CSCI 339

Distributed Systems

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Assignment 1: A Web Server

Due: Monday, Feb 18, 11:59pm (updated)

The goal of this project is to build a functional web server.  This assignment will teach you the basics of distributed programming, client/server structures, and issues in building high performance servers.  While the course lectures will focus on the concepts that enable network communication, it is also important to understand the structure of systems that make use of the global Internet.

This project should be done in teams of two (or three with prior approval).

Part 1: Build the Server

At a high level, a web server listens for connections on a socket (bound to a specific port on a host machine).  Clients connect to this socket and use a simple text-based protocol to retrieve files from the server.  For example, you might try the following command from a UNIX machine:

% telnet www.cs.williams.edu 80
GET /index.html HTTP/1.0

(Type two carriage returns after the "GET" command).  This will return to you (on the command line) the HTML representing the "front page" of the Williams Computer Science web page.

One of the key things to keep in mind in building your web server is that the server is translating relative filenames (such as index.html ) to absolute filenames in a local filesystem.  For example, you might decide to keep all the files for your server in ~08abc/cs339/server/files/, which we call the document root.  When your server gets a request for index.html (which is the default web page if no file is specified), it will prepend the document root to the specified file and determine if the file exists, and if the proper permissions are set on the file (typically the file has to be world readable).  If the file does not exist, a file not found error is returned.  If a file is present but the proper permissions are not set, a permission denied error is returned.  Otherwise, an HTTP OK message is returned along with the contents of a file. 

You should also note that since index.html is the default file, web servers typically translate "GET /" to "GET /index.html".  That way index.html is assumed to be the filename if no explicit filename is present.  This is also why the two URLs http://www.cs.williams.edu and http://www.cs.williams.edu/index.html return equivalent results.

When you type a URL into a web browser, the server retrieves the contents of the requested file.  If the file is of type text/html and HTTP/1.0 is being used, the browser will parse the html for embedded links (such as images) and then make separate connections to the web server to retrieve the embedded files.  If a web page contains 4 images, a total of five separate connections will be made to the web server to retrieve the html and the four image files.

Using HTTP/1.0, a separate connection is used for each requested file. This implies that the TCP connections being used never get out of the slow start phase. HTTP/1.1 attempts to address this limitation. When using HTTP/1.1, the server keeps connections to clients open, allowing for "persistent" connections and pipelining of client requests. That is, after the results of a single request are returned (e.g., index.html), the server should by default leave the connection open for some period of time, allowing the client to reuse that connection to make subsequent requests. One key issue here is determining how long to keep the connection open. This timeout needs to be configured in the server and ideally should be dynamic based on the number of other active connections the server is currently supporting. Thus if the server is idle, it can afford to leave the connection open for a relatively long period of time. If the server is busy servicing several clients at once, it may not be able to afford to have an idle connection sitting around (consuming kernel/thread resources) for very long. You should develop a simple heuristic to determine this timeout in your server.

For this assignment, you will need to support enough of the HTTP/1.0 and HTTP/1.1 protocols to allow an existing web browser (Firefox) to connect to your web server and retrieve the contents of the Willams CS front page from your server.  (Of course, this will require that you copy the appropriate files to your server's document directory.) Note that you DO NOT have to support script parsing (php, javascript), and you do not have to support HTTP POST requests. You should support images, and you should return appropriate HTTP error messages as needed.

At a high level, your web server will be structured something like the following:

Forever loop:
Listen for connections
    Accept new connection from incoming client
    Parse HTTP request
    Ensure well-formed request (return error otherwise)
    Determine if target file exists and if permissions are set properly (return error otherwise)
    Transmit contents of file to connect (by performing reads on the file and writes on the socket)
    Close the connection (if HTTP/1.0)

You will have three main choices in how you structure your web server in the context of the above simple structure:

1) A multi-threaded approach will spawn a new thread for each incoming connection.  That is, once the server accepts a connection, it will spawn a thread to parse the request, transmit the file, etc.

2) A multi-process approach (really only an option if you choose to do your development in C/C++) maintains a worker pool of active processes to hand requests off to from the main server.  This approach is largely appropriate because of its portability (relative to assuming the presence of a given threads package across multiple hardware/software platform).  It does face increased context-switch overhead relative to a multi-threaded approach.

3) An event-driven architecture will keep a list of active connections and loop over them, performing a little bit of work on behalf of each connection.  For example, there might be a loop that first checks to see if any new connections are pending to the server (performing appropriate bookkeeping if so), and then it will loop overall all existing client connections and send a "block" of file data to each (e.g., 4096 bytes, or 8192 bytes, matching the granularity of disk block size).  This event-driven architecture has the primary advantage of avoiding any synchronization issues associated with a multi-threaded model (though synchronization effects should be limited in your simple web server) and avoids the performance overhead of context switching among a number of threads.

You may choose from C or C++ to build your web server but you must do it in a Unix-like environment.  If you choose Java, you will be penalized 10 points. In C/C++, you will want to become familiar with the interactions of the following system calls to build your system: socket(), select(), listen(), accept(), connect() .  I outline a number of resources below with additional information on these system calls.  A good book is also available on this topic.

Part 2: Writeup

Once you have completed the implementation of your server, write a paper that describes your chosen architecture and implementation details. Be sure to clearly state anything that does not work correctly. Also describe any problems that you encountered and any help that you received. In addition to describing the structure of your server, include a discussion that addresses the following questions:

1) Web servers often use ".htaccess" files to restrict access to clients based on their IP address. How would you go about supporting .htaccess in your server?

2) Since your web servers are restricted to on campus use only, it is unlikely that you will notice any significant performance differences between HTTP/1.0 and HTTP/1.1. Can you think of a scenario in which HTTP/1.0 may perform better than HTTP/1.1? Can you think of a scenario when HTTP/1.1 outperforms HTTP/1.0? Think about bandwidth, latency, and file size. Consider some of the pros and cons of using a connection per session versus using a connection per object.  The difference between the two comes down to the following:

- Only  a single connection is established for all retrieved objects, meaning that slow start is only incurred once (assuming that the pipeline is kept full) and that the overhead of establishing and tearing down a TCP connection is also only incurred once.

- However, all objects must be retrieved in serial in HTTP/1.1 meaning that some of the benefits of parallelism are lost.

Part 3: Submission

Make the server document directory ( the directory which the webserver uses to serve files ) a command line option. The command line option must be specified as -document_root.

Make the port that the server listens on a command line option. The option must be specified as -port . Thus, I should be able to run your server as

$ ./server -document_root "/home/jeannie/assignment1_files" -port 8888
(Note that you should use ports 8888 or 9999 for testing purposes.)

To submit your assignment, email a tarball to jeannie@cs.williams.edu.

Please include the following files in your tarball.
1) Your writeup (preferably PDF).
2) All the files for your source code only. Please do not include any executables.
3) Your Makefile (or compile instructions).

Resources

I have collected a list of available resources to help you get started with various aspects of this assignment:

A short tutorial on socket programming, from the University of Massachusetts:
ftp://gaia.cs.umass.edu/cs653/sock.ps

An Introduction to Socket Programming
http://www.uwo.ca/its/doc/courses/notes/socket/

Additional Socket Programming links:
http://compnetworking.about.com/cs/socketprogramming/

HTTP 1.0 and 1.1:
http://www.jmarshall.com/easy/http/

w3c HTTP page:
http://www.w3.org/Protocols/

Thread Programming Examples (C):
http://www.cs.cf.ac.uk/Dave/C/node32.html

C Programming link:
http://www.cs.cf.ac.uk/Dave/C/CE.html

Java Thread Programming:
http://www.javaworld.com/channel_content/jw-threads-index.shtml

Duane's C tutorial
http://www.cs.williams.edu/~bailey/c.pdf

C++ for Java Programmers
http://www.cs.williams.edu/~lenhart/courses/cs371/c++forjava.html