week 10, remote procedure call

Remote Procedure CallCS-502, Operating Systems Fall 2007 (Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne, Modern Operating Systems, 2nd ed., by Tanenbaum, and Distributed Systems: Principles & Paradigms, 2nd ed. By Tanenbaum and Van Steen)

Remote Procedure Call

Message-oriented ProtocolsDifficult to design and implementEspecially with more sophisticated middlewareMany difficult implementation issues for each new implementationFormattingUniform representation of dataClient-server relationshipsMany still in widespread useTraditional TCP/IP and Internet protocols


Remote Procedure Call (RPC)The most common framework for newer protocols and for middlewareUsed both by operating systems and by applicationsNFS is implemented as a set of RPCsDCOM, CORBA, Java RMI, etc., are just RPC systems ReferenceBirrell, Andrew D., and Nelson, Bruce, Implementing Remote Procedure Calls, ACM Transactions on Computer Systems, vol. 2, #1, February 1984, pp 39-59. (.pdf)


Remote Procedure Call (RPC)Fundamental idea: Server process exports an interface of procedures or functions that can be called by client programssimilar to library API, class definitions, etc.Clients make local procedure/function calls As if directly linked with the server processUnder the covers, procedure/function call is converted into a message exchange with remote server process


Ordinary procedure/function callcount = read(fd, buf, bytes)


Remote Procedure CallWould like to do the same if called procedure or function is on a remote server


Solution a pair of StubsA client-side stub is a function that looks to the client as if it were a callable servicefunctionI.e., same API as the services implementation of the functionA service-side stub looks like a caller to the serviceI.e., like a hunk of code invoking the service functionThe client program thinks its invoking the servicebut its calling into the client-side stubThe service program thinks its called by the clientbut its really called by the service-side stubThe stubs send messages to each other to make the RPC happen transparently (almost!)


RPC StubsTanenbaum & Van Steen, Fig 4-7


RPC Stubs SummaryClient-side stubLooks like local server functionSame interface as local functionBundles arguments into a message, sends to server-side stubWaits for reply, un-bundles resultsreturnsServer-side stubLooks like local client function to serverListens on a socket for message from client stubUn-bundles arguments to local variablesMakes a local function call to serverBundles result into reply message to client stub


Result a very useful AbstractionThe hard work of building messages, formatting, uniform representation, etc., is buried in the stubsWhere it can be automated!Designers of client and server can concentrate on semantics of applicationPrograms behave in familiar way


RPC IssuesHow to make the remote part of RPC invisible to the programmer?What are semantics of parameter passing?E.g., pass by reference?How to bind (locate & connect) to servers?How to handle heterogeneity?OS, language, architecture, How to make it go fast?


RPC ModelA server defines the service interface using an interface definition language (IDL)the IDL specifies the names, parameters, and types for all client-callable server procedures

A stub compiler reads the IDL declarations and produces two stub functions for each server functionServer-side and client-side


RPC Model (continued)Linking:Server programmer implements the services functions and links with the server-side stubsClient programmer implements the client program and links it with client-side stubsOperation:Stubs manage all of the details of remote communication between client and server


Marshalling ArgumentsMarshalling is the packing of function parameters into a message packetthe RPC stubs call type-specific functions to marshal or unmarshal the parameters of an RPCClient stub marshals the arguments into a messageServer stub unmarshals the arguments and uses them to invoke the service functionon return:the server stub marshals return valuesthe client stub unmarshals return values, and returns to the client program


Issue #1 representation of dataBig endian vs. little endianSent by PentiumRecd by SPARCAfter inversion


Representation of Data (continued)IDL must also define representation of data on networkMulti-byte integersStrings, character codesFloating point, complex, example: Suns XDR (external data representation)Each stub converts machine representation to/from network representationClients and servers must not try to cast data!


Issue #2 Pointers and Referencesread(int fd, char* buf, int nbytes)Pointers are only valid within one address spaceCannot be interpreted by another processEven on same machine!

Pointers and references are ubiquitous in C, C++Even in Java implementations!


Pointers and References Restricted SemanticsOption: call by valueSending stub dereferences pointer, copies result to messageReceiving stub conjures up a new pointerOption: call by resultSending stub provides buffer, called function puts data into itReceiving stub copies data to callers buffer as specified by pointer


Pointers and References Restricted Semantics (continued)Option: call by value-resultCallers stub copies data to message, then copies result back to client bufferServer stub keeps data in own buffer, server updates it; server sends data back in reply

Not allowed:Call by referenceAliased arguments


Transport of Remote Procedure CallOption TCPConnection-based, reliable transmissionUseful but heavyweight, less efficientNecessary if repeating a call produces different resultAlternative UDPIf message fails to arrive within a reasonable time, callers stub simply sends it againOkay if repeating a call produces same result


Asynchronous RPCAnalogous to spawning a threadCaller must eventually wait for resultAnalogous to join


Asynchronous RPC (continued)Analogous to spawning a threadCaller must eventually wait for resultAnalogous to joinOr be interrupted (software interrupt)


RPC BindingBinding is the process of connecting the client to the serverthe server, when it starts up, exports its interfaceidentifies itself to a network name servertells RPC runtime that it is alive and ready to accept callsthe client, before issuing any calls, imports the serverRPC runtime uses the name server to find the location of the server and establish a connectionThe import and export operations are explicit in the server and client programs


Remote Procedure Call is used Between processes on different machinesE.g., client-server model

Between processes on the same machineMore structured than simple message passing

Between subsystems of an operating systemWindows XP (called Local Procedure Call)


Questions?


Practical RPC SystemsDCE (Distributed Computing Environment)Open Software FoundationBasis for Microsoft DCOMTanenbaum & Van Steen, 4.2.4Suns ONC (Open Network Computing)Very similar to DCEWidely usedrpcgenhttp://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-Q0R5B-TET1_html/TITLE.html


Practical RPC Systems (continued)Java RMI (Remote Method Invocation)java.rmi standard packageJava-oriented approach objects and methodsCORBA (Common Object Request Broker Architecture) Standard, multi-language, multi-platform middlewareObject-orientedHeavyweight


Implementation Model for DCE


Implementation Model for ONCprogram & version #program & version #rpcgenXDR


Validating a Remote ServicePurposeAvoid binding to wrong service or wrong versionDCEGlobally unique IDGenerated in template of IDL fileSun ONCProgram numbers registered with SunVersion # and procedure # administered locally


RPC Binding Sun ONCService registers with portmapper service on server OSProgram # and version #Optional static port #ClientMust know host name or IP addressclnt_create(host, prog, vers, proto)I.e., RPC to portmapper of host requesting to bind to prog, vers using protocol proto (tcp or udp)(Additional functions for authentication, etc.)Invokes remote functions by name


Sun ONC (continued)#include Header file for client and serverrpcgenThe stub compilerCompiles interface.xProduces .h files for client and service; also stubsSee alsorpcinfoRPC Programming Guide


Note on XDR(the Interface Definition Language for ONC)Much like C header fileExceptionsstring type maps to char *bool type maps to bool_t


Sun ONCOnline tutorialhttp://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-Q0R5B-TET1_html/TITLE.htmlCode sampleshttp://web.cs.wpi.edu/~rek/DCS/D04/SunRPC.htmlhttp://web.cs.wpi.edu/~goos/Teach/cs4513-d05/http://web.cs.wpi.edu/~cs4513/b05/week4-sunrpc.pdf Any other resources you can find


Questions?


Assumptions:Graduate levelOperating SystemsMaking Choices about operation systems

Why a micro-century? just about enough time for one concept

week 10, remote procedure call

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