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<!--
$FML: db_modules.sgml,v 1.1 2005/08/03 13:22:06 fukachan Exp $
-->
<!--
XXX-TODO: database
-->
<chapter id="db.module">
<title>
Database Related Modules
</title>
<sect1 id="db.module.ovewview">
<title>
Overview
</title>
<para>
ML driver needs to hold some data on mail. Some data exixsts
persistently but some data with expiration (data is discarded after
some interval).
</para>
<para>
An example of permisistent data is thread database.
The latter case, an exapmle of non persistent data, is message-id cache.
The thread database also has message-id cache in it, which is a
sub-set of all message-id set of incoming messages. We handle these
two data separately.
</para>
<para>
We should handle these data in integrated way.
<screen>
module A ---| A |--- persistent database
module B ---| da |
module C ---| pt |
modlue D ---| er |--- cache with expiration
</screen>
An example of this persistent DB, thread DB, is
<link linkend="threadtrack.db">
Mail::Message::DB
</link>
class. Examples of cache database are Tie::JournaledDir and
FML::Cache::Ring classes.
</para>
<para>
It is better to use RDBMS for important persistent data such as
address list. This chapter describes persistent data handling only for
&fml8; internal use.
</para>
</sect1>
<sect1 id="db.module.persistent">
<title>
Persistent Data Use
</title>
<para>
Handling of persistent data is important.
We should consider use of
<link linkend="dbms">
DBMS
</link>
<footnote>
<para>
DBMS = DataBase Management System
</para>
</footnote>
for important data.
</para>
<para>
This chapter describes only &fml8; internal use.
</para>
<para>
&fml8; provides two type of cache databases.
The type is time saving or space saving.
</para>
</sect1>
<sect1 id="db.module.Tie.JournaledFile">
<title>
Tie::JournaledFile Class
</title>
<para>
This class appends data into a file.
<ulink url="../../en/modules/Tie/JournaledFile.txt">
Tie::JournaledFile
</ulink>
class provides this function.
</para>
<para>
This class is append only. It is similar to LFS (Log Structured File
System).
</para>
<para>
Operation to extract a value is to get the latest (last appended) value.
That is, the last match.
</para>
<para>
In other words, data cache logs all data to enable back track.
</para>
<para>
We cannot restrict the maximum size of this cache.
This class holds data within some period without size limitation.
We control cache expiration by time.
</para>
<para>
To get the following structure as a result,
you can use get_all_values_as_hash_ref() method.
<screen>
key => [
log1 (line A of file 1),
log2 (line B of file 2),
log2 (line C of file 3),
]
</screen>
</para>
<warning>
<para>
Search algorithm is either of first match or last match. The last
match by default since the last appended data is latest.
</para>
</warning>
</sect1>
<sect1 id="db.module.FML.Cache.Ring">
<title>
FML::Cache::Ring Class
</title>
<para>
<ulink url="../../en/modules/File/CacheDir.txt">
FML::Cache::Ring
</ulink>
holds data with size limit but without time limitation.
</para>
<para>
It is suitable for debug since the debug data is expected not to eat
disk.
</para>
<para>
This class holds data in files in a directory such as
$ml_home_dir/var/db/.
For example, files named as db/1 .. db/100 are created.
These files are used repeatedly.
</para>
<warning>
<para>
RingBuffer class is integrated into FML::Cache::Ring class.
FML::Cache::Ring is derived from File::CacheDir class.
</para>
</warning>
</sect1>
</chapter>
|
