=head1 NAME Writing mod_perl Handlers and Scripts =head1 Description This chapter covers the mod_perl coding specifics, different from normal Perl coding. Most other perl coding issues are covered in the perl manpages and rich literature. =head1 Prerequisites =head1 Where the Methods Live mod_perl 2.0 has all its methods spread across many modules. In order to use these methods the modules containing them have to be loaded first. If you don't do that mod_perl will complain that it can't find the methods in question. The module C> can be used to find out which modules need to be used. =head1 Techniques =head2 Method Handlers In addition to function handlers method handlers can be used. Method handlers are useful when you want to write code that takes advantage of inheritance. To make the handler act as a method under mod_perl 2, use the C attribute. See the Perl I manpage for details on the attributes syntax (C). For example: package Bird::Eagle; @ISA = qw(Bird); sub handler : method { my ($class_or_object, $r) = @_; ...; } sub new { bless {}, __PACKAGE__ } and then register it as: PerlResponseHandler Bird::Eagle When mod_perl sees that the handler has a method attribute, it passes two arguments to it: the calling object or a class, depending on how it was called, and the request object, as shown above. If Cmethod> syntax is used for a C, e.g.: PerlResponseHandler Bird::Eagle->handler; the C<:method> attribute is not required. In the preceding configuration example, the C method will be called as a class (static) method. Also, you can use objects created at startup to call methods. For example: use Bird::Eagle; $Bird::Global::object = Bird::Eagle->new(); ... PerlResponseHandler $Bird::Global::object->handler In this example, the C method will be called as an instance method on the global object $C. =head2 Cleaning up It's possible to arrange for cleanups to happen at the end of various phases. One can't rely on C blocks to do the job, since these L until the interpreter quits, with an exception to the L handlers. Module authors needing to run cleanups after each HTTP request, should use C>. Module authors needing to run cleanups at other times can always register a cleanup callback via C> on the pool object of choice. Here are some examples of its usage: To run something at the server shutdown and restart use a cleanup handler registered on C> in F: #PerlPostConfigRequire startup.pl use Apache2::ServerUtil (); use APR::Pool (); warn "parent pid is $$\n"; Apache2::ServerUtil::server_shutdown_cleanup_register((\&cleanup); sub cleanup { warn "server cleanup in $$\n" } This is usually useful when some server-wide cleanup should be performed when the server is stopped or restarted. To run a cleanup at the end of each connection phase, assign a cleanup callback to the connection pool object: use Apache2::Connection (); use APR::Pool (); my $pool = $c->pool; $pool->cleanup_register(\&my_cleanup); sub my_cleanup { ... } You can also create your own pool object, register a cleanup callback and it'll be called when the object is destroyed: use APR::Pool (); { my @args = 1..3; my $pool = APR::Pool->new; $pool->cleanup_register(\&cleanup, \@args); } sub cleanup { my @args = @{ +shift }; warn "cleanup was called with args: @args"; } In this example the cleanup callback gets called, when C<$pool> goes out of scope and gets destroyed. This is very similar to OO C method. =head1 Goodies Toolkit =head2 Environment Variables mod_perl sets the following environment variables: =over =item * C<$ENV{MOD_PERL}> - is set to the mod_perl version the server is running under. e.g.: mod_perl/2.000002 If C<$ENV{MOD_PERL}> doesn't exist, most likely you are not running under mod_perl. die "I refuse to work without mod_perl!" unless exists $ENV{MOD_PERL}; However to check which version is used it's better to use the following technique: use mod_perl; use constant MP2 => ( exists $ENV{MOD_PERL_API_VERSION} and $ENV{MOD_PERL_API_VERSION} >= 2 ); # die "I want mod_perl 2.0!" unless MP2; =back mod_perl passes (exports) the following shell environment variables (if they are set) : =over =item * C - Executables search path. =item * C - Time Zone. =back Any of these environment variables can be accessed via C<%ENV>. =head2 Threaded MPM or not? If the code needs to behave differently depending on whether it's running under one of the threaded MPMs, or not, the class method Cis_threaded> can be used. For example: use Apache2::MPM (); if (Apache2::MPM->is_threaded) { require APR::OS; my $tid = APR::OS::current_thread_id(); print "current thread id: $tid (pid: $$)"; } else { print "current process id: $$"; } This code prints the current thread id if running under a threaded MPM, otherwise it prints the process id. =head2 Writing MPM-specific Code If you write a CPAN module it's a bad idea to write code that won't run under all MPMs, and developers should strive to write a code that works with all mpms. However it's perfectly fine to perform different things under different mpms. If you don't develop CPAN modules, it's perfectly fine to develop your project to be run under a specific MPM. use Apache2::MPM (); my $mpm = lc Apache2::MPM->show; if ($mpm eq 'prefork') { # prefork-specific code } elsif ($mpm eq 'worker') { # worker-specific code } elsif ($mpm eq 'winnt') { # winnt-specific code } else { # others... } =head1 Code Developing Nuances =head2 Auto-Reloading Modified Modules with Apache2::Reload META: need to port Apache2::Reload notes from the guide here. but the gist is: PerlModule Apache2::Reload PerlInitHandler Apache2::Reload #PerlPreConnectionHandler Apache2::Reload PerlSetVar ReloadAll Off PerlSetVar ReloadModules "ModPerl::* Apache2::*" Use: PerlInitHandler Apache2::Reload if you need to debug HTTP protocol handlers. Use: PerlPreConnectionHandler Apache2::Reload for any handlers. Though notice that we have started to practice the following style in our modules: package Apache2::Whatever; use strict; use warnings FATAL => 'all'; C 'all'> escalates all warnings into fatal errors. So when C is modified and reloaded by C the request is aborted. Therefore if you follow this very healthy style and want to use C, flex the strictness by changing it to: use warnings FATAL => 'all'; no warnings 'redefine'; but you probably still want to get the I warnings, but downgrade them to be non-fatal. The following will do the trick: use warnings FATAL => 'all'; no warnings 'redefine'; use warnings 'redefine'; Perl 5.8.0 allows to do all this in one line: use warnings FATAL => 'all', NONFATAL => 'redefine'; but if your code may be used with older perl versions, you probably don't want to use this new functionality. Refer to the I manpage for more information. =head1 Integration with Apache Issues In the following sections we discuss the specifics of Apache behavior relevant to mod_perl developers. =head2 HTTP Response Headers =head3 Generating HTTP Response Headers The best approach for generating HTTP response headers is by using the L. Some common headers have dedicated methods, others are set by manipulating the C> table directly. For example to set the I header you should call Ccontent_type|docs::2.0::api::Apache2::RequestRec/C_content_type_>>: use Apache2::RequestRec (); $r->content_type('text/html'); To C> a custom header I you should call: use Apache2::RequestRec (); use APR::Table; $r->headers_out->set(My-Header => "SomeValue"); If you are inside a registry script L the C> object. Howerever you can choose a slower method of generating headers by just printing them out before printing any response. This will work only if C> is in effect. For example: print "Content-type: text/html\n"; print "My-Header: SomeValue\n"; print "\n"; This method is slower since Apache needs to parse the text to identify certain headers it needs to know about. It also has several limitations which we will now discuss. When using this approach you must make sure that the C filehandle is not set to flush the data after each print (which is set by the value of a special perl variable C<$|>). Here we assume that STDOUT is the currently Ced filehandle and C<$|> affects it. For example this code won't work: local $| = 1; print "Content-type: text/html\n"; print "My-Header: SomeValue\n"; print "\n"; Having a true C<$|> causes the first print() call to flush its data immediately, which is sent to the internal HTTP header parser, which will fail since it won't see the terminating C<"\n\n">. One solution is to make sure that STDOUT won't flush immediately, like so: local $| = 0; print "Content-type: text/html\n"; print "My-Header: SomeValue\n"; print "\n"; Notice that we Cize that change, so it L. If you send headers line by line and their total length is bigger than 8k, you will have the header parser problem again, since mod_perl will flush data when the 8k buffer gets full. In which case the solution is not to print the headers one by one, but to buffer them all in a variable and then print the whole set at once. Notice that you don't have any of these problems with mod_cgi, because it ignores any of the flush attempts by Perl. mod_cgi simply opens a pipe to the external process and reads any output sent from that process at once. If you use C<$r> to set headers as explained at the beginning of this section, you won't encounter any of these problems. Finally, if you don't want Apache to send its own headers and you want to send your own set of headers (non-parsed headers handlers) use the Cassbackwards|docs::2.0::api::Apache2::RequestRec/C_assbackwards_>> method. Notice that registry handlers will do that for you if the script's name start with the C prefix. =head3 Forcing HTTP Response Headers Out Apache 2.0 doesn't provide a method to force HTTP response headers sending (what used to be done by C in Apache 1.3). HTTP response headers are sent as soon as the first bits of the response body are seen by the special core output filter that generates these headers. When the response handler sends the first chunks of body it may be cached by the mod_perl internal buffer or even by some of the output filters. The response handler needs to flush the output in order to tell all the components participating in the sending of the response to pass the data out. For example if the handler needs to perform a relatively long-running operation (e.g. a slow db lookup) and the client may timeout if it receives nothing right away, you may want to start the handler by setting the I header, following by an immediate flush: sub handler { my $r = shift; $r->content_type('text/html'); $r->rflush; # send the headers out $r->print(long_operation()); return Apache2::Const::OK; } If this doesn't work, check whether you have configured any third-party output filters for the resource in question. L may ignore the command to flush the data. =head2 Sending HTTP Response Body In mod_perl 2.0 a response body can be sent only during the response phase. Any attempts to do that in the earlier phases will fail with an appropriate explanation logged into the I file. This happens due to the Apache 2.0 HTTP architecture specifics. One of the issues is that the HTTP response filters are not setup before the response phase. =head2 Using Signal Handlers 3rd party Apache 2 modules should avoid using code relying on signals. This is because typical signal use is not thread-safe and modules which rely on signals may not work portably. Certain signals may still work for non-threaded mpms. For example C can be used under prefork MPM, but it won't work on any other MPM. Moreover the Apache developers don'tq guarantee that the signals that currently happen to work will continue to do so in the future Apache releases. So use them at your own risk. It should be possible to rework the code using signals to use an alternative solution, which works under threads. For example if you were using C to trap potentially long running I/O, you can modify the I/O logic for select/poll usage (or if you use APR I/O then set timeouts on the apr pipes or sockets). For example, Apache 1.3 on Unix made blocking I/O calls and relied on the parent process to send the SIGALRM signal to break it out of the I/O after a timeout expired. With Apache 2.0, APR support for timeouts on I/O operations is used so that signals or other thread-unsafe mechanisms are not necessary. CPU timeout handling is another example. It can be accomplished by modifying the computation logic to explicitly check for the timeout at intervals. Talking about C under prefork mpm, POSIX signals seem to work, but require Perl 5.8.x+. For example: use POSIX qw(SIGALRM); my $mask = POSIX::SigSet->new( SIGALRM ); my $action = POSIX::SigAction->new(sub { die "alarm" }, $mask); my $oldaction = POSIX::SigAction->new(); POSIX::sigaction(SIGALRM, $action, $oldaction ); eval { alarm 2; sleep 10 # some real code should be here alarm 0; }; POSIX::sigaction(SIGALRM, $oldaction); # restore original warn "got alarm" if $@ and $@ =~ /alarm/; For more details see: http://search.cpan.org/dist/perl/ext/POSIX/POSIX.pod#POSIX::SigAction. One could use the C<$SIG{ALRM}> technique, working for 5.6.x+, but it works B under DSO modperl build. Moreover starting from 5.8.0 Perl delays signal delivery, making signals safe. This change may break previously working code. For more information please see: http://search.cpan.org/dist/perl/pod/perl58delta.pod#Safe_Signals and http://search.cpan.org/dist/perl/pod/perlipc.pod#Deferred_Signals_%28Safe_Signals%29. For example if you had the alarm code: eval { local $SIG{ALRM} = sub { die "alarm" }; alarm 3; sleep 10; # in reality some real code should be here alarm 0; }; die "the operation was aborted" if $@ and $@ =~ /alarm/; It may not work anymore. Starting from 5.8.1 it's possible to circumvent the safeness of signals, by setting: $ENV{PERL_SIGNALS} = "unsafe"; as soon as you start your program (e.g. in the case of mod_perl in startup.pl). As of this writing, this workaround fails on MacOSX, POSIX signals must be used instead. For more information please refer to: http://search.cpan.org/dist/perl/pod/perl581delta.pod#Unsafe_signals_again_available and http://search.cpan.org/dist/perl/pod/perlrun.pod#PERL_SIGNALS. Though if you use perl 5.8.x+ it's preferrable to use the POSIX API technique explained earlier in this section. =head1 Perl Specifics in the mod_perl Environment In the following sections we discuss the specifics of Perl behavior under mod_perl. =head2 C Blocks Perl executes C blocks as soon as possible, at the time of compiling the code. The same is true under mod_perl. However, since mod_perl normally only compiles scripts and modules once, either in the parent server (at the server startup) or once per-child (on the first request using a module), C blocks in that code will only be run once. As the C manpage explains, once a C block has run, it is immediately undefined. In the mod_perl environment, this means that C blocks will not be run during the response to an incoming request unless that request happens to be the one that causes the compilation of the code, i.e. if it wasn't loaded yet. C blocks in modules and files pulled in via C or C will be executed: =over 4 =item * Only once, if pulled in by the parent process at the server startup. =item * Once per each child process or Perl interpreter if not pulled in by the parent process. =item * An additional time, once per each child process or Perl interpreter if the module is reloaded off disk again via C>. =item * Unpredictable if you fiddle with C<%INC> yourself. =back The C blocks behavior is different in C> and C> handlers, and their subclasses. =head2 C and C Blocks C and C blocks run when the source code compilation is complete, but before the program starts. C can mean "checkpoint" or "double-check" or even just "stop". C stands for "initialization". The difference is subtle; C blocks are run just after the compilation ends, C just before the runtime begins. (Hence the C<-c> command-line perl option runs C blocks but not C blocks.) Perl only calls these blocks during I, which mod_perl calls once at startup time. Under threaded mpm, these blocks will be called once per C>. Therefore C and C blocks don't work after the server is started, for the same reason these code samples don't work: % perl -e 'eval qq(CHECK { print "ok\n" })' % perl -e 'eval qq(INIT { print "ok\n" })' =head2 C Blocks As the C manpage explains, an C block is executed as late as possible, that is, when the interpreter exits. So for example mod_cgi will run its C blocks on each invocation, since on every invocation it starts a new interpreter and then kills it when the request processing is done. In the mod_perl environment, the interpreter does not exit after serving a single request (unless it is configured to do so) and hence it will run its C blocks only when it exits, which usually happens during the server shutdown, but may also happen earlier than that (e.g. a process exits because it has served a C number of requests). mod_perl does L for scripts running under C> and friends. The L section explains how to deal with cleanups for non-Registry handlers. C> API: C>, C> and C>, internally used by registry handlers, can be used to run C blocks at arbitrary times. =head2 Request-localized Globals mod_perl 2.0 provides two types of C handlers: C> and C>. Remember that the C directive is only relevant for the response phase handlers, it neither needed nor affects non-response phases. Under the handler: SetHandler perl-script several special global Perl variables are saved before the handler is called and restored afterwards. This includes: C<%ENV>, C<@INC>, C<$/>, C's C<$|> and C blocks array (C). Under: SetHandler modperl nothing is restored, so you should be especially careful to remember localize all special Perl variables so the local changes won't affect other handlers. =head2 C In the normal Perl code exit() is used to stop the program flow and exit the Perl interpreter. However under mod_perl we only want the stop the program flow without killing the Perl interpreter. You should take no action if your code includes exit() calls and it's OK to continue using them. mod_perl worries to override the exit() function with L which stops the program flow, and performs all the necessary cleanups, but doesn't kill the server. This is done by overriding: *CORE::GLOBAL::exit = \&ModPerl::Util::exit; so if you mess up with C<*CORE::GLOBAL::exit> yourself you better know what you are doing. You can still call C to kill the interpreter, again if you know what you are doing. One caveat is when C is called inside C -- L explains how to deal with this situation. =head1 C Handlers Family =head2 A Look Behind the Scenes If you have a CGI script F: #!/usr/bin/perl print "Content-type: text/plain\n\n"; print "Hello"; a typical registry family handler turns it into something like: package foo_bar_baz; sub handler { local $0 = "/full/path/to/test.pl"; #line 1 test.pl #!/usr/bin/perl print "Content-type: text/plain\n\n"; print "Hello"; } Turning it into an almost full-fledged mod_perl handler. The only difference is that it handles the return status for you. (META: more details on return status needed.) It then executes it as: foo_bar_baz::handler($r); passing the C> object as the only argument to the C function. Depending on the used registry handler the package is made of the file path, the uri or anything else. Check the handler's documentation to learn which method is used. =head2 Getting the C<$r> Object As explained in L the C<$r> object is always passed to the registry script's special function C as the first and the only argument, so you can get this object by accessing C<@_>, since: my $r = shift; print "Content-type: text/plain\n\n"; print "Hello"; is turned into: sub handler { my $r = shift; print "Content-type: text/plain\n\n"; print "Hello"; } behind the scenes. Now you can use C<$r> to call various mod_perl methods, e.g. rewriting the script as: my $r = shift; $r->content_type('text/plain'); $r->print(); If you are deep inside some code and can't get to the entry point to reach for C<$r>, you can use Crequest|docs::2.0::api::Apache2::RequestUtil/C_request_>>. =head1 Threads Coding Issues Under mod_perl The following sections discuss threading issues when running mod_perl under a threaded MPM. =head2 Thread-environment Issues The "only" thing you have to worry about your code is that it's thread-safe and that you don't use functions that affect all threads in the same process. Perl 5.8.0 itself is thread-safe. That means that operations like C, C, C, C<=>, C, C<+=>, etc. are thread-safe. Operations that involve system calls, may or may not be thread-safe. It all depends on whether the underlying C libraries used by the perl functions are thread-safe. For example the function C is not thread-safe when the implementation of C is not thread-safe. Other usually problematic functions include C, C, etc. Another important issue that shouldn't be missed is what some people refer to as I. Certain functions executed in a single thread affect the whole process and therefore all other threads running inside that process. For example if you C in one thread, all other thread now see the current working directory of that thread that C'ed to that directory. Other functions with similar effects include C, C, etc. Currently there is no cure for this problem. You have to find these functions in your code and replace them with alternative solutions which don't incur this problem. For more information refer to the I (I) manpage. =head2 Deploying Threads This is actually quite unrelated to mod_perl 2.0. You don't have to know much about Perl threads, other than L, to have your code properly work under threaded MPM mod_perl. If you want to spawn your own threads, first of all study how the new ithreads Perl model works, by reading the I, I (I) and I (I) manpages. Artur Bergman wrote an article which explains how to port pure Perl modules to work properly with Perl ithreads. Issues with C and other functions that rely on shared process' datastructures are discussed. I. =head2 Shared Variables Global variables are only global to the interpreter in which they are created. Other interpreters from other threads can't access that variable. Though it's possible to make existing variables shared between several threads running in the same process by using the function C. New variables can be shared by using the I attribute when creating them. This feature is documented in the I (I) manpage. =head1 Maintainers Maintainer is the person(s) you should contact with updates, corrections and patches. =over =item * =back =head1 Authors =over =item * =back Only the major authors are listed above. For contributors see the Changes file. =cut