NAME

       DBIx::Class::Storage::DBI::Replicated - BETA Replicated database support

SYNOPSIS

       The Following example shows how to change an existing $schema to a replicated storage
       type, add some replicated (read-only) databases, and perform reporting tasks.

       You should set the 'storage_type attribute to a replicated type.  You should also define
       your arguments, such as which balancer you want and any arguments that the Pool object
       should get.

         my $schema = Schema::Class->clone;
         $schema->storage_type( ['::DBI::Replicated', {balancer=>'::Random'}] );
         $schema->connection(...);

       Next, you need to add in the Replicants.  Basically this is an array of arrayrefs, where
       each arrayref is database connect information.  Think of these arguments as what you'd
       pass to the 'normal' $schema->connect method.

         $schema->storage->connect_replicants(
           [$dsn1, $user, $pass, \%opts],
           [$dsn2, $user, $pass, \%opts],
           [$dsn3, $user, $pass, \%opts],
         );

       Now, just use the $schema as you normally would.  Automatically all reads will be
       delegated to the replicants, while writes to the master.

         $schema->resultset('Source')->search({name=>'etc'});

       You can force a given query to use a particular storage using the search attribute
       'force_pool'.  For example:

         my $rs = $schema->resultset('Source')->search(undef, {force_pool=>'master'});

       Now $rs will force everything (both reads and writes) to use whatever was setup as the
       master storage.  'master' is hardcoded to always point to the Master, but you can also use
       any Replicant name.  Please see: DBIx::Class::Storage::DBI::Replicated::Pool and the
       replicants attribute for more.

       Also see transactions and "execute_reliably" for alternative ways to force read traffic to
       the master.  In general, you should wrap your statements in a transaction when you are
       reading and writing to the same tables at the same time, since your replicants will often
       lag a bit behind the master.

       If you have a multi-statement read only transaction you can force it to select a random
       server in the pool by:

         my $rs = $schema->resultset('Source')->search( undef,
           { force_pool => $db->storage->read_handler->next_storage }
         );

DESCRIPTION

       Warning: This class is marked BETA.  This has been running a production website using
       MySQL native replication as its backend and we have some decent test coverage but the code
       hasn't yet been stressed by a variety of databases.  Individual DBs may have quirks we are
       not aware of.  Please use this in first development and pass along your experiences/bug
       fixes.

       This class implements replicated data store for DBI. Currently you can define one master
       and numerous slave database connections. All write-type queries (INSERT, UPDATE, DELETE
       and even LAST_INSERT_ID) are routed to master database, all read-type queries (SELECTs) go
       to the slave database.

       Basically, any method request that DBIx::Class::Storage::DBI would normally handle gets
       delegated to one of the two attributes: "read_handler" or to "write_handler".
       Additionally, some methods need to be distributed to all existing storages.  This way our
       storage class is a drop in replacement for DBIx::Class::Storage::DBI.

       Read traffic is spread across the replicants (slaves) occurring to a user selected
       algorithm.  The default algorithm is random weighted.

NOTES

       The consistency between master and replicants is database specific.  The Pool gives you a
       method to validate its replicants, removing and replacing them when they fail/pass
       predefined criteria.  Please make careful use of the ways to force a query to run against
       Master when needed.

REQUIREMENTS

       Replicated Storage has additional requirements not currently part of DBIx::Class. See
       DBIx::Class::Optional::Dependencies for more details.

ATTRIBUTES

       This class defines the following attributes.

   schema
       The underlying DBIx::Class::Schema object this storage is attaching

   pool_type
       Contains the classname which will instantiate the "pool" object.  Defaults to:
       DBIx::Class::Storage::DBI::Replicated::Pool.

   pool_args
       Contains a hashref of initialized information to pass to the Balancer object.  See
       DBIx::Class::Storage::DBI::Replicated::Pool for available arguments.

   balancer_type
       The replication pool requires a balance class to provider the methods for choose how to
       spread the query load across each replicant in the pool.

   balancer_args
       Contains a hashref of initialized information to pass to the Balancer object.  See
       DBIx::Class::Storage::DBI::Replicated::Balancer for available arguments.

   pool
       Is a DBIx::Class::Storage::DBI::Replicated::Pool or derived class.  This is a container
       class for one or more replicated databases.

   balancer
       Is a DBIx::Class::Storage::DBI::Replicated::Balancer or derived class.  This is a class
       that takes a pool (DBIx::Class::Storage::DBI::Replicated::Pool)

   master
       The master defines the canonical state for a pool of connected databases.  All the
       replicants are expected to match this databases state.  Thus, in a classic Master / Slaves
       distributed system, all the slaves are expected to replicate the Master's state as quick
       as possible.  This is the only database in the pool of databases that is allowed to handle
       write traffic.

ATTRIBUTES IMPLEMENTING THE DBIx::Storage::DBI INTERFACE

       The following methods are delegated all the methods required for the
       DBIx::Class::Storage::DBI interface.

   read_handler
       Defines an object that implements the read side of BIx::Class::Storage::DBI.

   write_handler
       Defines an object that implements the write side of BIx::Class::Storage::DBI, as well as
       methods that don't write or read that can be called on only one storage, methods that
       return a $dbh, and any methods that don't make sense to run on a replicant.

   around: connect_info
       Preserves master's "connect_info" options (for merging with replicants.)  Also sets any
       Replicated-related options from connect_info, such as "pool_type", "pool_args",
       "balancer_type" and "balancer_args".

METHODS

       This class defines the following methods.

   BUILDARGS
       DBIx::Class::Schema when instantiating its storage passed itself as the first argument.
       So we need to massage the arguments a bit so that all the bits get put into the correct
       places.

   _build_master
       Lazy builder for the "master" attribute.

   _build_pool
       Lazy builder for the "pool" attribute.

   _build_balancer
       Lazy builder for the "balancer" attribute.  This takes a Pool object so that the balancer
       knows which pool it's balancing.

   _build_write_handler
       Lazy builder for the "write_handler" attribute.  The default is to set this to the
       "master".

   _build_read_handler
       Lazy builder for the "read_handler" attribute.  The default is to set this to the
       "balancer".

   around: connect_replicants
       All calls to connect_replicants needs to have an existing $schema tacked onto top of the
       args, since DBIx::Storage::DBI needs it, and any "connect_info" options merged with the
       master, with replicant opts having higher priority.

   all_storages
       Returns an array of of all the connected storage backends.  The first element in the
       returned array is the master, and the remainings are each of the replicants.

   execute_reliably ($coderef, ?@args)
       Given a coderef, saves the current state of the "read_handler", forces it to use reliable
       storage (e.g. sets it to the master), executes a coderef and then restores the original
       state.

       Example:

         my $reliably = sub {
           my $name = shift @_;
           $schema->resultset('User')->create({name=>$name});
           my $user_rs = $schema->resultset('User')->find({name=>$name});
           return $user_rs;
         };

         my $user_rs = $schema->storage->execute_reliably($reliably, 'John');

       Use this when you must be certain of your database state, such as when you just inserted
       something and need to get a resultset including it, etc.

   set_reliable_storage
       Sets the current $schema to be 'reliable', that is all queries, both read and write are
       sent to the master

   set_balanced_storage
       Sets the current $schema to be use the </balancer> for all reads, while all writes are
       sent to the master only

   connected
       Check that the master and at least one of the replicants is connected.

   ensure_connected
       Make sure all the storages are connected.

   limit_dialect
       Set the limit_dialect for all existing storages

   quote_char
       Set the quote_char for all existing storages

   name_sep
       Set the name_sep for all existing storages

   set_schema
       Set the schema object for all existing storages

   debug
       set a debug flag across all storages

   debugobj
       set a debug object

   debugfh
       set a debugfh object

   debugcb
       set a debug callback

   disconnect
       disconnect everything

   cursor_class
       set cursor class on all storages, or return master's

   cursor
       set cursor class on all storages, or return master's, alias for "cursor_class" above.

   unsafe
       sets the "unsafe" in DBIx::Class::Storage::DBI option on all storages or returns master's
       current setting

   disable_sth_caching
       sets the "disable_sth_caching" in DBIx::Class::Storage::DBI option on all storages or
       returns master's current setting

   lag_behind_master
       returns the highest Replicant "lag_behind_master" in DBIx::Class::Storage::DBI setting

   is_replicating
       returns true if all replicants return true for "is_replicating" in
       DBIx::Class::Storage::DBI

   connect_call_datetime_setup
       calls "connect_call_datetime_setup" in DBIx::Class::Storage::DBI for all storages

GOTCHAS

       Due to the fact that replicants can lag behind a master, you must take care to make sure
       you use one of the methods to force read queries to a master should you need realtime data
       integrity.  For example, if you insert a row, and then immediately re-read it from the
       database (say, by doing $row->discard_changes) or you insert a row and then immediately
       build a query that expects that row to be an item, you should force the master to handle
       reads.  Otherwise, due to the lag, there is no certainty your data will be in the expected
       state.

       For data integrity, all transactions automatically use the master storage for all read and
       write queries.  Using a transaction is the preferred and recommended method to force the
       master to handle all read queries.

       Otherwise, you can force a single query to use the master with the 'force_pool' attribute:

         my $row = $resultset->search(undef, {force_pool=>'master'})->find($pk);

       This attribute will safely be ignore by non replicated storages, so you can use the same
       code for both types of systems.

       Lastly, you can use the "execute_reliably" method, which works very much like a
       transaction.

       For debugging, you can turn replication on/off with the methods "set_reliable_storage" and
       "set_balanced_storage", however this operates at a global level and is not suitable if you
       have a shared Schema object being used by multiple processes, such as on a web application
       server.  You can get around this limitation by using the Schema clone method.

         my $new_schema = $schema->clone;
         $new_schema->set_reliable_storage;

         ## $new_schema will use only the Master storage for all reads/writes while
         ## the $schema object will use replicated storage.

AUTHOR

         John Napiorkowski <john.napiorkowski@takkle.com>

       Based on code originated by:

         Norbert CsongrXdi <bert@cpan.org>
         Peter SiklXsi <einon@einon.hu>

LICENSE

       You may distribute this code under the same terms as Perl itself.