Real Time Communication

Posted on August 21st, 2022

Talks aboutC#.NET

The vast majority of the web uses HTTP, which doesn't lend itself to real-time updates.Request-response isn't designed for clients to send data frequently and the server can't send a response without a request, so how can it update the client on its terms?

A History Lesson

We haven't always had a convenient way to update web applications in real-time. We had some jank, then some less jank, then we got there properly.

AJAX Polling

Using standard asynchronous requests, AJAX polling sends requests at a specific interval to create a two-way communication channel in which both the client and server can (optionally) send the latest data.

As above, HTTP isn't designed for this, so it's intensive to process so many HTTP requests given their relative size and complexity.

Long Polling

Long polling solves the intensity issue by delegating it to the server. When the client sends a request, the server waits for an update (polling) before responding to the client's request. Naturally, client requests can last indefinitely.

OrderUpdate? result;

while (true)
{
  result = await orderChecker.GetUpdate(orderId);

  if (result.HasValue) { return result; }

  await Task.Delay(3000); // polling interval
}

Long Polling example

Server Sent Events (SSE)

SSEs allow servers to send data to the client without an initial request.

The Good

No repetitious additional requests reduces overhead
Web standards
Easy to polyfill for older browsers
Auto-reconnection

The Bad

Most browsers have ~6 connection max
- 7 tabs and you're screwed
- Includes Chrome and Firefox
Only server-to-client

Web Sockets

Web sockets are a modern, standardised approach to implement real-time communication on the web, utilising a single TCP socket for two-way communication.

The web socket handshake first sends a request to 'upgrade' the connection. If the server supports web sockets, it responds with a 101 status code with a key to be included in the Accept header in subsequent communications:

// Request
Upgrade: websocket
Connection: Upgrade

Sec-WebSocket-Key: something-unique
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13
Sec-WebSocket-Extensions: deflate-stream

// Response
HTTP/1.1 101 Switching Protocols

Upgrade: websocket
Connection: Upgrade

Sec-WebSocket-Accept: something-unique
Sec-WebSocket-Protocol: chat
Sec-WebSocket-Key: Extensions: deflate-stream

Web Socket Handshake

SignalR

SignalR is the big daddy of real-time communication on the web. It supports polling, SSEs and web sockets, managed internally to automatically select the best technology available. It also provides an API for Remote Procedure Call (RPC) and scales to handle increasing traffic.

Hubs are a component of SignalR; server-side classes that exchange messages with the client. This includes a (de)serialisation protocol for parameters, JSON by default. It also supports 'MessagePack', a binary format more compact than JSON, and inherently faster to process.