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+++ b/Symbian3/SDK/Source/GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C.dita	Wed Mar 31 11:11:55 2010 +0100
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+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
+<!-- This component and the accompanying materials are made available under the terms of the License 
+"Eclipse Public License v1.0" which accompanies this distribution, 
+and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
+<!-- Initial Contributors:
+    Nokia Corporation - initial contribution.
+Contributors: 
+-->
+<!DOCTYPE concept
+  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
+<concept id="GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C" xml:lang="en"><title>Introduction
+to the client-server architecture</title><shortdesc>Provides a high-level description of the Symbian platform client-server
+architecture.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<ul>
+<li id="GUID-31FDC700-8C84-58C7-A18A-A3398A5BA6AD"><p> <xref href="GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C.dita#GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C/GUID-177DE161-3EAD-5623-93D7-7DE0A631F193">Why use a server?</xref>  </p> </li>
+<li id="GUID-23B5C6F8-6860-5A5F-9C89-BC25A3CAC6B9"><p> <xref href="GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C.dita#GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C/GUID-1B3F9BF2-D909-5C5F-8380-A2D9FDE0D05B">Symbian platform client/server</xref>  </p> </li>
+</ul>
+<section id="GUID-177DE161-3EAD-5623-93D7-7DE0A631F193"><title>Why use a server?</title> <p>It
+is usually appropriate to provide services through a server: </p> <ul>
+<li id="GUID-A16F540F-2A32-5FAE-82F7-A7DC06E694B9"><p>when the services involve
+management of one or more shared system resources. </p> <p>For example, the
+File Server manages all files and file systems on behalf of all client programs.
+The file server may therefore ensure that all file resources are correctly
+shared. Similarly, the Window Server allows all applications to share the
+machine's screen, keyboard, and pointer devices. </p> </li>
+<li id="GUID-A69E1C2D-350C-5B34-B448-C7FBC0DDDC47"><p>when a degree of isolation
+is required between client and server. </p> <p>Since client and server run
+in separate threads, they may also run in separate processes, with separate
+address spaces. This means there is no possibility of communication between
+them except by the interface provided by the server and its client interface.
+Thus, misbehaved clients cannot corrupt the server or any of the resources
+it manages on behalf of other clients. </p> <p>The File Server architecture
+is strongly motivated by this requirement. </p> </li>
+<li id="GUID-D7B7C91B-9F6B-53AC-8177-16CB4ECDF5A7"><p>when asynchronous services
+are required. </p> <p>Asynchronous requests can be handled using Kernel services
+(e.g. timers) and active objects within an application thread. However, when
+the completion of a request is some time in the future, and the application
+program making the request might no longer be present, a continuously-running
+server is appropriate. </p> </li>
+</ul> <p>Most servers are motivated by a combination of requirements such
+as these. </p> <p>There are three distinct aspects to the service provided
+by a server: </p> <ul>
+<li id="GUID-30ADCD43-2C5A-56C0-8992-160BEFA0F58A"><p>a server program, which
+runs in its own thread </p> </li>
+<li id="GUID-8B58B03C-B548-5B7C-B2AB-7491574353C4"><p>several client programs,
+each running in their own thread </p> </li>
+<li id="GUID-AD63FE65-65C0-5DF4-BEEE-B4DDF481521E"><p>a client interface,
+provided by the server, for use by each client program </p> </li>
+</ul> </section>
+<section id="GUID-1B3F9BF2-D909-5C5F-8380-A2D9FDE0D05B"><title>Symbian platform
+client/server</title> <p>In Symbian platform, the server defines a client
+interface API which the client uses to request specific services. The client
+and server programs run in different threads, often in different processes,
+and so cannot directly access each other's address space. They use a message
+passing protocol to communicate. </p> <p>All servers have a name which is
+passed to the server when it is started. A client finds a server through its
+name. </p> <p>The channel of communication between a client and a server is
+known as a <i>session</i>. To make a server request, the client sends a message
+to the server over the session. The message includes a 32-bit request type
+code, and up to four 32-bit parameters. The server may use data transfer services
+to send and receive additional data. When it has finished servicing the message,
+a 32-bit completion code is returned to the client. All client-server communication
+is mediated by the Kernel. </p> <p>A session can be used by all threads in
+a process, and an also be shared by all threads across all processes, and
+not just by the thread that connected to the server. Such a session is said
+to be sharable. </p> <p>A server must indicate that it can support sharable
+sessions. Default behaviour is always to create non-sharable sessions. </p> <p>The
+Symbian platform provides several servers. </p> </section>
+</conbody></concept>
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