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

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Chapter 4: Client server Programming

With RPC

Rufin Soh

INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

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Content

1. RPC fields of applications

2. Distributed systems challenges

3. Alternatives to DS challenges ?

4. RPC

a. Why RPC?

b. RPC: definitions

c. RPC working mode

d. RPC model

e. RPC architecture

f. Execution mode

g. Calling Mode

11. SUN RPCa. The remote program b. Programs Identificationc. The port mapperd. XDR

e. Parameters exchanges

f. Passing parameters

g. Messages format

h. Rpcgen

12. Development with RPC

13. RPC server types

14. Remote PC vs Local PC

15. Failures with RPC

16. RPC lacks

17. RMI

18. Idempotent operations

A.Obaid-Wilfried Probst – Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

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RPC fields of applications

Transport and delivery Inventories

Cash and billing

Accounting

manufacturing

EngineeringSales


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Content (1)

1. RPC possible fields of applications



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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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Content (2)





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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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Content (3)



3. Alternatives to DS challenges ?4. RPC

a. Why RPC?

b. RPC: definitions

c. RPC working mode

d. RPC model

e. RPC architecture

f. Execution mode

g. Calling Mode


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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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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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RPC working modeClient Request

1. The call is detected as an RPC call.2. On A, the Call parameters are passed and placed in a data structure

(Marshalling) waiting to be transferred on the network3. An RPC identifier is generated for this Call4. A timer is initialized at A and set5. Data are transferred on the network to B, through the Operating System


Caller program

Call

RPC Service

A

E

Low level protocols

NETWORK

Low level protocols

Call

RPC Service

B

D

Called procedure

return

C

Caller System (Client) Called System (Server)

1

5

1

2 Marshalling3 RPC identifier4 Timer

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RPC working mode (2)

Server process and response6. Prameters are extracted from the network buffer in a useful

format(Unmarshalling) for Call to be made7. RPC identifier is recorded8. The Call is processed at C and return to the caller at D9. The results to return are placed in a special data structure (Marshalling)

waiting for transfer10. The timer is initialized


Caller program

Call

RPC Service

A

E

Low level protocols

NETWORK

Low level protocols

Call

RPC Service

B

D

Called procedure

return

C


1

5

1

6 Unmarshalling7 Record RPC identifier

89 Marshalling10 Set Timer

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RPC working mode (3)

Client ACK11. Data are sent on the network (through the server operating system APIs) and

transferred to E12. Results are extracted from the network buffer in a data structure

(Unmarshalling)13. The Client-side timer is desactivated14. An acknowledgement is sent to the server with the corresponding RPC identifier.15. The server side timer is desactivated on D


Caller program

Call

RPC Service

A

E

Low level protocols

NETWORK

Low level protocols

Call

RPC Service

B

D

Called procedure

return

C


1

5

12 Unmarshalling13 Desactivate timer

8

15 Desactivate Timer

11

14

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RPC modelThe 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.


procA()

return

procB()

returnreturn

Procedure B (server)

Procedure A (server)Main Program

Machine 1 Machine 2Network Machine 3Network

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RPC architecture

Marshalling Unmarshalling

SendRequest() ReceiveResponse()

Application

Procedure call

Procedure return

Marshalling Unmarshalling

SendResponse() ReceiveRequest()

Procedure

Procedure return

Run procedure

Client stub Server stub

Client RPC Server


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Execution mode

Procedures calls are made on the following model:

local variables main() SP

local variables main()bytes

buff

fd

Return adress

local variablesread() SP

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

N = read(fd,buff,nbytes)


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Calling Mode

Client

Client Stub

Marshalling parameters

Unmarshalling results

core

Server

ServerStub

Marshalling results

Unmarshalling parameters

core

Call Return Return Call

The machine hosting the procedure on the network remains transparent to the client


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Content (4)




4. RPC

11. SUN RPCa. The remote program b. Programs Identificationc. The port mapperd. XDR

e. Parameters exchanges

f. Passing parameters

g. Messages format

h. rpcgen


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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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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

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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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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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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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 descriptionportmap 100000 port mapperrstat 100001 rstat, rup, perfmeterruserd 100002 remote usersnfs 100003 Network File Systemypserv 100004 Yellow pages (NIS)mountd 100005 mount, showmountdbxd 100006 debuggerypbind 100007 NIS binderetherstatd 100010 Ethernet snifferpcnfs 150001 NFS for PC


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The port mapper

RPC server Program

Port Mapper

The program transmits

the trio (ident, RPC, port)

socket allocated to the RPC program

Port Mapper’s socket = 111


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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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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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Communication between the client and the server is done through a message containing the name of the procedure, and grouped parameters (marshalling )

n=sum(4,7)

core

sum(I,j){ int I, j; return (i+j) ; }

sum

4

7

sum

4

7

core


Parameters exchanges

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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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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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rpcgenRpcgen 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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rpcgen (1)

Q.x

rpcgen

Q_clnt.c

Q.h

Q.xdr.c

Q.svc.c

compiler

Client application Client interface

client

compiler

remote procedures Server interface

server


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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:

o A file to include into both server and clients codes (Q.h)

o A server code skeletton (Q_svc.c)

o Procedures that actually handle client remote calls (Q_clnt.c)

o XDR filters (Q_xdr.c)

So the programmer will only have to write two files:

o 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.

o The client code that will be the “Q_clnt.c”, and in which the programmer will add

a main method.

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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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rpcgen (4)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

client stub for B2

client stub for B2

Dispatcher

server stub for B1

server 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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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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rpcgen (6)

Proc A

client Interface

Clientcomm. Proc B

ServerInterface

server comm.


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Content (5)




4. RPC5. SUN RPC

6. Development with RPC

7. RPC server types

8. Remote PC vs Local PC

9. Failures with RPC

10. RPC lacks

11. RMI

12. Idempotent operations


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Development with RPC

interface Definition interface Definition

Client StubClient Stub Server StubServer StubHeader fileHeader fileClient CodeClient Code Server CodeServer Code

Client ObjetClient Objet Client Stub Objet Client Stub Objet Server ObjetServer ObjetServer Stub ObjetServer Stub Objet

LibrairyLibrairy

CC

Linker Linker

CC CC CC

rpcgen

ClientClient ServerServer

LibrairyLibrairy


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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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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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Failures with RPC

Many failures types may occur:

1. The client is unable to locate the server

The server is not availaible

The Stubs versions are not adequat

2. The client request towards the server is lost:

After a timeout, the client retransmits the same request

3. 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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Failures with RPC (2)

4. 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.

Receives

Executes

Respond

Req

Rep

Receives

Executes

Failure

Req

Receives

Failure

Req


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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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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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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.


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

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