Writing Modules for mod_perl

Discover the flexibility and power of writing mod_perl modules instead of CGI programs.
Reducing Memory Usage

mod_perl is an exciting development that has already made a great many new applications possible. But there is a trade-off for everything, and mod_perl's additional functionality comes at the expense of greater memory usage. It is hard to calculate the additional memory needed for mod_perl, but keep in mind that Perl can be a bit of a memory hog.

In addition, while lexical (“my” or “temporary”) variables disappear after each invocation of a Perl module rule via mod_perl, global variables stick around across invocations. This can be an attractive way to keep track of state in your program, but it can also lead to larger memory allocations.

For example, if your module creates an array with 10,000 elements, that array will continue to consume memory even after the program is invoked. This might be useful in some cases, such as when a complicated data structure is referenced in each invocation. However, it also means the large structure will constantly eat up memory, as opposed to only when necessary.

You can reduce memory usage by forcing mod_perl to share memory among Apache child processes. When you run Apache as a web server, it “preforks” a number of processes so that incoming connections will not have to wait for a new server process to be created. Each of these preforked servers is considered a separate process by Linux, operating independently. However, Apache is smart enough to share some memory among server siblings, at least to a certain degree.

mod_perl takes advantage of this shared memory by allowing the various server processes to share Perl code as well. However, there is a catch: you must make sure the Perl code is brought into mod_perl before preforking takes place. Perl modules and code compiled after the split occurs will raise the memory requirement for each individual server process, without regard to whether the same code has been loaded by another process.

In order to load code before Apache forks off child processes, use the PerlModule directive in the configuration files.

If, for example, you use the statement

PerlModule Apache::DBI

in one of the *.conf files, then

use Apache::DBI;
in a PerlHandler module, the latter invocation does not actually load any new code. Rather, it uses the cached, shared version of Apache::DBI that was loaded at startup by mod_perl.

You can load multiple modules with PerlModule, using the syntax

PerlModule Apache::DBI Apache::DBII Apache::DBIII

However, you can load only ten modules this way. If you want to load more, you can use the PerlRequire directive. Strictly speaking, PerlRequire allows you to specify the name of a Perl program to be evaluated only when Apache starts up. For example,

PerlRequire /usr/local/apache/conf/startup.pl
will evaluate the contents of startup.pl before forking off Apache child processes. However, if you include a number of “use” statements in startup.pl, you can effectively get around PerlModule's ten-module limit.

Remember that PerlModule or PerlRequire is necessary for modules to be shared among the different Apache sibling server processes, but it is not sufficient. You will still have to import the module in your own program in order to reap the benefits.

Conclusion

When I first started to work with mod_perl, I thought it was useful for speeding up CGI programs and for running filters like Embperl. As I have grown more dependent on it in my own work, I am amazed and impressed by the power mod_perl offers programmers looking to harness the power of Apache without the overhead of external programs or the development time associated with C.

As you can see, writing mod_perl modules is not difficult and is limited only by your imagination. It does require that you think a bit more carefully about your programs than when you are working with CGI, since you can affect the Apache server in ways that will slow it down or otherwise hurt your system's performance.

Resources

Reuven M. Lerner (reuven@lerner.co.il) is an Internet and web consultant living in Haifa, Israel, who has been using the web since early 1993. His book Core Perl will be published by Prentice-Hall in the spring. The ATF home page, including archives and discussion forums, is at http://www.lerner.co.il/atf/.

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