NAME

       Rose::DB::Cache - A mod_perl-aware cache for Rose::DB objects.

SYNOPSIS

         # Usage
         package My::DB;

         use base 'Rose::DB';
         ...

         $cache = My::DB->db_cache;

         $db = $cache->get_db(...);

         $cache->set_db($db);

         $cache->clear;

         # Subclassing
         package My::DB::Cache;

         use Rose::DB::Cache;
         our @ISA = qw(Rose::DB::Cache);

         # Override methods as desired
         sub get_db          { ... }
         sub set_db          { ... }
         sub prepare_db      { ... }
         sub build_cache_key { ... }
         sub clear           { ... }
         ...

DESCRIPTION

       Rose::DB::Cache provides both an API and a default implementation of a caching system for
       Rose::DB objects.  Each Rose::DB-derived class references a Rose::DB::Cache-derived object
       to which it delegates cache-related activities.  See the new_or_cached method for an
       example.

       The default implementation caches and returns Rose::DB objects using the combination of
       their type and domain as the cache key.  There is no cache expiration or other cache
       cleaning.

       The only sophistication in the default implementation is that it is mod_perl- and
       Apache::DBI-aware.  When running under mod_perl, with or without Apache::DBI, the dbh
       attribute of each cached Rose::DB object is set to "undef" at the end of each request.
       Additionally, any db connections made in a pre-fork parent apache process are not cached.

       When running under Apache::DBI, the behavior described above will ensure that
       Apache::DBI's "ping" and rollback features work as expected, keeping the DBI database
       handles contained within each Rose::DB object connected and alive.

       When running under mod_perl without Apache::DBI, the behavior described above will use a
       single DBI database connection per cached Rose::DB object per request, but will discard
       these connections at the end of each request.

       Both mod_perl 1.x and 2.x are supported.  Under mod_perl 2.x, you should load Rose::DB on
       server startup (e.g., in your "startup.pl" file).  If this is not possible, then you must
       explicitly tell Rose::DB::Cache that apache has started up already by setting
       apache_has_started to a true value.

       Subclasses can override any and all methods described below in order to implement their
       own caching strategy.

CLASS METHODS

       apache_has_started [BOOL]
           Get or set a boolean value indicating whether or not apache has completed its startup
           process.  If this value is not set explicitly, a best guess as to the answer will be
           returned.

       build_cache_key PARAMS
           Given the name/value pairs PARAMS, return a string representing the corresponding
           cache key.  Calls to this method from within Rose::DB::Cache will include at least
           "type" and "domain" parameters, but you may pass any parameters if you override all
           methods that call this method in your subclass.

       default_use_cache_during_apache_startup [BOOL]
           Get or set a boolean value that determines the default value of the
           use_cache_during_apache_startup object attribute.  The default value is false.  See
           the use_cache_during_apache_startup documentation for more information.

       entry_class [CLASS]
           Get or set the name of the Rose::DB::Cache::Entry-derived class used to store cached
           Rose::DB objects on behalf of this class.  The default value is
           Rose::DB::Cache::Entry.

CONSTRUCTORS

       new PARAMS
           Constructs a new Rose::DB::Cache object based on PARAMS, where PARAMS are name/value
           pairs.  Any object method is a valid parameter name.

OBJECT METHODS

       clear
           Clear the cache entirely.

       db_cache_entries
           Returns a list (in list context) or reference to an array (in scalar context) of cache
           entries for each cached db object.

       db_cache_keys
           Returns a list (in list context) or reference to an array (in scalar context) of keys
           for each L <cache entries|Rose::DB::Cache::Entry>.

       get_db [PARAMS]
           Return the cached Rose::DB-derived object corresponding to the name/value pairs passed
           in PARAMS.  PARAMS are passed to the build_cache_key method, and the key returned is
           used to look up the cached object.

           If a cached object is found, the prepare_db method is called, passing the cached db
           object and its corresponding Rose::DB::Cache::Entry object as arguments.  The cached
           db object is then returned.

           If no such object exists in the cache, undef is returned.

       prepare_for_apache_fork
           Prepares the cache for the initial fork of the apache parent process by
           disconnect()ing all database handles and deleting all cache entries that were created
           during apache startup.  This call is only necessary if running under mod_perl and
           use_cache_during_apache_startup set set to true.  See the
           use_cache_during_apache_startup documentation for more information.

       prepare_db [DB, ENTRY]
           Prepare the cached Rose::DB-derived object DB for usage.  The cached's db object's
           Rose::DB::Cache::Entry object, ENTRY, is also passed.

           When NOT running under mod_perl, this method does nothing.

           When running under mod_perl (version 1.x or 2.x), this method will do the following:

           •   Any DBI database handle created inside a Rose::DB object during apache server
               startup will be marked as such.  Any attempt to use such an object after the
               apache startup process has completed (i.e., in a child apache process) will cause
               it to be discarded and replaced.  Note that you usually don't want it to come to
               this.  It's better to cleanly disconnect all such database handles before the
               first apache child forks off.  See the documentation for the
               use_cache_during_apache_startup and prepare_for_apache_fork methods for more
               information.

           •   All DBI database handles contained in cached Rose::DB objects will be cleared at
               the end of each request using a "PerlCleanupHandler".  This will cause
               DBI->connect to be called the next time a dbh is requested from a cached Rose::DB
               object, which in turn will trigger Apache::DBI's ping mechanism to ensure that the
               database handle is fresh.

           Putting all the pieces together, the following implementation of the init_db method in
           your Rose::DB::Object-derived common base class will ensure that database connections
           are shared and fresh under mod_perl and (optionally) Apache::DBI, but unshared
           elsewhere:

             package My::DB::Object;

             use base 'Rose::DB::Object';

             use My::DB; # isa Rose::DB
             ...

             BEGIN:
             {
               if($ENV{'MOD_PERL'})
               {
                 *init_db = sub { My::DB->new_or_cached };
               }
               else # act "normally" when not under mod_perl
               {
                 *init_db = sub { My::DB->new };
               }
             }

       set_db DB
           Add the Rose::DB-derived object DB to the cache.  The DB's domain, type, and the db
           object itself (under the parameter name "db") are all are passed to the
           build_cache_key method and the DB object is stored under the key returned.

           If running under mod_perl and the apache server is starting up and
           use_cache_during_apache_startup is set to true, then the DB object is not added to the
           cache, but merely returned.

       use_cache_during_apache_startup [BOOL]
           Get or set a boolean value that determines whether or not to cache database objects
           during the apache server startup process.  The default value is determined by the
           default_use_cache_during_apache_startup class method.

           DBI database handles created in the parent apache process cannot be used in child
           apache processes.  Furthermore, in the case of at least one one DBI driver class, you
           must also ensure that any database handles created in the apache parent process during
           server startup are properly disconnect()ed before you fork off the first apache child.
           Failure to do so may cause segmentation faults(!) in child apache processes.

           The upshot is that if use_cache_during_apache_startup is set to true, you should call
           prepare_for_apache_fork at the very end of the apache startup process (i.e., once all
           other Perl modules have been loaded and all other Perl code has run).  This is usually
           done by placing a call at the bottom of the traditional "startup.pl" file.  Assuming
           "My::DB" is your Rose::DB-derived class:

               My::DB->db_cache->prepare_for_apache_fork();

           A convenience method exists in Rose::DB as well, which simply translates into call
           shown above:

               My::DB->prepare_cache_for_apache_fork();

AUTHOR

       John C. Siracusa (siracusa@gmail.com)

LICENSE

       Copyright (c) 2010 by John C. Siracusa.  All rights reserved.  This program is free
       software; you can redistribute it and/or modify it under the same terms as Perl itself.