Quartz 4

Request and response model

3 min read

  • The first/most famous communication design pattern. It’s everywhere!
  • Web, HTTP, DNS, SSH
  • RPC (remote procedure call) → request is to a remote server, you cross the network
  • SQL and Database Protocols
    • send a query to the db → db parses/processes/executes → returns results
  • APIs (REST/SOAP/GraphQL)

Request response model

  • Client sends a request
  • Server parses the request
    • Understanding where the request begins/ends in the stream of data
    • The cost of parsing the data is not cheap.
      • Payload can be JSON/XML/protocol buffer
      • cost of parsing: XML > JSON > protocol buffers
  • Server processes the request (actually executing)
    • Deserialization (to something I can understand with my programming language)
  • Server sends a response
  • Client parses the response and consumes it

The anatomy of a request/response

  • Request/Response structure is defined by both client and server, defined by the protocol & the message format
    • Protocol → HTTP Fundamentals, message format → JSON/XML etc
    • The server understands it, parses it, and gives you back a response. This work is done for us by libraries (like Express.js uses HTTP library to parse).
      • JavaScript is the best for this because it’s native at parsing JSONs (unlike languages like C++ where u need an external JSON parser which takes like 2 secs)
GET /HTTP/1.1
Headers
<CRLF>
BODY
  • Request/Response has a boundary

Visual

  • Client writes the request at t0
    • writing the request also takes time, it also has a cost
  • Sending Request (client)
    • the network takes time to transfer my request
    • Most protocols (HTTP, SSH, etc) use TCP
      • data is divided into segments which fit into IP packets, which is routed in the internet
      • guarantees that all data chunks arrive
  • Processing Request (server)
    • server writes & sends the request at t30
    • client is waiting from t2 to t32

Building an upload image service

Every option is a request/response, but the style changes!

  • Some options?
    • Option 1
      • Send large request with the image (laaarge file)
      • If you fail, it fails entirely.
    • Option 2
      • Chunk image and send a request per chunk (resumable)
      • The server can receives each chunks even if the rest fails. There is a way to synchronize the states
  • It doesn’t work everywhere:
    • The backend has the knowledge and client doesnt. Client should keep asking!
      • If you have a notification/chatting service, the client should keep asking if I have a notification/message (polling). It doesn’t scale well.
    • If you have very long requests
      • better to use asynchronous requests
    • What if client disconnects?

curl demo

curl -v --trace out.txt http://google.com
cat out.txt

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