There are four objectives to this assignment:
In this assignment, you will be developing a real, working web server. To greatly simplify this project, we are providing you with the code for a very basic web server. This basic web server operates with only a single thread; it will be your job to make the web server multi-threaded so that it is more efficient.
Before describing what you will be implementing in this project, we will provide a very brief overview of how a web server works and the HTTP protocol. Our goal in providing you with a basic web server is that you should be shielded from all of the details of network connections and the HTTP protocol. The code that we give you already handles everything that we describe in this section. If you are really interested in the full details of the HTTP protocol, you can read the specification, but we do not recommend this for this initial project!
Web browsers and web servers interact using a text-based protocol called HTTP (Hypertext Transfer Protocol). A web browser opens an Internet connection to a web server and requests some content with HTTP. The web server responds with the requested content and closes the connection. The browser reads the content and displays it on the screen.
Each piece of content on the server is associated with a file. If a client requests a specific disk file, then this is referred to as static content. If a client requests that a executable file be run and its output returned, then this is dynamic content. Each file has a unique name known as a URL (Universal Resource Locator). For example, the URL www.cs.wisc.edu:80/index.html identifies an HTML file called "/index.html" on Internet host "www.cs.wisc.edu" that is managed by a web server listening on port 80. The port number is optional and defaults to the well-known HTTP port of 80. URLs for executable files can include program arguments after the file name. A '?' character separates the file name from the arguments and each argument is separated by a '&' character. This string of arguments will be passed to a CGI program as part of its "QUERY_STRING" environment variable.
An HTTP request (from the web browser to the server) consists of a request line, followed by zero or more request headers, and finally an empty text line. A request line has the form: [method] [uri] [version]. The method is usually GET (but may be other things, such as POST, OPTIONS, or PUT). The URI is the file name and any optional arguments (for dynamic content). Finally, the version indicates the version of the HTTP protocol that the web client is using (e.g., HTTP/1.0 or HTTP/1.1).
An HTTP response (from the server to the browser) is similar; it consists of a response line, zero or more response headers, an empty text line, and finally the interesting part, the response body. A response line has the form [version] [status] [message]. The status is a three-digit positive integer that indicates the state of the request; some common states are 200 for "OK", 403 for "Forbidden", and 404 for "Not found". Two important lines in the header are "Content-Type", which tells the client the MIME type of the content in the response body (e.g., html or gif) and "Content-Length", which indicates its size in bytes.
If you would like to see the HTTP protocol in action, you can connect to any web server using telnet. For example, run telnet www.cs.wisc.edu 80 and then type (note that there is an empty line at the end):
GET / HTTP/1.1 host: www.cs.wisc.edu
You will then see the HTML text for that web page!
Again, you don't need to know this information about HTTP unless you want to understand the details of the code we have given you. You will not need to modify any of the procedures in the web server that deal with the HTTP protocol or network connections.
The code for the web server is available from ~cs537-2/public/Projects/P2. You should copy over all of the files there into your own working directory. You should compile the files by simply typing "make". Compile and run this basic web server before making any changes to it!
When you run this basic web server, you need to specify the port number that it will listen on; you should specify port numbers that are greater than about 2000 to avoid active ports. When you then connect your web browser to this server, make sure that you specify this same port. For example, assume that you are running on royal21.cs and use port number 2003; copy your favorite html file to the directory that you start the web server from. Then, to view this file from a web browser (running on the same or a different machine), use the url: royal21.cs.wisc.edu:2003/favorite.html
The web server that we are providing you is only about 200 lines of C code, plus some helper functions. To keep the code short and very understandable, we are providing you with the absolute minimum for a web server. For example, the web server does not handle any HTTP requests other than GET, understands only a few content types, and supports only the QUERY_STRING environment variable for CGI programs. This web server is also not very robust; for example, if a web client closes its connection to the server (e.g., if the user presses the "stop") it may crash. We do not expect you to fix these problems!
The helper functions are simply wrappers for system calls that check the error codes of those system codes and immediately terminate if an error occurs. One should always check error codes! However, many programmer don't like to do it because they believe that it makes their code less readable. The solution is to use these wrapper functions. Note the common convention that we use of naming the wrapper function the same as the underlying system call, except capitalizing the first letter, and keeping the arguments exactly the same. We expect that you will write wrapper functions for the new system routines that you call.
One approach to building a multi-threaded server is to spawn a new thread for every new http request. The OS will then schedule these threads according to its own policy. The advantage of creating these threads is that now short requests will not need to wait for a long request to complete; further, when one thread is blocked (i.e., waiting for disk I/O to finish) the other threads can continue to handle other requests. However, the drawback of the one-thread-per-request approach is that the web server pays the overhead of creating a new thread on every request.
Therefore, the generally preferred approach for a multi-threaded server is to create a fixed-size pool of worker threads when the web server is first started. With the pool-of-threads approach, each thread is blocked until there is an http request for it to handle. Therefore, if there are more worker threads than active requests, then some of the threads will be blocked, waiting for new http requests to arrive; if there are more requests than worker threads, then those requests will need to be buffered until there is a ready thread.
In your implementation, you must have a master thread that begins by creating a pool of worker threads, the number of which is specified on the command line. Your master thread is then responsible for accepting new http connections over the network and placing the descriptor for this connection into a fixed-size buffer; in your basic implementation, the master thread should not read from this connection. The number of elements in the buffer is also specified on the command line. Note that the existing web server has a single thread that accepts a connection and then immediately handles the connection; in your web server, this thread should place the connection descriptor into a fixed-size buffer and return to accepting more connections. You should investigate how to create and manage posix threads with pthread_create and pthread_detach.
Each worker thread is able to handle both static and dynamic requests. A worker thread wakes when there is an http request in the queue; when there are multiple http requests available, then which request is handled depends upon the scheduling policy, described below. Once the worker thread wakes, it performs the read on the network descriptor, obtains the specified content (by either reading the static file or executing the CGI process), and then returns the content to the client by writing to the descriptor. The worker thread then waits for another http request.
Note that the master thread and the worker threads are in a producer-consumer relationship and require that their accesses to the shared buffer be synchronized. Specifically, the master thread must block and wait if the buffer is full; a worker thread must wait if the buffer is empty. In this project, you are encouraged to use condition variables. If your implementation performs any busy-waiting (or spin-waiting), you will be heavily penalized.
Side note: Do not be confused by the fact that the basic web server we provide forks a new process for each CGI process that it runs. Although, in a very limited sense, the web server does use multiple processes, it never handles more than a single request at a time; the parent process in the web server explicitly waits for the child CGI process to complete before continuing and accepting more http requests. When making your server multi-threaded, you should not modify this section of the code.
In this project, you will implement a number of different scheduling policies. Note that when your web server has multiple worker threads running (the number of which is specified on the command line), you will not have any control over which thread is actually scheduled at any given time by the OS. Your role in scheduling is to determine which http request should be handled by each of the waiting worker threads in your web server.
The scheduling policy is determined by a command line argument when the web server is started and are as follows:
You will also note that the HPSC, HPDC, and SFF policies require that something be known about each request (e.g., whether it is for static or dynamic content, or the size of the file) before the requests can be scheduled. Thus, to support these scheduling policies, you will need to do some initial processing of the request outside of the worker threads; you will probably want the master thread to perform this work, which requires that it read from the network descriptor.
For each request, you will record the following times or durations at the granularity of milliseconds. You may find gettimeofday() useful for gathering these statistics.
You should also keep the following statistics for each thread:
server [portnum] [threads] [buffers] [schedalg] [fairbound]The command line arguments to your web server are to be interpreted as follows.
For example, if you run your program as
server 5003 8 16 HPSC 20then your web server will listen to port 5003, create 8 worker threads for handling http requests, allocate 16 buffers for connections that are currently in progress (or waiting), use HPSC scheduling for arriving requests, and ensure that no request waits behind 20 or more requests.
As a second step, we recommend understanding how the code that we gave you works. All of the code is available from ~cs537-2/public/Projects/P2. We provide the following .c files:
We also provide you with a sample
The best way to learn about the code is to compile it and run it. Run the server we gave you with your preferred web browser. Run this server with the client code we gave you. You can even have the client code we gave you contact any other server (e.g., www.cs.wisc.edu). Make small changes to the server code (e.g., have it print out more debugging information) to see if you understand how it works.
We have provided a few comments, marked with "CS537", to point you to where we expect you will make changes for this project.
As a third step, we recommend making the server multi-threaded. Finally, add in the different scheduling algorithms, beginning with the easiest (FIFO).
We anticipate that you will find the following routines useful for creating and synchronizing threads: pthread_create, pthread_detach, pthread_mutex_init, pthread_mutex_lock, pthread_mutex_unlock, pthread_cond_init, pthread_cond_wait, pthread_cond_signal. To find information on these library routines, being with the manual pages (using the Unix command man).
You may find the following tutorials useful as well.
There are two steps for turning in your project this time.
First, hand in your source code. We have created a directory ~cs537-SECTION/handin/NAME, where SECTION is either 1 or 2 and NAME is your login name. For example, if you are in section 2 and your login is lab, your handin directory is ~cs537-2/handin/lab. Your handin directory has five subdirectories: p1, p2, p3, p4, and p5. For this assignment, use directory p2. You may use the directory of either project partner. Make sure that all of your files list both project partner's names
You should copy all of your server source files (*.c and *.h) and a Makefile to your p2 handin directory. Do not submit any .o files. You do not need to copy any files for creating clients or CGI programs. After the deadline for this project, you will be prevented from making any changes in these directory. Remember: No late projects will be accepted!
Second, send a brief, private e-mail to me (dusseau) rating your experience working with your project partner. Please use a scale of 1-5, with 1 being the worst and 5 being the best. If you rate your project partner with a 1 or a 2, you will also need to explain why it was such a poor experience. These e-mails will be kept confidential. In general, we hope to completely ignore these ratings; we will only use them if necessary to track a history of problems. In general, partners will receive the same grade on a project.