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SECURITY

Web Security : Automating with LibWWWPerl - Using Threading for Performance

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1. Problem

You want to have your Perl script issue multiple simultaneous requests. You could do this because you’re trying to test concurrency issues (what happens when several users work on the same part of the application?) or because you’re trying to increase the load that your server puts on the server. Either way, threads make a logical and convenient way to make simultaneous requests.

2. Solution

You must have threads for Perl enabled to use the solution in Example 1.

Example 1. Multithreaded fetching of web pages with Perl
#!/usr/bin/perl
use threads;
use LWP;

my $PAGE = "http://www.example.com/";

# Ten concurrent threads
my $numThreads = 10;
my @threadHandles = ();
my @results = ();

for ($i = 0; $i < $numThreads; $i++ ) {
# create a thread, give it its number as an argument
my $thread = threads->create( doFetch, $i, $PAGE );
push( @threadHandles, $thread );
}

# Run through all outstanding threads and record their results.
while( $#threadHandles > 0 ) {
my $handle = pop(@threadHandles);
my $result = $handle->join();
push( @results, $result );
print "result: $result\n";
}

sub doFetch {
my $threadNum = shift;
my $URL = shift;
my $browser = LWP::UserAgent->new;
my $response = $browser->get( $URL );
return "thread $i " . $response->status_line;
}


3. Discussion

The example in Example 1 is pretty minimal and just shows you the basic techniques. The threads execute in an unknown order, and they might take any amount of time to run. That’s part of the point: to simulate a bunch of unrelated web browsers hitting your application in random order.

One of the best uses of multithreading like this is to test applications where it is acceptable to have the same account logged in more than once. Build a subroutine that logs in, executes a function or two, and tests the resulting output or state, then exits. Now launch a bunch of threads executing that subroutine.

Threading is an optional module in Perl. It might be compiled into your Perl, or it might not. If it is not, you either have to get a threading Perl or give up on running this kind of test. Run the command perldoc perlthrtut to learn about threading and how to tell if your implementation has threads enabled.

Threading Is Hard!

Be aware that multithreaded and concurrent programming is hard for many, many reasons. If you don’t stray far from the example in Example 1, you’ll do alright, but it is very easy to imagine some simple modifications that are surprisingly hard to do correctly.

Make sure to use my on all the variables in your threads. Don’t try to pass data between threads. Writing to global variables is a sure way to get into trouble when you’re multithreading. Your programming language (Perl in this case) will let you do it, but you’ll almost certainly get unexpected results. Only pass data from the thread to the main program through the return value in the join() call. It’s not that there is no other way to pass data around; it’s just that this is the safest, simplest method if you’ve never done threads before.

A cobbled-together, threaded Perl script is not a substitute for a well-planned, properly executed performance test plan. Just because you can execute a whole bunch of threads, doesn’t mean you’ll accomplish a lot. You might bog down your Perl program so much that you actually don’t execute many concurrent processes at all. Do some experiments to see just how many threads you can realistically get going at the same time.

Be careful with things like the sleep command in Perl. In some operating systems, your whole process (i.e., all threads) may go to sleep and stop running if you call sleep in just one of them or in your main program.

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