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_type => '::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.

ATTRIBUTES IMPLEMENTING THE DBIx::Storage::DBI INTERFACE¶

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

METHODS¶

This class defines the following methods.

  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');

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

This attribute will safely be ignored 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.

FURTHER QUESTIONS?¶

Check the list of additional DBIC resources.

COPYRIGHT AND LICENSE¶

This module is free software copyright by the DBIx::Class (DBIC) authors. You can redistribute it and/or modify it under the same terms as the DBIx::Class library.