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Alzabo::Design(3) |
User Contributed Perl Documentation |
Alzabo::Design(3) |
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
"Alzabo::Create::ColumnDefinition"
documentation). This is only relevant if you are writing a schema creation
interface.
- "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 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 "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.
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.
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.
Dave Rolsky, <autarch@urth.org>
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