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Request - Response

Flow​

Request Response Flow

  1. Client sends a request (e.g. HTTP GET request)
  2. Server parses the request
  3. Server processes the request (e.g. database query or other business logic)
  4. Server sends a response (e.g. JSON)
  5. Client parses the response (e.g. parse JSON)
  6. Client processes the response (e.g. display data to user)

Use cases​

  • HTTP, DNS, SSH
  • RPC (remote procedure call)
  • APIs (REST, SOAP, GraphQL, etc.)
  • SQL and database protocols
  • many more

Structure​

  • Request structure is defined by a protocol (which both client and server must agree on) and a message format

HTTP GET request example:

GET /users HTTP/1.1
Host: example.com
Accept: application/json
... (other headers)
  • The same is true for the response

HTTP response example:

HTTP/1.1 200 OK
Server: example.com
Content-Type: application/json
... (other headers)
<CLRF>
[body]

Doesn't work well for ...​

  • Real-time communication (e.g. chat applications)
  • Notifications (e.g. push notifications) -> client would need to poll the server regularly
  • Long-running requests (e.g. file upload) -> client would need to wait for the response (what if the connection is lost?)

Demo​

Request:

curl -v google.com

Response:

* Trying 172.217.20.14:80...
* Connected to google.com (172.217.20.14) port 80 (#0)
> GET / HTTP/1.1
> Host: google.com
> User-Agent: curl/7.81.0
> Accept: */*
>

< HTTP/1.1 301 Moved Permanently
< Location: http://www.google.com/
< Content-Type: text/html; charset=UTF-8
< Cache-Control: public, max-age=2592000
< Server: gws
< Content-Length: 219
< X-XSS-Protection: 0
< X-Frame-Options: SAMEORIGIN
<
<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>301 Moved</TITLE></HEAD><BODY>
<H1>301 Moved</H1>
The document has moved
<A HREF="http://www.google.com/">here</A>.
</BODY></HTML>