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INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES. Chapter 4: Client server Programming With RPC Rufin Soh. Content. SUN RPC The remote program Programs Identification The port mapper XDR Parameters exchanges Passing parameters Messages format Rpcgen

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INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

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  1. INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES Chapter 4: Client server Programming With RPC Rufin Soh

  2. Content • SUN RPC • The remote program • Programs Identification • The port mapper • XDR • Parameters exchanges • Passing parameters • Messages format • Rpcgen • Development with RPC • RPC server types • Remote PC vs Local PC • Failures with RPC • RPC lacks • RMI • Idempotent operations • RPC fields of applications • Distributed systems challenges • Alternatives to DS challenges ? • RPC • Why RPC? • RPC: definitions • RPC working mode • RPC model • RPC architecture • Execution mode • Calling Mode A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  3. RPC fields of applications Sales Engineering Accounting manufacturing Cash and billing Transport and delivery Inventories A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  4. Content (1) • RPC possible fields of applications • Distributed systems challenges A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  5. Distributed systems challenges • Distributed systems are applications that run in different namespaces or hosting computers. Their Inherent problems are: • Communication: • Transmission of information between softwares. • Heterogenous environment: • Manage the diversity among hardwares and softwares that interact. • Intégration • Preserv the patrimony for legacy applications • Interoperability • Exchange data between distributed applications from different vendors. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  6. Content (2) • RPC possible fields of applications • Distributed systems challenges • Alternatives to DS challenges ? A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  7. Alternatives to DS challenges ? • To overcome the following challenges we need: • A normalized network infrastructure • OSI, TCP IP model. • Normalized communication protocols • IP (for packets transport), TCP IP connexions(for streaming e.g. sockets). • HTTP, FTP, CGI, at the application layer. • Normalized mechanism for components (applications) interaction • Remote procedure call is the fundamental paradigm for distributed programming as well as procedure call is for structured programming. • Normalized conversion functions • All this implemented on different hardware and software platforms, with the illusion of a unique platform • The distributed platform: the middleware, the RPC middleware. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  8. Content (3) • RPC possible fields of applications • Distributed systems challenges • Alternatives to DS challenges ? • RPC • Why RPC? • RPC: definitions • RPC working mode • RPC model • RPC architecture • Execution mode • Calling Mode A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  9. WhyRPC? • Advantages of RPC: • The goal is to inherit as far as possible the conventional procedure call semantic in a different environment. • Encapsulates the communication details, making no difference between local and a remote calls (or nearly…). • Widely used by programmers. • Lets you achieve nearly all the following distributed systems challenges without using many different development tools. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  10. Introduction to RPC • Remote Procedure Call (RPC) : is the technology used to execute procedures on remote environments. • A distributed application designer may work following two approaches: • Communication oriented design: • Define the application protocol(messages format and syntax ), that interwork between the client and the server • Design the client and server components and describe how they react to input messages and emit output. • e.g: HTTP, FTP. • Application oriented design: • Build a mono-computer conventional application. • Then divide it into many modules that will run on different machines. • e.g: RPC. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  11. Marshalling • RPC identifier • Timer Caller program RPC Service Low level protocols Low level protocols RPC Service Called procedure N E T W O R K 5 A B 1 C Call E D return Call 1 Caller System (Client) Called System (Server) RPC working mode • Client Request • The call is detected as an RPC call. • On A, the Call parameters are passed and placed in a data structure (Marshalling) waiting to be transferred on the network • An RPC identifier is generated for this Call • A timer is initialized at A and set • Data are transferred on the network to B, through the Operating System A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  12. 6 Unmarshalling 7 Record RPC identifier Caller program RPC Service Low level protocols Low level protocols RPC Service Called procedure N E T W O R K 5 A B 1 C 8 9 Marshalling 10 Set Timer Call E D return Call 1 Caller System (Client) Called System (Server) RPC working mode (2) • Server process and response • Prameters are extracted from the network buffer in a useful format(Unmarshalling) for Call to be made • RPC identifier is recorded • The Call is processed at C and return to the caller at D • The results to return are placed in a special data structure (Marshalling) waiting for transfer • The timer is initialized A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  13. 15 Desactivate Timer Caller program RPC Service Low level protocols Low level protocols RPC Service Called procedure 12 Unmarshalling 13 Desactivate timer N E T W O R K 5 A B 1 C 8 Call 11 E D return Call 14 Caller System (Client) Called System (Server) RPC working mode (3) • Client ACK • Data are sent on the network (through the server operating system APIs) and transferred to E • Results are extracted from the network buffer in a data structure (Unmarshalling) • The Client-side timer is desactivated • An acknowledgement is sent to the server with the corresponding RPC identifier. • The server side timer is desactivated on D A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  14. RPC model • The RPC model uses an «application oriented design» approach that enables many procedures to run on remote sites. • The remote procedure call is the client request, and the procedure return is the server response. • A procedure call follows a synchronous working mode(blocking tasks): the next instruction after the procedure call can not be executed before the called procedure ends. Procedure B (server) Procedure A (server) Main Program procA() procB() return return return Machine 1 Machine 3 Machine 2 Network Network A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  15. Client RPC Server Procedure Application Procedure return Run procedure Procedure call Procedure return Server stub Client stub Marshalling Unmarshalling Marshalling Unmarshalling ReceiveRequest() SendRequest() ReceiveResponse() SendResponse() RPC architecture A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  16. local variables main() local variables main() SP bytes buff fd Return adress local variables read() SP Execution mode • How to Pass parameters : • per value:initialize a procedure local variable • per reference: use a pointer, pointing on the parameter variable • by copy of reference: a copy at the call and a copy at the return Procedures calls are made on the following model: N = read(fd,buff,nbytes) A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  17. Calling Mode Client Server Call Return Return Call Client Stub Server Stub Marshalling parameters Unmarshalling results Marshalling results Unmarshalling parameters core core The machine hosting the procedure on the network remains transparent to the client A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  18. Content (4) • SUN RPC • The remote program • Programs Identification • The port mapper • XDR • Parameters exchanges • Passing parameters • Messages format • rpcgen • RPC possible fields of applications • Distributed systems challenges • Alternatives to DS challenges ? • RPC A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  19. SUN RPC • Sun Microsystems has developed an RPC technology named the « Sun RPC ». It is Todays de facto standard. NFS is built on top RPC. • Sun RPC defines: • The messages format that the caller(client) transmit to start the remote procedure on the server. • The parameters format • The results format. • UDP and TCP (OSI layer …?) protocols are used for communication and XDR presentation protcol (TCP-IP layer …? Give the answer before continuing) enables RPC to work in an heterogenous environment. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  20. insert procedure delete procedure lookup procedure Databases The remote program (!!! not procedure) • The remote program is the logical software or unit, running on a remote machine(??? A machine different from the one in front of you). • A remote program is a server with its own procedures and its own data. • E.g a Database server • Every remote program is identified by a unique 32-bits integer used by the caller. • A remote program procedures are sequecially identified by the integers 1,2,…,N A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  21. The remote program (2) • RPC communication mode «  at least one transmission»: • If a remote procedure call running on UDP does not return a value, the caller couldn’t know if the procedure has been executed or if the answer is lost (why?… because UDP is best effort, it never promises nothing as compare to who??? Its brother TCP). • Futhermore, nothing tells you that the procedure has not been executed many times, due to duplicated requests. • The communication mode is a client sever mode(Blocking or not???). • The caller specified the server address(IP, port). • But which server? Of course the one hosting the remote program. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  22. The remote program (3) • A port number is dynamically allocated to any remote program, and the corresponding RPC client is notified: • Every computer that offers RPC programs hosts a dynamic port association service: The port mapper. • When an RPC program starts on the server, it dynamically allocate a local port number, then inform the port mapper on the hosting machine, of the RPC program identifier\port number association. • The port mapper will maintains a databases table of the different associations. • Any client who want to contact an RPC program on a M machine will first ask for its associated communication port to the M machine’s port mapper. • The port mapper always runs on communication port 111. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  23. Programs Identification • A remote procedure is identified by the trio (prog, vers,proc) • Prog identifies the remote program • Vers is the version (of what??? Of prog…of course) • Proc is the procedure (which one ??? The one you may want to call if you are an RPC client). • Address groups for remote programs: Name identifier description portmap 100000 port mapper rstat 100001 rstat, rup, perfmeter ruserd 100002 remote users nfs 100003 Network File System ypserv 100004 Yellow pages (NIS) mountd 100005 mount, showmount dbxd 100006 debugger ypbind 100007 NIS binder etherstatd 100010 Ethernet sniffer pcnfs 150001 NFS for PC A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  24. The port mapper The program transmits RPC server Program Port Mapper the trio (ident, RPC, port) socket allocated to the RPC program Port Mapper’s socket = 111 A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  25. XDR • RPC messages format length is generally variable. Messages fields are specified in XDR (eXternal Data Representation) language. • XDR : Data representation as defined by SUN Microsystems • Defines how data are going to be routed on the network • Enables data exchange between computers with differents internal data representations. • E.g. a 32 bits integer with a value of 260 will have the 2 following representations: • 0014 for a « big endian » type machines, that is a machine with lower addresses on MSB and higher addresses on LSB. • 4100 for « little endian » type machines. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  26. XDR (2) • XDR data encoding format only contains represented data without any information on the data type (as compared to ASN1). • If an application work with 32-bits integer, the encoding result will be stored exactly on 32 bits with nothing to tell you its type (if it is an integer or …). • This encoding scheme implied that both the client and the server must agree on the exact data format to be exchanged. • An XDR conversion fonctions library enables applications designers to use a standard software on any type of computer. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  27. Parameters exchanges n=sum(4,7) sum(I,j) { int I, j; return (i+j) ; } sum sum 4 4 7 7 core core Communication between the client and the server is done through a message containing the name of the procedure, and grouped parameters (marshalling ) A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  28. Passing parameters • For a per value parameters passing, the parameters copies are sent in the message. • For a per reference parameters passing (e.g. a vector or table in C programming language), there are many solutions among which: • Tables may be sent as copies of their values. So the server will replace each of those values with a pointer, work with the pointer and return the table to the client who will use the initial pointer (copy/restore). A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  29. Messages format Message ID Message type RPC Version number REMOTE Program REMOTE program version REMOTE Procedure Authentication Procedure arguments The format is of variable length, because the number of arguments in the called procedure can not be pre determined. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  30. rpcgen • Rpcgen is a software generator tool, generating: • The client stub • The server skeleton • XDR procedures for results and parameters • A file containing common definitions • SUN provides a complete methodology with: • Conversion routines for simple types • XDR routines for formatting complex types (tables and structures), used in RPC message definition. • Run time RPC functions that makes possible a remote procedure call by a program, a service record on the port mapper, dispatching a request for procedure call to the corresponding procedure inside the remote program. • Example of run time function is A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  31. Client application Client interface Q_clnt.c Q.x compiler Q.h client server rpcgen Q.xdr.c compiler Q.svc.c remote procedures Server interface rpcgen (1) A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  32. rpcgen (2) • Rpcgen is a procompiler which generates many files from a synthetic description of a program in an “Q.x” file • The description language used resemble C • The resulting files are: • A file to include into both server and clients codes (Q.h) • A server code skeletton (Q_svc.c) • Procedures that actually handle client remote calls (Q_clnt.c) • XDR filters (Q_xdr.c) • So the programmer will only have to write two files: • A file containing the implementations of the procedures provided by the server, that will complete the “Q_svc.c” file which already contains the main function. • The client code that will be the “Q_clnt.c”, and in which the programmer will add a main method. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  33. rpcgen (3) • The methodology consist of developing a distributed application exactly as if you was developing a local conventional application, then defines the procedures which are going to be ran remotely. • This decoupling implied adding codes between the call of the procedure and the remote procedure itself: • On the server side the new code must: • Accept an RPC request • Decodes parameters according to the internal representation of the local machine • Dispatch the message towards the adequat procedure. • Build the response and encodes it. • Transmit the encoded message to the client • On the Client side the new code must: • Encode parameters • Create an RPC CALL message • Transmit this message to the remote program • Wait for the results and decodes them according to the internal representation of the local machine A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  34. rpcgen (4) Proc A1 Proc A2 • The stubs procedures replace the following conventionnal procedures: • Calling procedure on the client side • Procedure being called on the server side Dispatcher client stub for B2 server stub for B1 server stub for B2 client stub for B2 Proc B1 Proc B2 The « stub » procedure is exactly named as the original calling procedure. That helps to keep the same source in the distributed version. e.g. the « stub » procedure for B2 is named B2, and the « stub » procedure for B1 is named B1. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  35. rpcgen (5) • Rpcgen splits every « stub » in two parts: • An entity that is common to all the applications, and which use to build the client server communication. • An entity belonging to the application, providing an interface with the application. • This separation is justified because rpcgen uses «  communication package » procedure call convention whereas it allows the user to remote procedures call conventions. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  36. rpcgen (6) server comm. Proc A client Interface Server Interface Client comm. Proc B A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  37. Content (5) • RPC possible fields of applications • Distributed systems challenges • Alternatives to DS challenges ? • RPC • SUN RPC • Development with RPC • RPC server types • Remote PC vs Local PC • Failures with RPC • RPC lacks • RMI • Idempotent operations A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  38. interface Definition   rpcgen Header file Client Stub Server Stub Server Code Client Code CC CC CC CC Client Objet Client Stub Objet Server Stub Objet Server Objet Librairy Librairy Linker Linker Client Server Development with RPC A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  39. RPC server types • Two types of RPC servers : • Stateful: • The server keeps informations on the different clients transactions. Those informations may serv to repeat operations that have been already been executed once. • Stateless: • There is no memory of information on client operations. The client is responsible of that, it could makes it operation idempotent itself. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  40. Remote PC vs local PC • Differences between remote procedures and local procedures calls are: • The waist of time in the network (network delay) may generate very important processing delay. • A remote procedure call may contain pointer types parameters, not easy to handle as on local procedure call. • Access to Input and output are denied to remote procedures, thus they can not use local peripherals (e.g. unable to write error messages) A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  41. Failures with RPC • Many failures types may occur: • The client is unable to locate the server • The server is not availaible • The Stubs versions are not adequat • The client request towards the server is lost: • After a timeout, the client retransmits the same request • The server response from the server to the client is lost: • The client will retransmit the same request. • No problem if it is an idempotent operation A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  42. Failures with RPC (2) • The server is down after receiving the client request (3 cases): • At least once: • When the machine restarts, retransmits the requests many times until the response arrives. • At most once: • Abandon the operation. The operation is done either once or never. • Exactly once: • Keep in memory the state of the operations that have been executed. Not easy to implement. Req Req Receives Executes Respond Req Receives Executes Failure Receives Failure Rep A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  43. RPC lacks • RPC lacks: • RPC is for structured programming • Parameters and return values are primitives types • Depends on the server localisation • No remote pointer • Evolutions • CORBA • Multilanguage, multi-platform (architecture+OS), MuliVendors (Chapter 11) • Java RMI • mono-language : Java, multiplatform : work on top JVM (Chapter 9) • DCOM / Object RPC • multi-languages, mainly Win32 platform, il existe des • There are some (non-MicroSoft) implementations for Unix, but proprietaries • SOAP (Simple Access Object Protocol) • multi-languages, multi-platform • Uses XML (with SOAP DTD) in request and response • Messages are transported on HTTP A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  44. RMI: Remote Method Invocation • The Object Oriented Programming RPC is Java Remote Method Invocation (RMI). • It is an approach for executing code on other machines across a network. • You can send a message to the remote object and get a result as if the object lived on your local machine. • In Chapter 9 we will walk you through the steps necessary to create your own RMI objects. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

  45. Idempotent operations • Idempotent operations are the one that can be repeated without side effects: • Idempotent operation: • Read the 1024 first records of a file • Non Idempotent operation: • Transfer an amount of $2000 from an account to another. A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

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