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.

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¶

Dave Rolsky, <autarch@urth.org>