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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C" xml:lang="en"><title>Introduction

    13 to the client-server architecture</title><shortdesc>Provides a high-level description of the Symbian platform client-server

    14 architecture.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    15 <ul>

    16 <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>

    17 <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>

    18 </ul>

    19 <section id="GUID-177DE161-3EAD-5623-93D7-7DE0A631F193"><title>Why use a server?</title> <p>It

    20 is usually appropriate to provide services through a server: </p> <ul>

    21 <li id="GUID-A16F540F-2A32-5FAE-82F7-A7DC06E694B9"><p>when the services involve

    22 management of one or more shared system resources. </p> <p>For example, the

    23 File Server manages all files and file systems on behalf of all client programs.

    24 The file server may therefore ensure that all file resources are correctly

    25 shared. Similarly, the Window Server allows all applications to share the

    26 machine's screen, keyboard, and pointer devices. </p> </li>

    27 <li id="GUID-A69E1C2D-350C-5B34-B448-C7FBC0DDDC47"><p>when a degree of isolation

    28 is required between client and server. </p> <p>Since client and server run

    29 in separate threads, they may also run in separate processes, with separate

    30 address spaces. This means there is no possibility of communication between

    31 them except by the interface provided by the server and its client interface.

    32 Thus, misbehaved clients cannot corrupt the server or any of the resources

    33 it manages on behalf of other clients. </p> <p>The File Server architecture

    34 is strongly motivated by this requirement. </p> </li>

    35 <li id="GUID-D7B7C91B-9F6B-53AC-8177-16CB4ECDF5A7"><p>when asynchronous services

    36 are required. </p> <p>Asynchronous requests can be handled using Kernel services

    37 (e.g. timers) and active objects within an application thread. However, when

    38 the completion of a request is some time in the future, and the application

    39 program making the request might no longer be present, a continuously-running

    40 server is appropriate. </p> </li>

    41 </ul> <p>Most servers are motivated by a combination of requirements such

    42 as these. </p> <p>There are three distinct aspects to the service provided

    43 by a server: </p> <ul>

    44 <li id="GUID-30ADCD43-2C5A-56C0-8992-160BEFA0F58A"><p>a server program, which

    45 runs in its own thread </p> </li>

    46 <li id="GUID-8B58B03C-B548-5B7C-B2AB-7491574353C4"><p>several client programs,

    47 each running in their own thread </p> </li>

    48 <li id="GUID-AD63FE65-65C0-5DF4-BEEE-B4DDF481521E"><p>a client interface,

    49 provided by the server, for use by each client program </p> </li>

    50 </ul> </section>

    51 <section id="GUID-1B3F9BF2-D909-5C5F-8380-A2D9FDE0D05B"><title>Symbian platform

    52 client/server</title> <p>In Symbian platform, the server defines a client

    53 interface API which the client uses to request specific services. The client

    54 and server programs run in different threads, often in different processes,

    55 and so cannot directly access each other's address space. They use a message

    56 passing protocol to communicate. </p> <p>All servers have a name which is

    57 passed to the server when it is started. A client finds a server through its

    58 name. </p> <p>The channel of communication between a client and a server is

    59 known as a <i>session</i>. To make a server request, the client sends a message

    60 to the server over the session. The message includes a 32-bit request type

    61 code, and up to four 32-bit parameters. The server may use data transfer services

    62 to send and receive additional data. When it has finished servicing the message,

    63 a 32-bit completion code is returned to the client. All client-server communication

    64 is mediated by the Kernel. </p> <p>A session can be used by all threads in

    65 a process, and an also be shared by all threads across all processes, and

    66 not just by the thread that connected to the server. Such a session is said

    67 to be sharable. </p> <p>A server must indicate that it can support sharable

    68 sessions. Default behaviour is always to create non-sharable sessions. </p> <p>The

    69 Symbian platform provides several servers. </p> </section>

    70 </conbody></concept>