pub_sub 2.3.0

Keep application instances in sync with a simple pub/sub API.

pub_sub

Keep application instances in sync with a simple pub/sub API.

Installation

Add pub_sub as a dependency in your pubspec.yaml file:

dependencies:
  pub_sub: ^1.0.0

Then, be sure to run pub get in your terminal.

Usage

pub_sub is your typical pub/sub API. However, pub_sub enforces authentication of every request. It is very possible that pub_sub will run on both servers and in the browser, or on a platform like Flutter. Thus, there are provisions available to limit access.

Be careful to not leak any pub_sub client ID's if operating over a network. If you do, you risk malicious users injecting events into your application, which could ultimately spell disaster.

A pub_sub server can operate across multiple adapters, which take care of interfacing data over different media. For example, a single server can handle pub/sub between multiple Isolates and TCP Sockets, as well as WebSockets, simultaneously.

import 'package:pub_sub/pub_sub.dart' as pub_sub;

main() async {
  var server = new pub_sub.Server([
    new FooAdapter(...),
    new BarAdapter(...)
  ]);

  server.addAdapter(new BazAdapter(...));

  // Call `start` to activate adapters, and begin handling requests.
  server.start();
}

Trusted Clients

You can use package:pub_sub without explicitly registering clients, if and only if those clients come from trusted sources.

Clients via Isolate are always trusted.

Clients via package:json_rpc_2 must be explicitly marked as trusted (i.e. using an IP whitelist mechanism):

new JsonRpc2Adapter(..., isTrusted: false);

// Pass `null` as Client ID when trusted...
new pub_sub.IsolateClient(null);

Access Control

The ID's of all untrusted clients who will connect to the server must be known at start-up time. You may not register new clients after the server has started. This is mostly a security consideration; if it is impossible to register new clients, then malicious users cannot grant themselves additional privileges within the system.

import 'package:pub_sub/pub_sub.dart' as pub_sub;

main() async {
  // ...
  server.registerClient(const ClientInfo('<client-id>'));

  // Create a user who can subscribe, but not publish.
  server.registerClient(const ClientInfo('<client-id>', canPublish: false));

  // Create a user who can publish, but not subscribe.
  server.registerClient(const ClientInfo('<client-id>', canSubscribe: false));

  // Create a user with no privileges whatsoever.
  server.registerClient(const ClientInfo('<client-id>', canPublish: false, canSubscribe: false));

  server.start();
}

Isolates

If you are just running multiple instances of a server, use package:pub_sub/isolate.dart.

You'll need one isolate to be the master. Typically this is the first isolate you create.

import 'dart:io';
import 'dart:isolate';
import 'package:pub_sub/isolate.dart' as pub_sub;
import 'package:pub_sub/pub_sub.dart' as pub_sub;

main() async {
  // Easily bring up a server.
  var adapter = new pub_sub.IsolateAdapter();
  var server = new pub_sub.Server([adapter]);

  // You then need to create a client that will connect to the adapter.
  // Each isolate in your application should contain a client.
  for (int i = 0; i < Platform.numberOfProcessors - 1; i++) {
    server.registerClient(new pub_sub.ClientInfo('client$i'));
  }

  // Start the server.
  server.start();

  // Next, let's start isolates that interact with the server.
  //
  // Fortunately, we can send SendPorts over Isolates, so this is no hassle.
  for (int i = 0; i < Platform.numberOfProcessors - 1; i++)
    Isolate.spawn(isolateMain, [i, adapter.receivePort.sendPort]);

  // It's possible that you're running your application in the server isolate as well:
  isolateMain([0, adapter.receivePort.sendPort]);
}

void isolateMain(List args) {
  var client =
      new pub_sub.IsolateClient('client${args[0]}', args[1] as SendPort);

  // The client will connect automatically. In the meantime, we can start subscribing to events.
  client.subscribe('user::logged_in').then((sub) {
    // The `ClientSubscription` class extends `Stream`. Hooray for asynchrony!
    sub.listen((msg) {
      print('Logged in: $msg');
    });
  });
}

JSON RPC 2.0

If you are not running on isolates, you need to import package:pub_sub/json_rpc_2.dart. This library leverages package:json_rpc_2 and package:stream_channel to create clients and servers that can hypothetically run on any medium, i.e. WebSockets, or TCP Sockets.

Check out test/json_rpc_2_test.dart for an example of serving pub_sub over TCP sockets.

Protocol

pub_sub is built upon a simple RPC, and this package includes an implementation that runs via SendPorts and ReceivePorts, as well as one that runs on any StreamChannel<String>.

Data sent over the wire looks like the following:

// Sent by a client to initiate an exchange.
interface Request {
  // This is an arbitrary string, assigned by your client, but in every case,
  // the client uses this to match your requests with asynchronous responses.
  request_id: string,
  
  // The ID of the client to authenticate as.
  // 
  // As you can imagine, this should be kept secret, to prevent breaches.
  client_id: string,

  // Required for *every* request.
  params: {
    // A value to be `publish`ed.
    value?: any,

    // The name of an event to `publish`.
    event_name?: string,

    // The ID of a subscription to be cancelled.
    subscription_id?: string
  }
}

/// Sent by the server in response to a request.
interface Response {
  // `true` for success, `false` for failures.
  status: boolean,
  
  // Only appears if `status` is `false`; explains why an operation failed.
  error_message?: string,

  // Matches the request_id sent by the client.
  request_id: string,

  result?: {
    // The number of other clients to whom an event was `publish`ed.
    listeners:? number,

    // The ID of a created subscription.
    subscription_id?: string
  }
}

When sending via JSON_RPC 2.0, the params of a Request are simply folded into the object itself, for simplicity's sake. In this case, a response will be sent as a notification whose name is the request_id.

In the case of Isolate clients/servers, events will be simply sent as Lists:

['<event-name>', value]

Clients can send the following (3) methods:

• subscribe (event_name:string): Subscribe to an event.
• unsubscribe (subscription_id:string): Unsubscribe from an event you previously subscribed to.
• publish (event_name:string, value:any): Publish an event to all other clients who are subscribed.

The client and server in package:pub_sub/isolate.dart must make extra provisions to keep track of client ID's. Since SendPorts and ReceivePorts do not have any sort of guaranteed-unique ID's, new clients must send their SendPort to the server before sending any requests. The server then responds with an id that must be used to identify a SendPort to send a response to.