River

⚠️ Not production ready, while Replit is using parts of River in production, we are still going through rapid breaking changes. First production ready version will be 1.x.x ⚠️

River allows multiple clients to connect to and make remote procedure calls to a remote server as if they were local procedures.

Long-lived streaming remote procedure calls

River provides a framework for long-lived streaming Remote Procedure Calls (RPCs) in modern web applications, featuring advanced error handling and customizable retry policies to ensure seamless communication between clients and servers.

River provides a framework similar to tRPC and gRPC but with additional features:

JSON Schema Support + run-time schema validation
full-duplex streaming
service multiplexing
result types and error handling
snappy DX (no code generation)
transparent reconnect support for long-lived sessions
over any transport (WebSockets and Unix Domain Socket out of the box)

See PROTOCOL.md for more information on the protocol.

Prerequisites

Before proceeding, ensure you have TypeScript 5 installed and configured appropriately:

Ensure your tsconfig.json is configured correctly:

You must verify that:
- compilerOptions.moduleResolution is set to "bundler"
- compilerOptions.strictFunctionTypes is set to true
- compilerOptions.strictNullChecks is set to true
or, preferably, that:
- compilerOptions.moduleResolution is set to "bundler"
- compilerOptions.strict is set to true
Like so:
```
{
  "compilerOptions": {
    "moduleResolution": "bundler",
    "strict": true
    // Other compiler options...
  }
}
```
If these options already exist in your tsconfig.json and don't match what is shown above, modify them. River is designed for "strict": true, but technically only strictFunctionTypes and strictNullChecks being set to true is required. Failing to set these will cause unresolvable type errors when defining services.
Install River and Dependencies:

To use River, install the required packages using npm:
```
npm i @replit/river @sinclair/typebox
```

Writing services

Concepts

Router: a collection of services, namespaced by service name.
Service: a collection of procedures with a shared state.
Procedure: a single procedure. A procedure declares its type, a request data type, a response data type, optionally a response error type, and the associated handler. Valid types are:
- rpc, single request, single response
- upload, multiple requests, single response
- subscription, single request, multiple responses
- stream, multiple requests, multiple response
Transport: manages the lifecycle (creation/deletion) of connections and multiplexing read/writes from clients. Both the client and the server must be passed in a subclass of Transport to work.
- Connection: the actual raw underlying transport connection
- Session: a higher-level abstraction that operates over the span of potentially multiple transport-level connections
Codec: encodes messages between clients/servers before the transport sends it across the wire.

A basic router

First, we create a service using ServiceSchema:

import { ServiceSchema, Procedure, Ok } from '@replit/river';
import { Type } from '@sinclair/typebox';

export const ExampleService = ServiceSchema.define(
  // configuration
  {
    // initializer for shared state
    initializeState: () => ({ count: 0 }),
  },
  // procedures
  {
    add: Procedure.rpc({
      requestInit: Type.Object({ n: Type.Number() }),
      responseData: Type.Object({ result: Type.Number() }),
      requestErrors: Type.Never(),
      // note that a handler is unique per user RPC
      async handler({ ctx, reqInit: { n } }) {
        // access and mutate shared state
        ctx.state.count += n;
        return Ok({ result: ctx.state.count });
      },
    }),
  },
);

Then, we create the server:

import http from 'http';
import { WebSocketServer } from 'ws';
import { WebSocketServerTransport } from '@replit/river/transport/ws/server';
import { createServer } from '@replit/river';

// start websocket server on port 3000
const httpServer = http.createServer();
const port = 3000;
const wss = new WebSocketServer({ server: httpServer });
const transport = new WebSocketServerTransport(wss, 'SERVER');

export const server = createServer(transport, {
  example: ExampleService,
});

export type ServiceSurface = typeof server;

httpServer.listen(port);

In another file for the client (to create a separate entrypoint),

import { WebSocketClientTransport } from '@replit/river/transport/ws/client';
import { createClient } from '@replit/river';
import { WebSocket } from 'ws';

const transport = new WebSocketClientTransport(
  async () => new WebSocket('ws://localhost:3000'),
  'my-client-id',
);

const client = createClient(
  transport,
  'SERVER', // transport id of the server in the previous step
  { eagerlyConnect: true }, // whether to eagerly connect to the server on creation (optional argument)
);

// we get full type safety on `client`
// client.<service name>.<procedure name>.<procedure type>()
// e.g.
const result = await client.example.add.rpc({ n: 3 });
if (result.ok) {
  const msg = result.payload;
  console.log(msg.result); // 0 + 3 = 3
}

Logging

To add logging, you can bind a logging function to a transport.

import { coloredStringLogger } from '@replit/river/logging';

const transport = new WebSocketClientTransport(
  async () => new WebSocket('ws://localhost:3000'),
  'my-client-id',
);

transport.bindLogger(console.log);
// or
transport.bindLogger(coloredStringLogger);

You can define your own logging functions that satisfy the LogFn type.

Connection status

River defines two types of reconnects:

Transparent reconnects: These occur when the connection is temporarily lost and reestablished without losing any messages. From the application's perspective, this process is seamless and does not disrupt ongoing operations.
Hard reconnect: This occurs when all server state is lost, requiring the client to reinitialize anything stateful (e.g. subscriptions).

Hard reconnects are signaled via sessionStatus events.

If your application is stateful on either the server or the client, the service consumer should wrap all the client-side setup with transport.addEventListener('sessionStatus', (evt) => ...) to do appropriate setup and teardown.

transport.addEventListener('sessionStatus', (evt) => {
  if (evt.status === 'connect') {
    // do something
  } else if (evt.status === 'disconnect') {
    // do something else
  }
});

// or, listen for specific session states
transport.addEventListener('sessionTransition', (evt) => {
  if (evt.state === SessionState.Connected) {
    // switch on various transition states
  } else if (evt.state === SessionState.NoConnection) {
    // do something
  }
});

Custom Handshake

River allows you to extend the protocol-level handshake so you can add additional logic to validate incoming connections.

You can do this by passing extra options to createClient and createServer and extending the ParsedMetadata interface:

declare module '@replit/river' {
  interface ParsedMetadata {
    userId: number;
  }
}

const schema = Type.Object({ token: Type.String() });
createClient<typeof services>(new MockClientTransport('client'), 'SERVER', {
  eagerlyConnect: false,
  handshakeOptions: createClientHandshakeOptions(schema, async () => ({
    // the type of this function is
    // () => Static<typeof schema> | Promise<Static<typeof schema>>
    token: '123',
  })),
});

createServer(new MockServerTransport('SERVER'), services, {
  handshakeOptions: createServerHandshakeOptions(
    schema,
    (metadata, previousMetadata) => {
      // the type of this function is
      // (metadata: Static<typeof<schema>, previousMetadata?: ParsedMetadata) =>
      //   | false | Promise<false> (if you reject it)
      //   | ParsedMetadata | Promise<ParsedMetadata> (if you allow it)
      // next time a connection happens on the same session, previousMetadata will
      // be populated with the last returned value
    },
  ),
});

You can then access the ParsedMetadata in your procedure handlers:

async handler(ctx, ...args) {
  // this contains the parsed metadata
  console.log(ctx.metadata)
}

Further examples

We've also provided an end-to-end testing environment using Next.js, and a simple backend connected with the WebSocket transport that you can play with on Replit.

You can find more service examples in the E2E test fixtures

Developing

npm i -- install dependencies
npm run check -- lint
npm run format -- format
npm run test -- run tests
npm run publish -- cut a new release (should bump version in package.json first)