tính toán song song thoại nam distributedsystem 15 communication sinhvienzone com

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Distributed Systems
Thoai Nam

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Faculty of Computer Science and Engineering
HCMC University of Technology

SinhVienZone.com

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Issues in communication
Message-oriented Communication

Remote Procedure Calls

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Chapter 2: Communication

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– Transparency but poor for passing references

Remote Method Invocation

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– RMIs are essentially RPCs but specific to remote objects
– System wide references passed as parameters

Stream-oriented Communication

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Protocols are agreements/rules on communication
Protocols could be connection-oriented or
connectionless

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Communication Protocols

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A typical message as it appears on the network.

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2-2

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Layered Protocols

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2-4 operation of TCP.
Normal
Transactional TCP.

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Client-Server TCP

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Middleware Protocols

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Middleware: layer that resides between an OS and
an application

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– May implement general-purpose protocols that warrant
their own layers. Ex: distributed commit

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Structure: group of servers offering service to
clients
Based on a request/response paradigm
Techniques:

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– Socket, remote procedure calls (RPC), Remote Method
Invocation (RMI)
client
kernel

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file
server

process
server

terminal
server

kernel

kernel

kernel

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Client-Server Communication
Model

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Addressing
Blocking versus non-blocking
Buffered versus unbuffered
Reliable versus unreliable
Server architecture: concurrent versus sequential
Scalability

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Issues in Client-Server
Communication

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user

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– Machine address and process
address are known a priori
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user

server

Broadcast-based

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– Server chooses address from a
sparse address space
– Client broadcasts request
– Can cache response for future
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server

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Question: how is the server
located?
Hard-wired address

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Addressing Issues

NS

user

Locate address via name server
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server


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Blocking versus Non-blocking
Blocking communication (synchronous)

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– Send blocks until message is actually sent
– Receive blocks until message is actually
received

Non-blocking communication (asynchronous)

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– Send returns immediately
– Return does not block either

Examples

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Buffering Issues

Unbuffered communication

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Buffered communication

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– Client send to a mailbox
– Server receives from a mailbox
user

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server

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– Server must call receive before clientuser
can call send

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server


Reliability

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Reliable channel

User

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– Reply acts as ACK for request
– Explicit ACK for response

ACK

Reliable communication on unreliable
channels

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request
reply

– Transport protocol handles lost messages
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Server

ACK

Server

ACK
reply

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– Need acknowledgements (ACKs)
– Applications handle ACKs
– ACKs for both request and reply

request

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Unreliable channel

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Goal: Make distributed computing look like
centralized computing
Allow remote services to be called as procedures

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Remote Procedure Calls

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– Transparency with regard to location, implementation,
language

Issues

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– How to pass parameters
– Bindings
– Semantics in face of errors
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Two classes: integrated into prog, language and
separate
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b) The stack while the called
procedure is active

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Parameter passing in a
local procedure call: the
stack before the call to
read

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Conventional Procedure Call

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Parameter Passing
Local procedure parameter passing

Remote procedure calls simulate this through:

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– Call-by-value
– Call-by-reference: arrays, complex data structures

Related issue: global variables are not allowed in
RPCs

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– Stubs – proxies
– Flattening – marshalling

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Principle of RPC between a client and server
program.

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Client and Server Stubs

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Client makes procedure call (just like a local
procedure call) to the client stub
Server is written as a standard procedure
Stubs take care of packaging arguments and
sending messages
Packaging parameters is called marshalling
Stub compiler generates stub automatically from
specs in an Interface Definition Language (IDL)

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Stubs

– Simplifies programmer task

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Client procedure calls client stub in normal way
Client stub builds message, calls local OS
Client's OS sends message to remote OS
Remote OS gives message to server stub
Server stub unpacks parameters, calls server
Server does work, returns result to the stub
Server stub packs it in message, calls local OS
Server's OS sends message to client's OS
Client's OS gives message to client stub
Stub unpacks result, returns to client

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Steps of a Remote Procedure Call

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Example of an RPC

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Marshalling

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Problem: different machines have different data
formats

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– Intel: little endian, SPARC: big endian

Solution: use a standard representation
Problem: how do we pass pointers?

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– Example: external data representation (XDR)
– If it points to a well-defined data structure, pass a copy and the
server stub passes a pointer to the local copy

What about data structures containing pointers?

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– Prohibit
– Chase pointers over network
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Marshalling: transform parameters/results into a byte
stream
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Binding
Problem: how does a client locate a server?
Server

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– Use Bindings

Client

– First RPC: send message to binder to import server
interface
– Binder: check to see if server has exported interface
» Return handle and unique identifier to client

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– Export server interface during initialization

– Send name, version no, unique identifier, handle
(address) to binder

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TCP: stream is divided into records
UDP: max packet size < 8912 bytes
UDP: timeout plus limited number of retransmissions
TCP: return error if connection is terminated by server

Multiple arguments marshaled into a single structure
At-least-once semantics if reply received, at-least-zero
semantics if no reply. With UDP tries at-most-once
Use SUN’s eXternal Data Representation (XDR)

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One of the most widely used RPC systems
Developed for use with NFS
Built on top of UDP or TCP

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Case Study: SUNRPC

– Big endian order for 32 bit integers, handle arbitrarily large data
structures
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Server start-up: create port
Server stub calls
svc_register to register
prog. #, version # with local
port mapper
Port mapper stores prog #,
version #, and port
Client start-up: call
clnt_create to locate server
port

Upon return, client can call
procedures at the server

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Binder: Port Mapper

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Rpcgen: generating stubs

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Q_xdr.c: do XDR conversion
Detailed example: later in this course
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Many RPCs occur between client and server on
same machine

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Lightweight RPCs

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Server S exports interface to remote procedures
Client C on same machine imports interface
OS kernel creates data structures including an
argument stack shared between S and C

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– Need to optimize RPCs for this special case => use a
lightweight RPC mechanism (LRPC)

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