DBD::Multi(3pm)

NAME
DBD::Multi - Manage Multiple Data Sources with Failover and Load Balancing

SYNOPSIS
  use DBI;

  my $other_dbh = DBI->connect(...);

  my $dbh = DBI->connect( 'dbi:Multi:', undef, undef, {
      dsns => [ # in priority order
          10 => [ 'dbi:SQLite:read_one.db', '', '' ],
          10 => [ 'dbi:SQLite:read_two.db', '', '' ],
          20 => [ 'dbi:SQLite:master.db',   '', '' ],
          30 => $other_dbh,
          40 => sub {  DBI->connect },
      ],
      # optional
      failed_max    => 1,     # short credibility
      failed_expire => 60*60, # long memory
      timeout       => 10,    # time out connection attempts after 10 seconds.
  });

  $dbh->prepare(...);  # Works like any other DBI handle.

  $dbh->multi_do_all(  # Loops through every single DB handle.
    sub {
        my $dbh = shift;
        ...
    } );

DESCRIPTION
This software manages multiple database connections for failovers and also simple load balancing. It acts as a proxy between your code and your database connections, transparently choosing a connection for each query, based on your preferences and present availability of the DB server.

This module is intended for read-only operations (where some other application is being used to handle replication).

This software does not prevent write operations from being executed. This is left up to the user. See "SUGGESTED USES" below for ideas.

The interface is nearly the same as other DBI drivers except that it allows you to specify multiple connections for a single handle.

CONFIGURING CONNECTIONS

  Configuring DSNs
Specify an attribute to the connect() constructor, "dsns". This is a list of DSNs to configure. The configuration is given in pairs. First comes the priority of the DSN. Second is the DSN.

The priorities specify which connections should be used first (lowest to highest). As long as the lowest priority connection is responding, the higher priority connections will never be used. If multiple connections have the same priority, then one connection will be chosen randomly for each operation. Note that the random DB is chosen when the statement is prepared. Therefore executing multiple queries on the same prepared statement handle will always run on the same connection.

The second parameter can a DBI object, a code ref which returns a DBI object, or a list of parameters to pass to the DBI connect() instructor. If a set of parameters or a code ref is given, then DBD::Multi will be able to attempt re-connect in the event that the connection is lost. If a DBI object is used, the DBD::Multi will give up permanently once that connection is lost.

These connections are lazy loaded, meaning they aren't made until they are actually used.

  Configuring Failures
By default, after a data source fails three times, it will not be tried again for 5 minutes. After that period, the data source will be tried again for future requests until it reaches its three failure limit (the cycle repeats forever).

To change the maximum number of failures allowed before a data source is deemed failed, set the "failed_max" parameter. To change the amount of time we remember a data source as being failed, set the "failed_expire" parameter in seconds.

  Timing out connections.
By default, if you attempt to connect to an IP that isn't answering, DBI will hang for a very long period of time. This behavior is not desirable in a multi database setup. Instead, it is better to give up on slow connections and move on to other databases quickly.

DBD::Multi will give up on connection attempts after 5 seconds and then try another connection. You may set the "timeout" parameter to change the timeout time, or set it to 0 to disable the timeout feature completely.

EXTRA METHODS

  multi_do_all
Loops through every database handle, executing an arbitrary coderef passing the current database handle as the first parameter and the original connection parameters as the second parameter.

If a database is unreachable, multi_do_all will skip over it.

    use Data::Dumper;
    my $expected_value = ...;
    $dbh->multi_do_all(
        sub {
            my $dbh = shift;
            my $source = shift;
            my($value) = $dbh->selectrow_array("SELECT ...");
            unless ( $value eq $expected_value ) {
                die "Unexpected value, $value found. (Expected $expected_value).  Data Source:\n", Dumper( $source );
            }
        } );

SUGGESTED USES
Here are some ideas on how to use this module effectively and safely.

It is important to remember that "DBD::Multi" is not intended for read-write operations. One suggestion to prevent accidental write operations is to make sure that the user you are connecting to the databases with has privileges sufficiently restricted to prevent updates.

Read-write operations should happen through a separate database handle that will somehow trigger replication to all of your databases. For example, your read-write handle might be connected to the master server that replicates itself to all of the subordinate servers.

Read-only database calls within your application would be updated to explicitly use the read-only (DBD::Multi) handle. It is not necessary to find every single call that can be load balanced, since they can safely be sent through the read/write handle as well.

TODO
There really isn't much of a TODO list for this module at this time. Feel free to submit a bug report to github <https://github.com/dwright/DBD-Multi/issues> if you think there is a feature missing.

Although there is some code intended for read/write operations, this should be considered not supported and not actively developed at this time. The actual read/write code remains un-documented because in the event that I ever do decide to work on supporting read/write operations, the API is not guaranteed to stay the same. The focus of this module is presently limited to read-only operations.

TESTING
DBD::Multi has it's own suite of regression tests. But, suppose you want to verify that you can slip DBD::Multi into whatever application you already have written without breaking anything.

Thanks to a feature of DBI, you can regression test DBD::Multi using any existing tests that already use DBI without having to update any of your code. Simply set the environment variable DBI_AUTOPROXY to 'dbi:Multi:' and then run your tests. DBD::Multi should act as a silent pipe between your application and whatever database driver you were previously using. This will help you verify that you aren't currently using some feature of the DBI that breaks DBD::Multi (If you are, please do me a favor and submit a bug report so I can fix it).

SEE ALSO
There are other modules that have similar, but different objectives. Depending on your specific needs these may be more or less suitable for your task:

CGI::Application::Plugin::DBH
    A plugin for the CGI::Application framework which makes it easy to support two database handles, and also supports lazy-loading.

DBD::Multiplex
    The original inspiration for this module. It doesn't support as many connection configurations options at this module. It does try to support write options in a single master, multiple slave configuration. It does this by parsing your SQL and trying to decide if you were doing a read or write operation.

DBIx::HA
    Written after this module. Built for high availability rather than load balancing. It purposely ignores some DBI features in favor of producing the fastest results for the most common operations. It doesn't utilize the standard DBI->connect() API, which means it will not work as a drop-in auto proxy.

DBI, perl - You should probably already know about these before using this module.

AUTHOR
Initially written by Casey West and Dan Wright for pair Networks, Inc. (www.pair.com)

Maintained by Dan Wright. <DWRIGHT@CPAN.ORG>.