This document discusses two common models for distributed computing communication: message passing and remote procedure calls (RPC). It describes the basic primitives and design issues for each model. For message passing, it covers synchronous vs asynchronous and blocking vs non-blocking primitives. For RPC, it explains the client-server model and how stubs are used to convert parameters and return results between machines. It also discusses binding, parameter passing techniques, and ensuring error handling and execution semantics.

1. Preetha/ME-CSE/I

2. INTRODUCTION
• High level constructs [Helps the program in
using underlying communication network]
• Two Types of Communication Models
– Message passing
– Remote Procedure Calls
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3. Message Passing Method
• Two basic communication primitives
 SEND - Message & its Destination
 RECEIVE - Source of msg &Buffer for storing the
message
• Client-Server Computation Model
 Client sends Message to server and waits
 Server replies after computation
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4. Design issues
Blocking vs Non blocking primitives
Nonblocking
• SEND primitive return the control to the user process as soon as the message is
copied from user buffer to kernel buffer.
• Advantage : Programs have maximum flexibility in performing
computation and communication in any order.
• Drawback : Programming becomes tricky and difficult .
Blocking
• SEND primitive does not return the control to the user process until message has
been sent or acknowledgement has been received.
• Advantage : Program behavior is predictable.
• Drawback : Lack of flexibility in programming.Preetha/ME-CSE/I

5. Cont…
Synchronous vs Asynchronous Primitives
Synchronous
• SEND primitive is blocked until corresponding RECEIVE
primitive is executed at the target computer.
• Also known as rendezvous.
Asynchronous
• Messages are buffered.
• SEND primitive does not block even if there is no
corresponding execution of the RECEIVE primitive.
• The corresponding RECEIVE primitive can be either blocking
or non-blocking.
• Drawback : buffering message is more complex , as it involves
creating , manging &destroying buffers.Preetha/ME-CSE/I

6. Details to be handled in Message Passing
• Pairing of Response with Requests
• Data Representation
• Sender should know the address of Remote
machine
• Communication and System failures
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7. Remote procedure call (RPC)
• RPC is a protocol that one program can use to
request a service from a program located in
another computer in a network without having
to understand network details.
• RPC uses the client/server model.
• The requesting program is a client and the
service-providing program is the server.
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8. Cont..
• The main idea of an RPC is to allow a local
computer (client) to remotely call procedures
on a remote computer (server).
• RPC is an interaction between a client and a
server.
• Client invokes procedure on sever.
• Server executes the procedure and pass the
result back to client.
• Calling process is suspended and proceeds
only after getting the result from server.
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10. RPC design issues
• Structure
• Binding
• Parameter and Result Passing
• Error handling, semantics and Correctness
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11. Structure
Stub : It is a piece of code used for converting parameters passed
during a RPC.
Client stub procedure :
• constructs a message containing identity of remote procedures & parameters.
• sends the message to remote server machine.
Server stub procedures :
• receives the message at remote machine side & passes the parameter to the
procedure specified.
• Upon completion, it passes the result back to the client-stub procedure ,which
returns the result to the client.
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12. Binding
It is a process that determines the remote
procedure & the machine on which it will be
executed, upon a remote procedure invocation.
Binding server :
 Client-server model uses this server.
 It stores the server machine addresses along
with the services they provide.
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13. Parameter and result passing
1. Stub procedures convert the parameters to remote machine
format and pack them into a buffer . After the message is
received , it must be unpacked.
Problems :Conversion becomes expensive if it has to be done
on every call.
2. Send the parameters along with a code identifying the format
used so that receiver can do the conversion.
Problems :Machine should know how to convert all the
formats that can possibly be used and poor portability
because whenever new representation is introduced into the
system, existing s/w need to be updated.
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14. Cont..
 Each data type may have standard format in
the message .Here ,sender convert data to
standard format & receiver convert from
standard format to its local representation.
 Problem : arises if both sender and receiver
use the same internal representation.
 Another issue is passing parameter by value &
reference .Passing parameter by value is
simple but other is complicated
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15. Error handling ,semantics & correctness
RPC fails for two reasons:
 Computer failure
 Communication failure
Semantics play a significant role in development of
programs for distributes computation.
RPC semantics :
 “At least once” semantics
 “exactly once” semantics
 “At most once” semantics
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16.  “At least once” semantics
If RPC succeeds,it implies that atleast one execution of remote
procedure has taken place at remote machine.
Else 0,partial,one or more execution.
 “exactly once” semantics
If RPC succeeds,it implies that exactly one execution of
remote procedure has taken place at remote machine.
Else 0,partial,one execution.
 “At most once” semantics
Similar to exactly once semantics , in addition calls that do not
terminate do not produce any side effects and also referred as
Zero-or-one semantics.
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