Ubuntu Manpage: Alzabo::Design - Documentation on Alzabo's design

NAME

       Alzabo::Design - Documentation on Alzabo's design

DESCRIPTION

       This document describes some of the Alzabo's design.

ARCHITECTURE

There are objects representing the schema, which contains table objects. Table objects contain column,
foreign key, and index objects. Column objects contain column definition objects. A single column
definition may be shared by multiple columns, but has only one owner.

This is a diagram of these inheritance relationships:

Alzabo::* (::Schema, ::Table, ::Column, ::ColumnDefinition, ::ForeignKey, ::Index)
/ \
is parent to
/ \
Alzabo::Create::* Alzabo::Runtime::*

This a diagram of how objects contain other objects:

Schema - makes--Alzabo::SQLMaker subclass object (many)
/ \
contains contains--Alzabo::Driver subclass object (1)
| \
Table (0 or more) Alzabo::RDBMSRules subclass object (1)
/ \ (* Alzabo::Create::Schema only)
/ \
contains--------------------
/ \ \
/ \ \
ForeignKey Column (0 or more) Index (0 or more)
(0 or more) |
contains
|
ColumnDefinition (1)

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

Other classes
• "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.

• "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.

• "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.

• "Alzabo::Runtime::Row" and "Alzabo::Runtime::RowState::*"

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

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

• "Alzabo::Runtime::JoinCursor" and "Alzabo::Runtime::RowCursor"

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

• "Alzabo::Runtime::UniqueRowCache"

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

• "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.

• "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 successfully
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 "Alzabo::Create::Schema".

• "Alzabo::MethodMaker"

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

• "Alzabo::Exceptions"

This object creates the exception subclasses used by Alzabo.

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

       There are several reasons for doing this:

       •   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.

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

       •   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
           "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.

RATIONALE FOR CURSORS

       Using cursors is definitely more complicated.  However, there are two excellent reasons for  using  them:
       speed  and memory savings.  As an example, I did a test with the old code (which returned all its objects
       at once) against a table with about  8,000  rows  using  the  "Alzabo::Runtime::Table->all_rows"  method.
       Under  the  old  implementation,  it  took  significantly  longer  to  return  the  first row.  Even more
       importantly than that, the old implementation used up about 10MB of memory versus about 4MB!  Now imagine
       that with a 1,000,000 row table.

       Thus Alzabo uses cursors so it can scale better.  This is a particularly big win in the  case  where  you
       are  working  through  a  long list of rows and may stop before the end is reached.  With cursors, Alzabo
       creates only as many rows as you need.  Plus the start up time on your loop is much,  much  quicker.   In
       the end, your program is quicker and less of a memory hog.  This is good.

AUTHOR