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    10 <!DOCTYPE concept

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

    12 <concept id="GUID-67718111-07FB-59CC-A9C2-86593F868C2C" xml:lang="en"><title>Features

    13 of the client-server architecture</title><shortdesc>Describes the features of the client-server architecture.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <section id="GUID-733332EE-A689-4D9A-AAE3-3980B915412B"><title>Context</title> <p>Each session maintains its own context.

    15 This allows sessions to be owned by different active objects within the same

    16 thread, by different components within the same thread, etc., without interference. </p> </section>

    17 <section id="GUID-35A7DB04-C9D5-47FF-A793-29533CD766A4"><title>Context switching</title> <p>Client-server communication requires

    18 context switching: </p> <ul>

    19 <li id="GUID-50EC1E74-44B3-52C7-A2E0-CA129561E25B"><p>messages are sent via

    20 the Kernel </p> </li>

    21 <li id="GUID-94E8A131-0E5C-5034-A903-E903A1121954"><p>handling the message

    22 involves a switch from client thread, to server thread, and back to client

    23 thread </p> </li>

    24 <li id="GUID-702DA432-6985-5046-92EB-C3159EAE7C2A"><p>inter-thread data transfer

    25 can never be done with simple C++ pointers: it always involves data copying.

    26 Furthermore, it may involve cross-address-space data transfer, if the threads

    27 are in different processes. </p> </li>

    28 </ul> <p>Compared with a simple function call or memory copy, context switching

    29 is a relatively expensive operation, and should be minimised. Servers whose

    30 performance is critical use buffering to minimise context switches. Sometimes,

    31 this is transparent. Often, the client interface design is affected by the

    32 requirement for buffering. </p> </section>

    33 <section id="GUID-BC1416C3-DD32-438E-A574-21802747220B"><title>Thread-based</title> <p>The basic architecture of servers

    34 is thread-based. This gives the implementers of a system the flexibility to

    35 package the server threads into whatever processes they choose, depending

    36 on the balance of requirements for security and economy. </p> <p>This can

    37 be contrasted with other systems in which all, or most, servers run as part

    38 of the Kernel. On Symbian platform this is not necessary, so security is better. </p> <p>Client-server

    39 policies can be contrasted with systems which require servers to have their

    40 own process. This uses more memory (e.g. for address translation tables),

    41 and has worse performance (inter-thread data transfer involves translation

    42 as well as copying). </p> </section>

    43 <section id="GUID-15B1DC2E-AB35-48DB-8EC4-11127DF974B0"><title>Cleanup</title> <p>When a client process terminates, all server

    44 resources associated with it should be cleaned up. </p> <p>When a session

    45 is ended, servers must clean up all objects associated with it, and clients

    46 must consider any handles associated with it as invalid, and perform any necessary

    47 client-side cleanup. </p> <p>For a non-sharable session, if the client thread

    48 dies, then the Kernel performs thread-death cleanup and sends a disconnect

    49 message to the server end of all sessions associated with that client thread. </p> <p>For

    50 a sharable session, the death of any or all client threads does not trigger

    51 closure of the session. This is because the session is process relative. To

    52 close a shared session, either the process must terminate, or the session

    53 must be explicitly closed through a client side call to <codeph>Close()</codeph> on

    54 the client-side session handle, <codeph>RSessionBase</codeph>. </p> </section>

    55 </conbody></concept>