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

2. Content SUN RPC RPC fields of applications 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
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3. RPC fields of applications
Sales
Engineering
Accounting
manufacturing
Transport and delivery
Cash
and billing
Inventories
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4. Content (1) RPC possible fields of applications
Distributed systems challenges
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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.
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6. Content (2) RPC possible fields of applications
Distributed systems challenges
Alternatives to DS challenges ?
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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.
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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
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9. Why RPC? 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.
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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.
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11. 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
Caller program
Call
RPC
Service A E
Low level protocols N T W O R K B D
Called procedure
return C
Caller System (Client)
Called System (Server) 1 5
Marshalling
RPC identifier
Timer
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12. 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
Caller program
Call
RPC
Service A E
Low level protocols N T W O R K B D
Called procedure
return C
Caller System (Client)
Called System (Server) 1 5
6 Unmarshalling
7 Record RPC
identifier 8
9 Marshalling
10 Set Timer
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13. 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
Caller program
Call
RPC
Service A E
Low level protocols N T W O R K B D
Called procedure
return C
Caller System (Client)
Called System (Server) 1 5
12 Unmarshalling
13 Desactivate
timer 8
15 Desactivate Timer 11 14
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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 A
(server)
Procedure B
(server)
Main Program
procA()
procB()
return
return
return
Machine 1
Network
Machine 2
Machine 3
Network
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15. RPC architecture Client RPC Server Server stub Client stub Marshalling
Unmarshalling
SendRequest()
ReceiveResponse()
Application
Procedure
call
return
SendResponse()
ReceiveRequest()
Run
procedure
Client stub
Server stub
Client
RPC
Server
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16. 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:
local
variables
main() SP
bytes
buff fd
Return
adress
read()
N = read(fd,buff,nbytes)
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17. Calling Mode Client Server Call Return Return Call Client 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
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18. Content (4) SUN RPC The remote program Programs Identification
RPC possible fields of applications
Distributed systems challenges
Alternatives to DS challenges ?
RPC
SUN RPC
The remote program
Programs Identification
The port mapper
XDR
Parameters exchanges
Passing parameters
Messages format
rpcgen
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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.
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20. The remote program Databases insert procedure delete procedure
(!!! 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
insert procedure
delete procedure
lookup procedure
Databases
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
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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.
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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.
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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
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24. The port mapper The program transmits the trio (ident, RPC, port)
RPC server
Program
Port Mapper
the trio (ident, RPC, port)
socket allocated
to the RPC program
Port Mapper’s
socket = 111
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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.
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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.
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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 )
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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).
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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.
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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
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31. rpcgen (1) Client application Client interface Q_clnt.c Q.x compiler
Q.h
Q.xdr.c
Q.svc.c
compiler
Client application
Client interface
client
remote procedures
Server interface
server
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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.
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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 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
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
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34. rpcgen (4) Proc A1 Proc A2 Dispatcher Proc B1 Proc B2
The stubs procedures replace the following conventionnal procedures:
Calling procedure on the client side
Procedure being called on the server side
Proc A1
Proc A2
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.
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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.
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36. rpcgen (6) Proc A Proc B server comm. client Interface Server
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37. Content (5) Development with RPC RPC server types
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
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38. Development with RPC interface Definition Client Stub Server Stub
Header file
Client Code
Server Code
Client Objet
Client Stub Objet
Server Objet
Server Stub Objet
Librairy CC
Linker
rpcgen
Client
Server
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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.
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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)
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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
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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
Receives
Executes
Respond
Req
Req
Receives
Executes
Failure
Receives
Failure
Rep
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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
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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.
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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.
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