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<!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>