--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Symbian3/SDK/Source/GUID-67718111-07FB-59CC-A9C2-86593F868C2C.dita	Thu Jan 21 18:18:20 2010 +0000
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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-67718111-07FB-59CC-A9C2-86593F868C2C" xml:lang="en"><title>Features
+of the client-server architecture</title><shortdesc>Describes the features of the client-server architecture.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<section id="GUID-733332EE-A689-4D9A-AAE3-3980B915412B"><title>Context</title> <p>Each session maintains its own context.
+This allows sessions to be owned by different active objects within the same
+thread, by different components within the same thread, etc., without interference. </p> </section>
+<section id="GUID-35A7DB04-C9D5-47FF-A793-29533CD766A4"><title>Context switching</title> <p>Client-server communication requires
+context switching: </p> <ul>
+<li id="GUID-50EC1E74-44B3-52C7-A2E0-CA129561E25B"><p>messages are sent via
+the Kernel </p> </li>
+<li id="GUID-94E8A131-0E5C-5034-A903-E903A1121954"><p>handling the message
+involves a switch from client thread, to server thread, and back to client
+thread </p> </li>
+<li id="GUID-702DA432-6985-5046-92EB-C3159EAE7C2A"><p>inter-thread data transfer
+can never be done with simple C++ pointers: it always involves data copying.
+Furthermore, it may involve cross-address-space data transfer, if the threads
+are in different processes. </p> </li>
+</ul> <p>Compared with a simple function call or memory copy, context switching
+is a relatively expensive operation, and should be minimised. Servers whose
+performance is critical use buffering to minimise context switches. Sometimes,
+this is transparent. Often, the client interface design is affected by the
+requirement for buffering. </p> </section>
+<section id="GUID-BC1416C3-DD32-438E-A574-21802747220B"><title>Thread-based</title> <p>The basic architecture of servers
+is thread-based. This gives the implementers of a system the flexibility to
+package the server threads into whatever processes they choose, depending
+on the balance of requirements for security and economy. </p> <p>This can
+be contrasted with other systems in which all, or most, servers run as part
+of the Kernel. On Symbian platform this is not necessary, so security is better. </p> <p>Client-server
+policies can be contrasted with systems which require servers to have their
+own process. This uses more memory (e.g. for address translation tables),
+and has worse performance (inter-thread data transfer involves translation
+as well as copying). </p> </section>
+<section id="GUID-15B1DC2E-AB35-48DB-8EC4-11127DF974B0"><title>Cleanup</title> <p>When a client process terminates, all server
+resources associated with it should be cleaned up. </p> <p>When a session
+is ended, servers must clean up all objects associated with it, and clients
+must consider any handles associated with it as invalid, and perform any necessary
+client-side cleanup. </p> <p>For a non-sharable session, if the client thread
+dies, then the Kernel performs thread-death cleanup and sends a disconnect
+message to the server end of all sessions associated with that client thread. </p> <p>For
+a sharable session, the death of any or all client threads does not trigger
+closure of the session. This is because the session is process relative. To
+close a shared session, either the process must terminate, or the session
+must be explicitly closed through a client side call to <codeph>Close()</codeph> on
+the client-side session handle, <codeph>RSessionBase</codeph>. </p> </section>
+</conbody></concept>
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