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                  ColumnDefinition (1)

Note that more than one column I<may> share a single definition object
(this is explained in the
L<C<Alzabo::Create::ColumnDefinition>|Alzabo::Create::ColumnDefinition>
documentation).  This is only relevant if you are writing a schema
creation interface.

=head2 Other classes

=over 4

=item * C<Alzabo::Driver>

These objects handle all the actual communication with the database,
using a thin wrapper over DBI.  The subclasses are used to implement
functionality that must be handled uniquely for a given RDBMS, such as
creating new values for sequenced columns.

=item * C<Alzabo::SQLMaker>

These objects handle the generation of all SQL for runtime operations.
The subclasses are used to implement functionality that varies between
RDBMS's, such as outer joins.

=item * C<Alzabo::RDBMSRules>

These objects perform several funtions.  First, they validate things
such as schema or table names, column type and length, etc.

Second they are used to generate SQL for creating and updating the
database and its tables.

And finally, they also handle the reverse engineering of an existing
database.

=item * C<Alzabo::Runtime::Row> and C<Alzabo::Runtime::RowState::*>

The C<Alzabo::Runtime::Row> class represents a single row.  These
objects are created by
L<C<Alzabo::Runtime::Table>|Alzabo::Runtime::Table>,
L<C<Alzabo::Runtime::RowCursor>|Alzabo::Runtime::RowCursor>, and
L<C<Alzabo::Runtime::JoinCursor>|Alzabo::Runtime::JoinCursor> objects.
It is the sole interface by which actual data is retrieved, updated,
or deleted in a table.

The various C<RowState> classes are used in order to change a row's
behavior depending on whether it is live, live and cached, potential,
or deleted.

=item * C<Alzabo::Runtime::JoinCursor> and C<Alzabo::Runtime::RowCursor>

These objects are cursor that returns row objects.  Using a cursor
saves a lot of memory for big selects.

=item * C<Alzabo::Runtime::UniqueRowCache>

Loading this class turns on Alzabo's simple row caching mechanism.

=item * C<Alzabo::Config>

This class is generated by Makefile.PL during installation and
contains information such as what directory contains saved schemas and
other configuration information.

=item * C<Alzabo::ChangeTracker>

This object provides a method for an object to register a series to
backout from multiple changes.  This is done by providing the
ChangeTracker object with a callback after a change is succesfully
made to an object or objects.  If a future change in a set of
operations fail, the tracker can be told to back the changes out. This
is used primarily in
L<C<Alzabo::Create::Schema>|Alzabo::Create::Schema>.

=item * C<Alzabo::MethodMaker>

This module can auto-generate useful methods for you schema, table,
and row objects based on the structure of your schema.

=item * C<Alzabo::Exceptions>

This object creates the exception subclasses used by Alzabo.

=back

=head1 WHY THE SUBDIVISION BETWEEN Alzabo::*, Alzabo::Create::*, and Alzabo::Runtime::*?

There are several reasons for doing this:

=over 4

=item *

In some environments (mod_perl) we would like to optimize for memory.
For an application that uses an existing schema, all we need is to be
able read object information, rather than needing to change the
schema's definition.  This means there is no reason to have the
overhead of compiling all the methods used when creating and modifying
objects.

=item *

In other environments (for example, when running as a separately
spawned CGI process) compile time is important.

=item *

Many people using Alzabo will use the schema creation GUI and then
write an application using that schema.  At the simplest level, they
would only need to learn how to instantiate C<Alzabo::Runtime::Row>
objects and how that class's methods work.  For more sophisticated
users, they can still avoid having to ever look at documentation on
methods that alter the schema and its contained objects.