Hypertext transfer protocl

Phase:

• Connection: Open connection to the server
• Request: Make a request (GET, POST, PUT, DELETE)
• Response: Receive a response from the server
• Close connection: Close the connection to the server

HTTP Is stateless

• HTTP does not keep track of what happened in prev connections
• APplications backend must do this on its own through use of databases
• Application can deposit cookies in the browser to maintain states

Request types

• GET: Request for a specific document (Can also be used to send data)
• POST: Request for server to accept data from browser
• PUT: Replace a document with data provided by browser
• DELETE: Remove a document
• HEAD: Retrive only header of document

Documents are in the form of HTML files:

• Consists of
  • Text that can be marked up to format
  • Hyperlinks: Links to other documetns possibly on other servers
  • Scripts: Programs that will be run on your browser

MIME Types

• Multipurpose internet Mail extension (MIME) types:
  • Extensible. Can define new MIME types
  • Common MIME types: - application/pdf
    • application/json
    • image/gif
    • image/jpg
• When request for a document using GET or POST, we shld specify the expected document type in the header. E.g Content-Type: application/json

Creating a HTTP server in python

• Flask

Message queuing telemetry transport

• ISO standard
• Advantages over HTTP:
  • Lightweight, less overheads
  • Public subscribe model - Centralised server
    • Clients subscribe to topics
    • When someone publishes messages in a topic, all interested client are notified
• Contrast with HTML
  • Essentially a point to point (client to server) model
  • Client needs to continually poll the server using a client-side script to get updates.
  • Some imporvements are available to sovle this (Webscokets)

Implementing

• Use Mosquitto
• sudo apt-get install mosquitto mosquitto-clients
• pip3 install paho-mqtt
• sudo service mosquitto start

Programming

• Declare two listeners
  • on_connect: Called when the client has attempted to connect to a broker. Must get a result code of 0 for success
  • on_message: Called when the client has recieved a message on the topic it is subscribing to.
• Example Code: GIven in the slides

See slides on how to use mosquitto

Databases

• We want to store this data
  • As flat files: Difficult to search
  • As a database - More complex solution, but easier to search for particular images or pieces of data. E.g data read from a certain range of dates

MySQL

• Establish relationship between tables
• Eastablish tight rules to govern the relationship
  • ie, if parent is deleted, any data that is related can be deleted

NoSql

Securing MQTT

• masquitto_passwd -U <password file>

Check the slides to see more data

Slides