Will the Guru

Pull Technology

#1. Short Polling

• Client sends a request to retrieve data from the broker every n seconds
  -> Server(Broker) responds back immediately with the data it has.
  • It can be either none, one, several messages
• Consumes resources
  • TCP connection management
  • Client authentication/authorization
  • Rate Limiting
  • TLS termination
  • …

#2. Long Polling

• Client sends request to retrieve data from the broker
  -> Server(Broker) responds back immediately only if it has data. Otherwise, it does not respond back until it has data
• To avoid endless wait on both client and server, the client specifies wait time in the request parameter.
  • If wait time reaches, it closes connection

Short Polling vs Long Polling

• Typically Long Polling is preferred
  • Long Polling reduces the number of empty responses.
  • However, if the client is polling with a single thread, it’s better to use short polling
• Short Polling is better in a usecase if the client expects an immediate response

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Push Technology

#1. WebSocket

• Message-based protocol
• Works over HTTP ports 80 and 443
• Re-uses TCP connection initially established for an HTTP request

How does WebSocket work?

1. Client sends request with ‘Upgrade‘ header
   GET /chat HTTP/1.1
Host: server.example.com
Connection: Upgrade
Upgrade: websocket // THIS!!
...
   
2. Server responds back
   HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: websocket
...

Advantages of WebSocket

• Fast
• Bi-directional
  • Server can send request to the client
• Natively supported by browsers
• Provides an alternative to raw TCP sockets when they are not allowed for security reasons

#2. Server-Sent Events

• Browser(Client) opens up a persistent HTTP connection with the server
  • Using keep-alive option
• Server uses the connection to send data to the client
• If the connection drops, client automatically tries to re-connect
• ex) Mail notification system
  • When there’s a new email inbox, server sends notification to the browser(client)

Advantages & Disadvantages of Server-Sent Events

Advantages

• Simplicity (Both client-server)
• No need for a custom protocol

Disadvantages

• Mono-directional (server can only push to client)
• Limited to UTF-8 (cannot transmit binary data)
• Browsers limit the number of open connections

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Reference

• https://leetcode.com/explore/featured/card/system-design-for-interviews-and-beyond/728/how-to-build-efficient-communication-in-distribute/4784/