system-design-refresher

Application Level Protocols

In this section we’ll go over some of the most commonly used application level protocols.

Contents

• HTTP
• WebSockets
• Email Related Protocols
• File Transfer Protocols
• Real-time Communications
• RPC

HTTP

Hyper Text Transfer Protocol

• Built on TCP/IP
• Request-Response protocol (no memory)

Status Codes

• 2XX Success Codes
  • 200 Ok
  • 201 Created
  • 204 No content
• 3XX Redirection Codes
  • 301 Moved permanently
  • 302 Found but temporarily in another URI
  • 304 Not Modified - Cached version is still valid
• 4XX Client Error Codes
  • 400 Bad Request
  • 401 Unauthorized
  • 403 Forbidden
  • 404 Not Found
  • 429 Too many requests
• 5XX Server Error Codes
  • 500 Internal Server Error
  • 501 Not Implemented
  • 503 Service Unavailable

Methods

Most commonly used methods are:

• GET to fetch data
• POST to create data
• PUT to update an entire resource
• PATCH to partially update a resource
• DELETE to delete a resource

WebSockets

Two-way communication channel over a single long-lived connection. It allows servers to “push” real time updates to clients.

Email Related Protocols

• SMTP Email transmission across the internet (Simple Mail Transfer Protocol)
• IMAP Used to retrieve emails from a server (Internet Message Access Protocol). Ideal for users who need to access their emails from different devices
• POP3 Used for downloading emails from a server (Post Office Protocol v3). Typically used when emails are managed from a single device

File Transfer Protocols

• FTP For transfering files over the internet
• SSH For command-line login and file transfer

Real-time Communications

• WebRTC Enables browser-to-browser applications for voice calling, video chat and file sharing.
• MQTT Lightweight messaging protocol (Message Queuing Telemetry Transport)
• AMQP Protocol for message-oriented middleware (Advanced Message Queuing Protocol)

RPC

Remote Procedure Calls allows a program on one computer to execute code on a server or another computer. It is a method used to invoke a function as if it were a local call, but in reality it is executed on the remote machine.