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2. Remote ProcedureCall (RPC)

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2.1 Introduction

RPC is a protocol that one program can use to request a

service from a program located in another comp

RPC is anIPC technology that allows a program to cause aprocedure to execute in another address space.

When this mechanism is implemented using object-oriented

principles, it is called Remote Method Invocation (RMI)

RPC is a popular way of implementing client-server model ofDistributed Computing

where, Requesting program is a Client

Service providing program is a Server

RPC is used by both, an O.S. & by applications..

NFS is implemented as a set of RPCs

DCOM , CORBA, Java RMI are RPCs

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RPC is an extension ofprocedure callmechanism

It enables a call to be made to a procedure thatdoes not reside in the address space of the callingprocedure

The called procedure (remote) can be on the samecomputer as that of the calling procedure or on adifferent computer

In this case, as the caller and callee processes have

different address spaces , there is no shared dataHence, RPC uses message passing scheme for

communication.

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Caller Callee

(Client) (Server)

Caller

Proc blocked

Proc

executes

Resume

Exec.

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Issues in Transparency..

A transparent RPC mechanism is the one in

which, local procedure and remote procedure

are indistinguishable to the user.

This requires two types of transparencies. Syntactic transparency

Semantic transparency

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2.2 Implementation of RPC To achieve the goal of transparency, implementation of RPC is

based on concept of STUBs

Which provide a normal procedure call abstraction by

concealing from programs the interface to the underlying

RPC system.

RPC involves two processes : Client & Server

Hence to achieve transparency, a separatestub process is

associated with each of the two processes

Also, to hide the existence and functional details of the

underlying network, RPC communication package called

RPC Runtime is used on both the sides.

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Elements involved in implementation of RPC

Mechanism :

The Client

The Client StubRPC Runtime

The Server Stub

The Server

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Client Server

Client stub Server stub

RPC Runtime RPC Runtime

Return Call

Unpack Pack

Receive Send

Call Return

Unpack Pack

Receive Send

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Client :

A user process that initiates a remote procedure call Client Stub :

Responsible for two jobs :

On receipt of a request call from client,

It packs specifications of the target procedure &

arguments into a message

Then asks the local RPC Runtime routine to

send it to the server stub

On receipt of result of procedure execution,

it unpacks the result and passes it to the client.

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RPC Runtime:

Responsible for transmission of messages across the

network between the client and server.

On Client Machine --

It receives a call request message from the client stub &

sends it to the server machine.

It also receives message containing result of procedure

execution from server machine & passes it to the client

stub.

On Server Machine --

It receives message containing the result of procedure

execution from server stub & sends it to the client

machine.

It also receives call request message from the client

machine & passes it to the server stub.

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Server Stub :

Responsible for two jobs :

On receipt of a request call message from local RPCruntime,

It unpacks it and makes a normal call to invoke

appropriate procedure in the server.

On receipt of result of procedure execution, it packs the result into a message and then asks the local

RPC Runtime to send it to the client stub.

Server :

It executes appropriate procedure & returns result ofprocedure execution to the server stub

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2.3 RPC Messages :

Types of messages involved in implementation of RPC :

Call Messages: sent by client to server requesting

execution of a remote procedure

Reply Messages:sent by server to client for returning result

of remote procedure execution

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1. Call Messages:

Components of a call message :-

Identification information of the remote procedure to beexecuted i.e.Remote procedure identifier.

Thearguments necessary for execution of the procedure

Additional Fields:- Message identification field : has a sequence no. for

identifying lost messages and duplicate messages.

Client identification field : to allow the server toidentify the client.

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Message Type field : used to distinguish call message

from reply message

0 call message

1 reply message

RPCcall message format:

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2. Reply Messages:

When the server of an RPC receives a call message, it could

be faced with one of the following conditions:

Call message violates RPC protocol

Action: Server rejects such a call.

Server detects by scanning clients id. Finds that it is notauthorized to use the service

Action: Server returns an unsuccessful reply.

Remote procedure specified in the remote procedure id.Field of the call message is not available with it

Action:Server returns an unsuccessful reply.

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Incompatible RPC interface being used by the client and

server.

Action: Server returns an unsuccessful reply.

An exception condition (Division by zero) occurs during

execution of remote procedure

Action: Server returns an unsuccessful reply.

The specified procedure is executed successfully

Action: Successful reply.

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Fields in theReply Message :

Message Identifier Field : same as call message.

Message Type :Indicates it is a reply message.

Reply status : 0Successful reply

1 / non-zero value Unsuccessful reply

Result : Result of procedure execution.

Reason : Reason for failure

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Asuccessfulreply message:

An unsuccessfulreply message:

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2.4 Marshalling arguments and results:

Implementation of RPC involves transfer of arguments from the client process to the server process and, Transfer of results from server process to client process.

These arguments and results are transferred in form of

message data between two computers.

In RPC, packing parameters into a message is known as

marshalling

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Steps in Marshalling :

1. Taking arguments (of client) or result (of server) that

will form message data to be sent to remote process.

2.Encoding the message data.

3.Decoding the message data.

For successful marshalling, the client & server ,both

should know the methods used.

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Marshalling procedures:

Provided as a part of RPC software

Best way

User-defined methods

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2.5 Server Implementation :

Based on the style of implementation used, servers can beof two types:

Stateful servers

Stateless servers

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Stateful server:

A stateful server maintains the clients state

information from one remote procedure call to thenext.

i.e. in case of two subsequent calls, some state info.

related to service performed for the client as a result of

first service call execution is stored by the server

process.

This clients state information is subsequently used at

the time of execution of second call

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Stateless server:

A stateless serverdoes not maintain any client

state information.

Hence, every request from the client MUST beaccompanied with all necessary parameters to

successfully carry out the desired operation.

Server doesnt keep track of any informationresulting from previous operation.client has tokeep the track

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Advantage of Stateful over stateless server:

They relieve the clients from keeping track of the

state information

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Advantage of Stateless over stateful:

They have an advantage in terms of failure.

E.g. with stateful server, if the server crashes, & thenrestarts, the state information that it was holding may

be lost and, client process might continue its task

unaware of the crash, producing unwanted results.

Hence, the client of a stateful server should bedesigned to detect server crashes , to do necessary error

handling.

Whereas, with stateless, a client has to only retry a

request until the server responds; it does not need toknow that the server has crashed.

The choice is purely APPLICATION DEPENDENT.

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2.6 Client-Server Binding:

It is necessary for a client to know the location ofa server before a remote procedure call can take

place between them.

The process by which a client becomes associated

with a server so that calls can take place is known

asBinding

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Issues in Client-server binding process:

How does a client specify a server to which it wants to get

bound?

How does the binding process locate the specified server ?

When is it proper to bind a client to a server?

Is it possible for a client to change a binding duringexecution ?

Can a client be simultaneously bound to multiple serversthat provide the same service ?

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2.6.1 How does a client specify a server to which it wants to

get bound?

Server Naming :

The specification of a client by a server with which it

wants to communicates a naming issue.

Solution.Use of Interface. Two parts of Interface name:

Type

Instance.

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2.6.2 How does the binding process locate the

specified server ?

Server Locating :

When a client specifies the interface name of a

server for making a RPC, the server must be

locatedbefore the clients request message can besent to it.

Two methods: Broadcasting

Binding Agent

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Broadcasting:

In this method, a message to locate the desired server is

broadcast to all the nodes The nodes on which the desired server is located return a

response message

The desired server may be replicated on several nodes, so

the client node will receive a response from all thesenodes.

Normally, the first response that is received at the clients

node is given to the client process & all the subsequent

responses are discarded

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Advantage:

Easy to implement

Suitable for small networks

Disadvantage:

Expensive for large networks.because of

the increase in message traffic due toinvolvement of all nodes in the broadcasting

process.

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Binding Agent:

It is a name server used to bind a client to a server

by providing the client with the locationinformation of the desired server.

A binding agent maintains a binding table , which

is mapping of a servers interface name to its

locations.

All the servers register themselves as a part of

their initialization process.

To register with the binding agent, a server givesthe binder its identification information& a handle

used to locate it.

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A server can also deregister with the binding

agent when it is no longer prepared to offerservice.

The binding agent can even poll the servers

periodically , automatically deregistering any

server that fails to respond

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Locating process:

To locate a server, a client contacts the bindingagent..lookup

If the server is registered with the agent, it

returns the handle of the server to the client.

To begin with, the binding agents location is

known to all the nodes.

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Binding Agent

Server processClient process

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Advantages:

This method can support multiple servers having the

same interface type so that any of the availableservers may be used to service the clients request

This helps to achieve a degree of fault tolerance.

Since the bindings are done by the binding agent,

when multiple servers provide the same service, theclients can be spread evenly over the servers to

balance the load.

The binding mechanism can be extended to allowthe servers to specify the list of clients who may use

its service.

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Disadvantages:

The overhead involved in binding clients to servers is large

& becomes significant when many client processes are shortlived.

It is necessary that the binding agent should be robust

against failures & should not become a performance

bottleneck. Distributing the binding function among several binding

agents & replicating information among them can satisfy

both these criteria.

But, replication involves extra overhead of keeping themultiple replicas consistent.

Hence, the functionality offered by many binding agents

is lower than what is expected.

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2.6.3 When is it proper to bind a client to a server?

Binding Time:

A client may be bound to the server at

Compile time

Link time

Call time

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Binding at Compile time:

Here, the client and server are programmed as ifthey were intended to be linked together.

E.g. the servers network address can be

compiled into the client code by the

programmer and then it can be found bylooking up the servers name in a file.

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Binding at Link time:

In this method, a server process exports its service

by registering itself with the binding agent as partof initialization process.

A client then makes an import request to the

binding agent for the service before making a call.

The binding agent binds the client & server by

returning to the client the servers handle.

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Binding at call time:

In this method, a client is bound to a server at the

time when it calls the server for the first timeduring its execution.

A commonly used approach for binding at call

time is indirect call method

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Binding Agent

Server processClient process

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2.6.4 Is it possible for a client to change a binding

during execution ?

Changing bindings: Basically, Binding is a connection establishment

between a client & a server.

The client or server of a connection may wish to

change the binding at some instance due to somereason For e.g. A client getting service from a server can

change the binding to another server of the same typewhen a call to already connected server fails.

The server of a binding may want to alter the binding& connect the client to another server when the serviceneeds to move to another node.

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2.6.5 Can a client be simultaneously bound to multiple

servers that provide the same service ?

Multiple simultaneous bindings:

In a system, a service may be provided by multiple

servers.

Also, a client is bound to a single server out of a numberof servers.

But, there may be situations when it is advantageous for a

client to be bound simultaneously to all or multiple servers

of the same type.

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Binding of this sort gives rise to multicast communication

as all the servers bound to the client for that service will

receive and process the call.

E.g. a client may wish to update multiple copies of a file that isreplicated at several nodes.

For this, the client can be bound simultaneously to all the nodes

where the replica of the file is located.

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2.7 RPC Semantics in case offailure:

The goal of RPC is to hide the communication bymaking remote calls look just like local ones.

But, when an error occurs, the differences

between the local & remote calls become difficult

to mask.

Failure handling code is a part ofRPC Runtimemechanism.

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Five different classes of failure :

1. The client is unable to locate the server.

2. The request message from the client to server is

lost.

3. The reply message from server to client is lost.

4. The server crashes after receiving a request.

5. The client crashes after sending a request.

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2.7.1. The client is unable to locate the

server

Solution .

Adding a new error typecannot locate

server

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2.7.2. The request message from the clientto server is lost.

To deal with this problem, kernel can be designedto start a TIMER when sending a request.

If the timer expires before a reply or acknowledgecomes back, the kernel sends the message again

Two possibilities: If the message was actually lost, the server is unable to

identify retransmission.

Kernel gives up after a number of tries and concludesthatthe server is down.

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2.7.3 The reply message from server to

client is lost.

More difficult scene. Clients kernel may not understand what is the exact

problem

Whether request is lost, or reply is lost, or the server is slow.

Some operations can be safely repeated without causing

any harm..Idempotent operations

E.g. Reading data from a file

Consider an example of transferring money in banking

server..Non idempotent operationas repeating the

operation will harm the data.

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One way of solving this problem is structure all the

requests in an Idempotent way.

not possible for all requests, like banking.

Another way is to have the clients kernel assign eachrequest a sequence no. same request is retransmitted

with same sequence no. if there is no problem from the

server side , then it can understand the retransmission , &

can refuse to carry out the request second time.

2 7 4 Th h ft i i t

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2.7.4. The server crashes after receiving a request.

ReceiveExecute

Reply

Req

Rep

Normal Case

ReceiveExecuteCrash

Req

No RepCrash after exec.

Receive

Crash

Req

No Rep

Crash before exec.

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At least once semantics.

At most once semantics.

Guarantee nothing

2.7.5 The client crashes after sending a

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2.7.5 The client crashes after sending a

request.

In this scene, the client sends a request to theserver & crashes before the server replies.

At this point, the computation is active and no

parent is waiting for the result.such

unwanted computation is called ORPHAN

Dealing with orphans:

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Dealing with orphans:

Extermination:

Before a client stub sends a request, it makes a log entry.

After the reboot, the log is checked & the orphan is explicitly

killed off. This solution is called Extermination

Reincarnation:

This scheme divides the time up into sequentially numberedepochs.

When a client reboots, it broadcasts a message to all the machines

declaring the start of a new epoch.

When such a broadcast comes in, all the remote computations are

killed.

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Gentle reincarnation:

When an epoch broadcast comes in, eachmachine checks to see if it has any remote

computations, & if so, tries to locate theirowner.

Only if the owner can not be fund, is thecomputation is killed.

Expiration:

Here, each RPC is given a standard amount oftime T, to do the job.

If not finished within time, RPC is expired.