and no cold starts.
• Fault Tolerant: Built-in error handling and recovery. Actors automatically restart on failure while preserving state integrity and continuing operations.

When to Use Rivet Actors

• AI agents & sandboxes: multi-step toolchains, conversation memory, sandbox orchestration.
• Multiplayer or collaborative apps: CRDT docs, shared cursors, realtime dashboards, chat.
• Workflow automation: background jobs, cron, rate limiters, durable queues, backpressure control.
• Data-intensive backends: geo-distributed or per-tenant databases, in-memory caches, sharded SQL.
• Networking workloads: WebSocket servers, custom protocols, local-first sync, edge fanout.

Minimal Project

Backend

index.ts

Client Docs

Use the client SDK that matches your app:

• JavaScript Client
• React Client
• Swift Client

Actor Quick Reference

In-Memory State

Persistent data that survives restarts, crashes, and deployments. State is persisted on Rivet Cloud or Rivet self-hosted, so it survives restarts if the current process crashes or exits.

Documentation

Keys

Keys uniquely identify actor instances. Use compound keys (arrays) for hierarchical addressing:

Don't build keys with string interpolation like "org:${userId}" when userId contains user data. Use arrays instead to prevent key injection attacks.

Documentation

Input

Pass initialization data when creating actors. Input is only available in createState and onCreate, so store it in state if you need it later.

Documentation

Temporary Variables

Temporary data that doesn't survive restarts. Use for non-serializable objects (event emitters, connections, etc).

Documentation

Actions

Actions are the primary way clients and other actors communicate with an actor.

Documentation

Events & Broadcasts

Events enable real-time communication from actors to connected clients.

Documentation

Connections

Access the current connection via c.conn or all connected clients via c.conns. Use c.conn.id or c.conn.state to securely identify who is calling an action. c.conn is only available for actions invoked through a connected client; stateless actor-handle calls run without a connection, so guard against that. Connection state is initialized via connState or createConnState, which receives parameters passed by the client on connect.

Documentation

Queues

Use queues to process durable messages in order inside a run loop.

Documentation

Workflows

Use workflows when your run logic needs durable, replayable multi-step execution.

Documentation

Actor-to-Actor Communication

Actors can call other actors using c.client().

Documentation

Scheduling

Schedule actions to run after a delay or at a specific time. Schedules persist across restarts, upgrades, and crashes.

Documentation

Destroying Actors

Permanently delete an actor and its state using c.destroy().

Documentation

Lifecycle Hooks

Actors support hooks for initialization, background processing, connections, networking, and state changes. Use run for long-lived background loops, and use c.aborted or c.abortSignal for graceful shutdown.

Documentation

Context Types

When writing helper functions outside the actor definition, use *ContextOf<typeof myActor> to extract the correct context type. Helpers like ActionContextOf, CreateContextOf, ConnContextOf, and ConnInitContextOf are exported from "rivetkit". Do not manually define your own context interface. Always derive it from the actor definition.

Documentation

Errors

Use UserError to throw errors that are safely returned to clients. Pass metadata to include structured data. Other errors are converted to generic "internal error" for security.

Documentation

Low-Level HTTP & WebSocket Handlers

For custom protocols or integrating libraries that need direct access to HTTP Request/Response or WebSocket connections, use onRequest and onWebSocket.

HTTP Handler Documentation · WebSocket Handler Documentation

Icons & Names

Customize how actors appear in the UI with display names and icons. It's recommended to always provide a name and icon to actors in order to make them easier to distinguish in the dashboard.

import { actor } from "rivetkit";

const chatRoom = actor({
	options: {
		name: "Chat Room",
		icon: "💬", // or FontAwesome: "comments", "chart-line", etc.
	},
	// ...
});

Documentation

Client Documentation

Find the full client guides here:

• JavaScript Client
• React Client
• Swift Client

Common Patterns

Actors scale naturally through isolated state and message-passing. Structure your applications with these patterns:

Documentation

Actor Per Entity

Create one actor per user, document, or room. Use compound keys to scope entities:

Coordinator & Data Actors

Data actors handle core logic (chat rooms, game sessions, user data). Coordinator actors track and manage collections of data actors—think of them as an index.

Run Loop

Use a run loop for continuous background work inside an actor. Process queue messages in order, run logic on intervals, stream AI responses, or coordinate long-running tasks.

Workflow Loop

Use this pattern for long-lived, durable workflows that initialize resources, process commands in a loop, then clean up.

Documentation

Actions vs Queues

• Actions are not durable. Use them for realtime reads, ephemeral data, and low-latency communication like player input.
• Queues are durable. Use them to serialize mutations through the run loop, avoiding race conditions with SQLite and other local state. Callers can still wait for a response from queued work.

Authentication, Security, & CORS

• Validate credentials in onBeforeConnect or createConnState and throw an error to reject unauthorized connections.
• Use c.conn.state to securely identify users in actions rather than trusting action parameters.
• For cross-origin access, validate the request origin in onBeforeConnect.

Authentication Documentation · CORS Documentation

Versions & Upgrades

When deploying new code, set a version number so Rivet can route new actors to the latest runner and optionally drain old ones. Use a build timestamp, git commit count, or CI build number as the version. It is very important to configure versioning before deploying to production. Without versioning, actors can regress by running on older runner versions, and existing actors will never be forced to migrate to new runners. They will continue running indefinitely on the old runners until they exit.

Documentation

Anti-Patterns

Never build a "god" actor

Do not put all your logic in a single actor. A god actor serializes every operation through one bottleneck, kills parallelism, and makes the entire system fail as a unit. Split into focused actors per entity.

Never create an actor per request

Actors are long-lived and maintain state across requests. Creating a new actor for every incoming request throws away the core benefit of the model and wastes resources on actor creation and teardown. Use actors for persistent entities and regular functions for stateless work.