=head1 NAME

DBIx::Class::Storage::DBI::Replicated::Introduction - Minimum Need to Know

=head1 SYNOPSIS

This is an introductory document for L<DBIx::Class::Storage::DBI::Replicated>.

This document is not an overview of what replication is or why you should be
using it. It is not a document explaining how to setup MySQL native replication
either. Copious external resources are available for both. This document
presumes you have the basics down.

=head1 DESCRIPTION

L<DBIx::Class> supports a framework for using database replication. This system
is integrated completely, which means once it's setup you should be able to
automatically just start using a replication cluster without additional work or
changes to your code. Some caveats apply, primarily related to the proper use
of transactions (you are wrapping all your database modifying statements inside
a transaction, right ;) ) however in our experience properly written DBIC will
work transparently with Replicated storage.

Currently we have support for MySQL native replication, which is relatively
easy to install and configure.  We also currently support single master to one
or more replicants (also called 'slaves' in some documentation).  However the
framework is not specifically tied to the MySQL framework and supporting other
replication systems or topographies should be possible.  Please bring your
patches and ideas to the #dbix-class IRC channel or the mailing list.

For an easy way to start playing with MySQL native replication, see:
L<MySQL::Sandbox>.

If you are using this with a L<Catalyst> based application, you may also want
to see more recent updates to L<Catalyst::Model::DBIC::Schema>, which has
support for replication configuration options as well.

=head1 REPLICATED STORAGE

By default, when you start L<DBIx::Class>, your Schema (L<DBIx::Class::Schema>)
is assigned a storage_type, which when fully connected will reflect your
underlying storage engine as defined by your chosen database driver.  For
example, if you connect to a MySQL database, your storage_type will be
L<DBIx::Class::Storage::DBI::mysql>  Your storage type class will contain
database specific code to help smooth over the differences between databases
and let L<DBIx::Class> do its thing.

If you want to use replication, you will override this setting so that the
replicated storage engine will 'wrap' your underlying storages and present
a unified interface to the end programmer.  This wrapper storage class will
delegate method calls to either a master database or one or more replicated
databases based on if they are read only (by default sent to the replicants)
or write (reserved for the master).  Additionally, the Replicated storage
will monitor the health of your replicants and automatically drop them should
one exceed configurable parameters.  Later, it can automatically restore a
replicant when its health is restored.

This gives you a very robust system, since you can add or drop replicants
and DBIC will automatically adjust itself accordingly.

Additionally, if you need high data integrity, such as when you are executing
a transaction, replicated storage will automatically delegate all database
traffic to the master storage.  There are several ways to enable this high
integrity mode, but wrapping your statements inside a transaction is the easy
and canonical option.

=head1 PARTS OF REPLICATED STORAGE

A replicated storage contains several parts.  First, there is the replicated
storage itself (L<DBIx::Class::Storage::DBI::Replicated>).  A replicated storage
takes a pool of replicants (L<DBIx::Class::Storage::DBI::Replicated::Pool>)
and a software balancer (L<DBIx::Class::Storage::DBI::Replicated::Balancer>).
The balancer does the job of splitting up all the read traffic amongst the
replicants in the Pool. Currently there are two types of balancers, a Random one
which chooses a Replicant in the Pool using a naive randomizer algorithm, and a
First replicant, which just uses the first one in the Pool (and obviously is
only of value when you have a single replicant).

=head1 REPLICATED STORAGE CONFIGURATION

All the parts of replication can be altered dynamically at runtime, which makes
it possibly to create a system that automatically scales under load by creating
more replicants as needed, perhaps using a cloud system such as Amazon EC2.
However, for common use you can setup your replicated storage to be enabled at
the time you connect the databases.  The following is a breakdown of how you
may wish to do this.  Again, if you are using L<Catalyst>, I strongly recommend
you use (or upgrade to) the latest L<Catalyst::Model::DBIC::Schema>, which makes
this job even easier.

First, you need to get a C<$schema> object and set the storage_type:

  my $schema = MyApp::Schema->clone;
  $schema->storage_type([
    '::DBI::Replicated' => {
      balancer_type => '::Random',
      balancer_args => {
        auto_validate_every => 5,
        master_read_weight => 1
      },
      pool_args => {
        maximum_lag =>2,
      },
    }
  ]);

Then, you need to connect your L<DBIx::Class::Schema>.

  $schema->connection($dsn, $user, $pass);

Let's break down the settings.  The method L<DBIx::Class::Schema/storage_type>
takes one mandatory parameter, a scalar value, and an option second value which
is a Hash Reference of configuration options for that storage.  In this case,
we are setting the Replicated storage type using '::DBI::Replicated' as the
first value.  You will only use a different value if you are subclassing the
replicated storage, so for now just copy that first parameter.

The second parameter contains a hash reference of stuff that gets passed to the
replicated storage.  L<DBIx::Class::Storage::DBI::Replicated/balancer_type> is
the type of software load balancer you will use to split up traffic among all
your replicants.  Right now we have two options, "::Random" and "::First". You
can review documentation for both at:

L<DBIx::Class::Storage::DBI::Replicated::Balancer::First>,
L<DBIx::Class::Storage::DBI::Replicated::Balancer::Random>.

In this case we will have three replicants, so the ::Random option is the only
one that makes sense.

'balancer_args' get passed to the balancer when it's instantiated.  All
balancers have the 'auto_validate_every' option.  This is the number of seconds
we allow to pass between validation checks on a load balanced replicant. So
the higher the number, the more possibility that your reads to the replicant
may be inconsistent with what's on the master.  Setting this number too low
will result in increased database loads, so choose a number with care.  Our
experience is that setting the number around 5 seconds results in a good
performance / integrity balance.

'master_read_weight' is an option associated with the ::Random balancer. It
allows you to let the master be read from.  I usually leave this off (default
is off).

The 'pool_args' are configuration options associated with the replicant pool.
This object (L<DBIx::Class::Storage::DBI::Replicated::Pool>) manages all the
declared replicants.  'maximum_lag' is the number of seconds a replicant is
allowed to lag behind the master before being temporarily removed from the pool.
Keep in mind that the Balancer option 'auto_validate_every' determines how often
a replicant is tested against this condition, so the true possible lag can be
higher than the number you set.  The default is zero.

No matter how low you set the maximum_lag or the auto_validate_every settings,
there is always the chance that your replicants will lag a bit behind the
master for the supported replication system built into MySQL.  You can ensure
reliable reads by using a transaction, which will force both read and write
activity to the master, however this will increase the load on your master
database.

After you've configured the replicated storage, you need to add the connection
information for the replicants:

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

These replicants should be configured as slaves to the master using the
instructions for MySQL native replication, or if you are just learning, you
will find L<MySQL::Sandbox> an easy way to set up a replication cluster.

And now your $schema object is properly configured!  Enjoy!

=head1 FURTHER QUESTIONS?

Check the list of L<additional DBIC resources|DBIx::Class/GETTING HELP/SUPPORT>.

=head1 COPYRIGHT AND LICENSE

This module is free software L<copyright|DBIx::Class/COPYRIGHT AND LICENSE>
by the L<DBIx::Class (DBIC) authors|DBIx::Class/AUTHORS>. You can
redistribute it and/or modify it under the same terms as the
L<DBIx::Class library|DBIx::Class/COPYRIGHT AND LICENSE>.

