Networking, Web, Java, Monsoon How do programs on different machines - - PowerPoint PPT Presentation

Duke CPS 108 17.1

Networking, Web, Java, Monsoon

• How do programs on different machines communicate?

➤ Same OS? Different OS? Programming language? ➤ Through browser? Application? Other? ➤ How to programs find out about each other?

• Clients and Servers

➤ The server accepts requests from the client or clients,

processes and communicates, centralized (e.g., database), typically one or small number of clients

➤ The client makes requests of the server, potentially LOTS

• f clients

➤ Possible to have middle-level as well, or proxy server

Duke CPS 108 17.2

Low-level communication

• A socket is a two-way pipe that can be opened between

programs

➤ Low-level communication, basically byte-at-a-time ➤ Possible to send strings as formatted byte sequence

• Protocol must be explicit between client and server, and

protocol is low-level

➤ If string read is this, do that ➤ Expect a string, process it

• Socket uses host address and port number, agreed on by client

and server

➤ In Java it’s easy to create a socket, get reader/writer ➤ Can do this in C/C++, write a class to re-use

• See Client.java and Server.java example

Duke CPS 108 17.3

Other methods of communication

• Applet – client side program that can talk to server

➤ Applet is Java code downloaded over the web, run in JVM

that’s part of browser on the client side

➤ Applet cannot (by default) communicate with client

computer, but can communicate back to host/server

➤ Advantages of applets? Drawbacks?

• RMI – remote method invocation

➤ Objects on different machines communicate directly by

method calls between objects, just like “regular” programs

➤ Java takes care of turning method calls/parameters into

low-level bytes as needed

➤ Client/Server roles mixed, calls go each way ➤ Need lots of infrastructure, Java supplies most of it

Duke CPS 108 17.4

Web architectures for client/server

• CGI: Common Gateway Interface

➤ Web server communicates with CGI programs ➤ CGI programs written in any language, web server

mediates between the CGI server program and the browser/client

• Typically each CGI request starts a new process

➤ Process is heavyweight (compared to thread), doesn’t scale ➤ FastCGI: shares CGI programs (don’t restart, reuse?),

mod_perl, execute CGI scripts/programs in web server

• ASP: Active Server Pages

➤ Combines HTML, scripts, server-side programs (COM) ➤ For practical purposes limited to Windows web servers

Duke CPS 108 17.5

Servlets

• Java program that takes HTTP requests, processes them,

returns HTTP result

➤ Executes as server-side program, similar to CGI ➤ Doesn’t start new process, executes within JVM as a thread ➤ Benefits from Java security model within JVM ➤ Portable (in theory) to many platforms ➤ In some ways like a server-side applet

• Many libraries available as part of Java and via third parties

for extending functionality of Servlets

➤ Need servlet engine that interfaces with Web server ➤ In fourth quarter 99, Apache web server has 60% of

market, has Servlet engine as an add on