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CSE331�Lecture 19.1 - 19.2

HTTP Servers Basics

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Ardi Madadi �Summer 2021�(Based on slides by Kevin Zatloukal, Mike Ernst, Dan Grossman, and many others)

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Event-driven programming

An event-driven program is designed to wait for events:

    • program initializes then enters the event loop
    • abstractly:

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do {

e = getNextEvent();

process event e;

} while (e != quit);

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Server Programming

  • Servers sit around waiting for events like:
    • new client connections
    • new data from the client (high scale servers)

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  • Simple version (normal scale):

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while (true) {

wait for a client to connect

process the request; send a response back

}

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    • probably want to use a new thread for processing
    • high scale web servers might look quite different

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Example: Chat Server

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ChatServer.java

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Server Sockets & Ports

  • Server creates a “server socket” and waits for a connection
    • each connection comes with an individual socket
    • allows reading from / writing to that client

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  • Servers on the same machine distinguished by a port number
    • numbers below 1024 require admin privileges

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ServerSocket ssock = new ServerSocket(80);

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  • Clients indicate the port when trying to connect:

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Socket sock = new Socket(“attu”, 80);

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Ports & Protocols

  • Sockets API allows reading & writing of byte data
    • like the File API

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  • Each server can define its own protocol for communication
    • the language it uses to speak to clients

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  • By convention, ports are associated with particular protocols
    • 80 = HTTP
    • 443 = HTTPS
    • 25 = SMTP relay
    • …

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  • Client that wants to talk HTTP can try connecting to 80

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Protocols

  • HTTP (Hyper-Text Transfer Protocol) is the most important
    • initially created for retrieving HTML documents
    • simple, text-based protocol

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  • Trend moving away from new protocols toward re-use of HTTP
    • Google (2010s) used HTTP for almost everything

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  • Allows for re-use of libraries for creating HTTP servers…
    • use of libraries reduces bugs, saves time, etc.
    • do not write your own HTTP server

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HTTP

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HTTP Request 1

GET /index.html HTTP/1.1

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  • Request ends with a blank line

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  • Between GET and blank are optional headers of the form

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Name: Value

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    • similar to Java properties files
    • common example would be User-Agent to describe client

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HTTP Response 1

HTTP/1.1 200 OK

content-length: 124

content-type: text/html; charset=UTF-8

Date: Wed, 27 May 2020 18:30:00 GMT

Connection: close

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<html>

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  • 200 status code indicates successful
  • 400s for error that is the client’s fault
  • 500s for errors on the server’s end

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Demo

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(command-line HTTP request)

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HTTP Request 2

POST /register HTTP/1.1

content-type: application/x-www-form-urlencoded

content-length: 25

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fname=Kevin&userid=kevinz

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  • POST request includes client content

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  • 25 bytes of content after the blank line
    • newlines are just another byte

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HTTP

  • GET & POST requests are by far the most common
    • other types like DELETE also exist

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  • See CSE 333 for a more complete discussion
    • (no need to memorize the details here)

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Uniform Resource Locators (URLs)

  • Tells the browser what to get and how to get it

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http://attu:8080/index.html

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Connect to server attu on port 8080

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Send GET request

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GET /index.html HTTP/1.1

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Uniform Resource Locators (URLs)

  • Port is optional (default is 80 for HTTP)

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  • Optional “?a=b&c=d” part of path is called query string
    • “&”-separated key=value pairs
    • useful for passing arguments to the server-side code…

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  • Fragment is only kept in the browser
    • client can use this to record its place in the document
    • allows back/forward buttons to work on a single page

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http://attu:8080/cse331/test?a=b&c=d#whatever

protocol

hostname

port

path

query string

fragment