Alzabo::Design - Documentation on Alzabo's design

This document describes some of the Alzabo's design.

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 CWAlzabo::Create::ColumnDefinition documentation). This is only relevant if you are writing a schema creation interface.

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. 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. 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. The CWAlzabo::Runtime::Row class represents a single row. These objects are created by CWAlzabo::Runtime::Table, CWAlzabo::Runtime::RowCursor, and CWAlzabo::Runtime::JoinCursor objects. It is the sole interface by which actual data is retrieved, updated, or deleted in a table. The various CWRowState classes are used in order to change a row's behavior depending on whether it is live, live and cached, potential, or deleted. These objects are cursor that returns row objects. Using a cursor saves a lot of memory for big selects. Loading this class turns on Alzabo's simple row caching mechanism. This class is generated by Makefile.PL during installation and contains information such as what directory contains saved schemas and other configuration information. 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 CWAlzabo::Create::Schema. This module can auto-generate useful methods for you schema, table, and row objects based on the structure of your schema. This object creates the exception subclasses used by Alzabo.

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 CWAlzabo::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.

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 CWAlzabo::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.

Dave Rolsky, <autarch@urth.org>