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+<?xml version="1.0"?>
+<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
+]>
+<chapter xml:id="sharing">
+	<title>Sharing PKCS#11 modules</title>
+
+	<section xml:id="sharing-problem">
+		<title>Multiple consumers of PKCS#11 in a process</title>
+
+		<para>As more and more applications and libraries use PKCS#11 we run
+		into a very basic problem. The PKCS#11 modules cannot be initialized and
+		finalized properly without coordination between the various consumers.
+		</para>
+
+		<para>An example: An application might use GnuTLS for
+		TLS connections, and use libgcr for display of certificates. Both of
+		these want to load (and initialze) the same PKCS#11 modules. There are
+		many places where this situation occurs, including large applications
+		like Evolution which due to their dependencies end up using both NSS and
+		GnuTLS.</para>
+
+		<para>Consumer A loads a PKCS#11 module and uses the module's
+		C_Initialize function to initialize it, which works as expected.
+		When consumer B initializes the module (also using C_Initialize),
+		the error code <literal>CKR_CRYPTOKI_ALREADY_INITIALIZED</literal>
+		is correctly returned. This is normal PKCS#11 specification
+		defined behavior for when a module is initalized twice in the
+		same process. If consumer B is aware of this situation they may
+		choose to ignore this error code.</para>
+
+		<para>However when the consumer A is done with its use of the
+		PKCS#11 module it finalizes the module using the module's
+		C_Finalize function. This is expected of a well behaved PKCS#11
+		consumer. This then causes errors and/or crashes for consumer B,
+		which cannot know that the module has now been finalized out
+		from underneath it.</para>
+
+		<para>It is necessary for the two consumers to coordinate their
+		initialization and finalization in some fashion. In
+		<literal>p11-kit</literal> we provide this coordination in a
+		loosely coupled, backwards compatible, and flexible way.</para>
+	</section>
+
+	<section xml:id="sharing-initialize">
+		<title>Solution: p11-kit</title>
+
+		<para><literal>p11-kit</literal> provides functions to
+		coordinate initialization and finalization of any PKCS#11
+		module. A module may be initialized any number of times using
+		the p11_kit_initialize_module() function. The first time that
+		p11_kit_initialize_module() is called for a module, that module's
+		C_Initialize function is used. Later invocations for the same
+		module cause p11-kit to increment an internal initialization
+		count, rather than calling C_Initialize again.</para>
+
+		<para>The p11_kit_finalize_module() is used to finalize a module.
+		Each time it is called it decrements the internal initialization
+		count for that module. When the internal initialization count
+		reaches zero, the module's C_Finalize function is called.</para>
+
+		<para>This is done in a thread-safe manner. These functions can
+		be used on modules that the consumer loads themselves.</para>
+	</section>
+
+	<section xml:id="sharing-module">
+		<title>Solution: proxy module</title>
+
+		<para>When an application is aware of the fact that coordination
+		is necessary between multiple consumers of a PKCS#11 module, it
+		can link to p11-kit and use the functions there to provide
+		this coordination.</para>
+
+		<para>However most current consumers of PKCS#11 are ignorant of
+		this problem, and do not link to p11-kit. In order to solve this
+		multiple initialization problem for all applications,
+		<literal>p11-kit</literal> provides a proxy compatibility
+		module.</para>
+
+		<para>This proxy module acts like a normal PKCS#11 module, but
+		internally loads a preconfigured set of PKCS#11 modules and
+		coordinates their initialization and finalization. Each slot
+		in the configured modules is exposed as a slot of the
+		<literal>p11-kit</literal> proxy module. The proxy module is
+		then used as a normal PKCS#11 module would be. It can be loaded by
+		crypto libraries like NSS and behaves as expected.</para>
+
+		<para>The proxy module bends the PKCS#11 rules slightly. It does
+		not return the <literal>CKR_CRYPTOKI_ALREADY_INITIALIZED</literal>
+		error code as specified in PKCS#11. However this is a small
+		price to pay for this compatibility.</para>
+	</section>
+</chapter>