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<!-- Module Developer's Guide -->

<chapter>
	
	<title>&develguide;</title>
	<section><title>Introduction</title>
		<para>
		Kamailio uses a database API for requests of numerous different
		types of data. Four primary operations are supported:
			<itemizedlist>
				<listitem><para>query</para></listitem>
				<listitem><para>insert</para></listitem>
				<listitem><para>update</para></listitem>
				<listitem><para>delete</para></listitem>
			</itemizedlist>
		</para>
		<para>
			This module relays these database requests to user implemented
			Perl functions.
		</para>
	</section>
	<section><title>Base class Kamailio::VDB</title>
		<para>
			A client module has to be configured to use the db_perlvdb module in conjunction
			with a Perl class to provide the functions. The configured class needs to
			inherit from the base class <literal role="code">Kamailio::VDB</literal>.
		</para>
		<para>
			Derived classes have to implement the necessary
			functions "query", "insert", "update" and/or "delete". The client module
			specifies the necessary functions.
			To find out which functions are called from a module, its processes may
			be evaluated with the <literal role="code">Kamailio::VDB::Adapter::Describe</literal> class which will
			log incoming requests (without actually providing any real functionality).
		</para>
		<para>
			While users can directly implement their desired functionality in a class
			derived from Kamailio::VDB, it is advisable to split the implementation into
			an Adapter that transforms the relational structured parameters into pure
			Perl function arguments, and add a virtual table (VTab) to provide the
			relaying to an underlying technology.
		</para>
	</section>
	<section><title>Data types</title>
		<para>
			Before introducing the higher level concepts of this module, the used
			datatypes will briefly be explained.
			The Kamailio Perl library includes some data types that have to be used
			in this module:
		</para>
		<section><title>Kamailio::VDB::Value</title>
		<para>
			A value includes a data type flag and a value. Valid data types are
			DB_INT, DB_DOUBLE, DB_STRING, DB_STR, DB_DATETIME, DB_BLOB, DB_BITMAP.
			A new variable may be created with <programlisting>
my $val = new Kamailio::VDB::Value(DB_STRING, "foobar");
</programlisting>
			Value objects contain the type() and data() methods to get or set the type
			and data attributes.
		</para>
		</section>
		<section><title>Kamailio::VDB::Pair</title>
		<para>
			The Pair class is derived from the Value class and additionally contains a
			column name (key).
			A new variable may be created with <programlisting>
my $pair = new Kamailio::VDB::Pair("foo", DB_STRING, "bar");
</programlisting>
			where foo is the key and bar is the value.
			Additionally to the methods of the Value class, it contains a key() method to
			get or set the key attribute.
		</para>
		</section>
		<section><title>Kamailio::VDB::ReqCond</title>
		<para>
			The ReqCond class is used for select condition and is derived from the Pair
			class. It contains an additional operator attribute.
			A new variable may be created with <programlisting>
my $cond = new Kamailio::VDB::ReqCond("foo", ">", DB_INT, 5);
</programlisting>
			where foo is the key, "greater" is the operator and 5 is the value to compare.
			Additionally to the methods of the Pair class, it contains an op() method to
			get or set the operator attribute.
		</para>
		</section>
		<section><title>Kamailio::VDB::Column</title>
		<para>
			This class represents a column definition or database schema. It contains an
			array for the column names and an array for the column types. Both arrays need
			to have the same length.
			A new variable may be created with <programlisting>
my @types = { DB_INT, DB_STRING };
my @names = { "id", "vals" };
my $cols = new Kamailio::VDB::Column(\@types, \@names);
</programlisting>
			The class contains the methods type() and name() to get or set the type and name
			arrays.
		</para>
		</section>
		<section><title>Kamailio::VDB::Result</title>
		<para>
			The Result class represents a query result. It contains a schema (class Column)
			and an array of rows, where each row is an array of Values. The object methods
			coldefs() and rows() may be used to get and set the object attributes.
		</para>
		</section>
	</section>
	<section><title>Adapters</title>
		<para>
			Adapters should be used to turn the relational structured database request into
			pure Perl function arguments. The alias_db function alias_db_lookup for example 
			takes a user/host pair, and turns it into another user/host pair. The Alias
			adapter turns the ReqCond array into two separate scalars that are used as parameters
			for a VTab call.
		</para>
		<para>
			Adapter classes have to inherit from the Kamailio::VDB base class and may provide
			one or more functions with the names insert, update, replace, query and/or delete,
			depending on the module which is to be used with the adapter. While modules such as
			alias_db only require a query function, others -- such as siptrace -- depend
			on inserts only.
		</para>
		<section><title>Function parameters</title>
			<para>
				The implemented functions need to deal with the correct data types. The
				parameter and return types are listed in this section.
			</para>
			<para>
				<emphasis>insert()</emphasis> is passed an array of Kamailio::VDB::Pair objects.
				It should return an integer value.
			</para>
			<para>
				<emphasis>replace()</emphasis> is passed an array of Kamailio::VDB::Pair objects.
				This function is currently not used by any publicly available modules.
				It should return an integer value.
			</para>
			<para>
				<emphasis>delete()</emphasis> is passed an array of Kamailio::VDB::ReqCond objects.
				It should return an integer value.
			</para>
			<para>
				<emphasis>update()</emphasis> is passed an array of Kamailio::VDB::ReqCond objects
				(which rows to update) and an array of Kamailio::VDB::Pair objects
				(new data).
				It should return an integer value.
			</para>
			<para>
				<emphasis>query()</emphasis> is passed an array of Kamailio::VDB::ReqCond objects
				(which rows to select), an array of strings (which column names to return)
				and a single string by which column to sort.
				It should return an object of type Kamailio::VDB::Result.
			</para>
		</section>
	</section>
	<section><title>VTabs</title>
		<para>
			VTabs (virtual tables) provide a particular implementation for an adapter. The Alias
			adapter e.g. calls a function with two parameters (user, host) and expects a hash to
			be returned with the two elements username and domain, or undef (when no result
			is found).
			A sample VTab implementation for the Alias adapter demonstrates this technique with
			a Perl hash that contains the alias data.
		</para>
		<para>
			The standard Adapter/VTab pattern lets the user choose between three options on how to
			implement VTabs:
			<itemizedlist>
				<listitem><para><emphasis>Single function</emphasis>. When a function is used
				as a virtual table, it is passed the operation name (insert, replace, update,
				query, delete) as its first parameter. The function may be implemented
				in the main namespace.</para></listitem>
			</itemizedlist>
			<itemizedlist>
				<listitem><para><emphasis>Package/class</emphasis>. The defined class needs
				to have an init() function. It will be called during the first call of that
				VTab.
				Additionally, the package has
				to define the necessary functions insert, replace, update, delete and/or query.
				These functions will be called in a function context (first parameter is the
				class name).
				</para></listitem>
			</itemizedlist>
			<itemizedlist>
				<listitem><para><emphasis>Object</emphasis>. The defined class needs
				to have a new() function which will return a reference to the newly
				created object. This object needs to define the necessary functions insert,
				replace, update, delete and/or query.
				These functions will be called in a method context (first parameter is
				a reference to the object).
				</para></listitem>
			</itemizedlist>
		</para>
	</section>
</chapter>

