►Accelerated I
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•Client Programs
On this page:
Registration
Receiving Messages
Sending Messages
A complete example
Warning
6.11.0.4
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Client Programs

This write-up is a concise introduction to the creation of world programs that communicate with a server. It is not a guide to the design of client-server systems.

Registration

A world program that wishes to communicate with a server program must register with the server. defterm{Registering} requires three additional clauses in the big-bang description of the world:

The register clause informs big-bang from where to expect messages and where to send them to. Your programs must register with

"dictionary.ccs.neu.edu"

which is a server that knows how to communicate with your programs.

The port clause specifies which entry point of the server you wish to use. In general, these points are addressed via numbers between 0 and 65,000 (approximately). Most of these ports are usually locked so that the Russian spies can’t steal your email and blackmail you when you become president of Northeastern University. Your programs must choose one of the following ports:

10000, 10001, 10002, 10003, 10004, 10005, 10006, 10007, 10008, 10009, 10010, 10011

For assignments on the Piazza project, the name clause sends your credentials to your server. Otherwise it will not allow you to communicate. Your program must send a string of the following shape:

last-name1.first-name1-initial:0123+last-name2.first-name2-initial:4567

The digits represent the last four digits of your student id. For the few of you who end up working alone for a short while, the credentials are just the last name, initial, and last four digits:

last-name.first-name-initial:0123

Here are some made-up examples:

partner 1

   

partner 2

   

credentials

name

   

id

   

name

   

id

Matt Mac

   

4444

   

Alan Misery

   

3344

   

"mac.m:4444+misery.a:3344"

Leena Bot

   

1221

   

Nadia Comm

   

9999

   

"bot.l:221+comm.n:9999"

Al Sam-To

   

6789

   

Becca Fell

   

5642

   

"sam-to.a:6789+fell.b:5642"

Ma Fell

   

1234

   

"fell.m:5642"

The server will not share the digits of your student ids with its clients.

Receiving Messages

A world program that wishes to receive message must specify the message handler with a on-receive clause. The handler consumes two arguments: the current state of the world and the message that the server sent.

Sending Messages

To send a message, a handler must produce a Package, which consists of two parts: the next state of the world and a message to be sent to the server on behalf of the world program. Any handler can return packages.

For example, your program might contain this handler definition for mouse events:
(define (mouse-handler w x y me)
  (make-package w "hello world"))
It would ignore the nature of the mouse event (x, y, and me). Instead, it would return a package that says “the state of the world stays the same” (w) and “please send the "hello world" string” out.

A complete example

; PURPOSE of PROGRAM
 
; This world program connects to a server that sends out one word from a
; dictionary per second. When the client receives a string, it uses it as its
; own state and displays it in the world canvas.
 
; If this client sends a string message to the server, the latter appends it
; to the strings it sends from now on (to this client). The words that a client
; sends out accumulates at the server.  
 
(define the-server "dictionary.ccs.neu.edu")
(define the-port   123456)
 
; Client is String
; interpretation The state of the client is a word.
; It changes every time when a message (string) arrives from the server.
 
; String -> String
; just shows the last word received from the
(define (client s)
  (big-bang s
    [name       "matthias:1234"]
    [register   the-server]
    [port       the-port]
    [on-tick    send-word 3]
    [to-draw    render-word]
    [on-receive receive-word]))
 
; Client -> Package (of String and String)
; send the word "dd" to the server
(check-expect (send-word "hello") (make-package "hello" "dd"))
(check-expect (send-word "world") (make-package "world" "dd"))
(define (send-word s)
  (make-package s "dd"))
 
; Client -> Image
; render the state of the client as a text image with a 22 font in red
(check-expect (render-word "bye") (text "bye" 22 "red"))
(define (render-word s)
  (text s 22 "red"))
 
; Client String -> Client
; turn the received word into the state of the client
(check-expect (receive-word "bye" "good") "good")
(define (receive-word s m)
  m)

Warning

Do not attempt to run your main function until your handlers pass a solid suite of test cases. Otherwise you will experience nothing but frustration.

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