Hermes: A Tutorial and Reference Manual - Researcher - IBM
Hermes: A Tutorial and Reference Manual - Researcher - IBM
Hermes: A Tutorial and Reference Manual - Researcher - IBM
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604.1.ExpressionBlocks anexpressionblockifyouneedtowriteastatement. blockwasn'treallynecessary.Theoriginalversionwasjustasconvenient. builtwithintheexpressionblock.Inthisparticularcase,theexpression However,insomecontexts,suchasafterwhereinaselector,youmustuse statement.Theoper<strong>and</strong>ofinsertistherecordCurrentWindowwhichis Inthisversionoftheprogram,theblockconsistsofasingleinsert<br />
4.2Send Anothercommonidiomistosendsomedatatoanotherprocess<strong>and</strong>not waitforareply.Youc<strong>and</strong>othisbydeninganinterfaceinwhichthedata youwishtosendisinitializedonentry<strong>and</strong>uninitializedonexit.However, thereisstilltheproblemthatthereceivingprocessmaychoosetowait<br />
statementsendvariabletoportcausesthevaluecurrentlyinvariableto indenitelybeforereceivingyourdata.Itmayevenrunforever<strong>and</strong>never process,<strong>Hermes</strong>providesanasynchronouscommunicationmechanism.The beremoved<strong>and</strong>senttothequeueattheinputportconnectedtoport. deadlocked,itisIwhoisblocked<strong>and</strong>notS. <strong>and</strong>returnstoS.ThenIcallstheeventualreceiverR.IfRisdelayedor receiveyourdata.<br />
Becauseofthesendoperation,inputportscanbedenedtoholdany intermediateprocessI.ScallsI,whichimmediatelyreceivesthedata, Ratherthanrequiringtheprogrammertoexplicitlycodeanintermediate Apossible<strong>Hermes</strong>solutionisthis:thesendingprocess,Screatesan<br />
datatype,notjustcallmessages. Herearetwopracticalusesofsend: Communicatingwithuntrustedprocesses:SupposeIwishtoload<strong>and</strong> Ieithermustspawnaprocesstoinitialize<strong>and</strong>calltheuntrusted willneverreturnmycallmessage,<strong>and</strong>Iwillblockforever.Tobesafe, thisifIdonottrusttheprocessIamloading.Perhapstheprocess statement. process,orImustdelivertheinitializationparametersusingasend aswedidinearlierexamples.However,thereissomerisktodoing someports.Icouldpassthisinformationwithacallstatement,just executeaprocess<strong>and</strong>passitsomeinitializationinformation|possibly<br />
Forwardingcalls:SupposeIamwritingaprocesswhosejobitisto tuallyservicethecall.Ifmyprocessreceivesthecallmessage<strong>and</strong>then receivecalls<strong>and</strong>routethemtoanappropriateprocesswhichwillac-<br />
callstheeventualdestination,itmustwaituntilthecallisserviced. originalcaller,<strong>and</strong>nottomyprocess. However,ifinsteadmyprocesstransfersthecallmessageusingsend, myprocessisfreetorouteothercalls.Whentheeventualdestination issuesareturnstatement,thecallmessagewillbereturnedtothe
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