COMP 655: Distributed/Operating Systems

COMP 655:Distributed/Operating

SystemsSummer 2011

Dr. Chunbo ChuWeek 2, 3: Communication

04/20/23 1Distributed Systems - COMP 655

Agenda• 6:00 - 6:30, system summary

presentation• 6:30 - 8:00, Lab 1• 8:00 - 8:15, Break• 8:15 – 9:30, Communication• Wrap up

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Communication essentials

• Communication patterns• Communication structure: OSI reference

model and TCP/IP coverage• Major middleware communication

services– RPC– RMI– http– I hate to wait …– Message passing– (skip) Streams

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Communication patterns

Producer Consumer

query

response

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Communication patterns with broker

Producer 2

query

response

Producer 1Consumer 2

Consumer 1

Consumer 3

Subscriptionservice

Client 2

Client 1

Server 2

Server 1

Server 3

Requestbroker

queryresponse

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Communication patterns -other variations

• Blocking or non-blocking• Connection-based or connectionless• Transient or persistent

Examples:• blocking transient query-response ==

RPC• non-blocking persistent

producer/consumer == message queuing

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A note about blocking• Blocking communication blocks the

thread that makes the call• Other work may continue in other

threads in the same process• More about threads and processes

next week

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Communication essentials

• Communication patterns• Communication structure: OSI

reference model and TCP/IP coverage• Major middleware communication services

– RPC– RMI– http– I hate to wait …– Message passing

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OSI Reference Model

2-1

TCP/IP

sending bits

e.g. Ethernet

connectionless IP

TCP, UDP

not used

not used

HTTP, FTP, …

Layered Protocols• A typical message as it appears on

the network.

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Middleware Protocols

2-5

RPC

RMI

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Where does http fit?• Originally, it looked like an

application-level protocol, where the application was fetching and viewing HTML pages

• As the Web has matured, it has been used increasingly as middleware.

• It’s the foundation for– Web applications– Web services

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Communication essentials

• Communication patterns• Communication structure: OSI reference

model and TCP/IP coverage• Major middleware communication services

– RPC– RMI– http– I hate to wait …– Message passing

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RPC is all about• Allowing a client to make a

procedure call that is processed on a remote machine

• Without the client’s having to care (much)

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Ordinary procedure call

process

caller

callee(callee is atanh)

0.98 2.3

Conventional Procedure Call

Parameter passing in a local procedure call: the stack before the call to read. (b) The stack while the called procedure is active.

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RPC: call the procedure across a network

clientprocess

serverprocess

network

caller callee

clientstub

serverstub

0.98

Atanh, 0.98

message

0.98

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RPC: call the procedure across a network

clientprocess

serverprocess

network

caller callee

clientstub

serverstub

2.3

OK, 2.3

message

2.3

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RPC Summary

clientprocess

serverprocess

network

caller

clientstub

callee

serverstub

•client stub pretends to be the callee

•server stub pretends to be the caller•caller and callee are written as if they were on the same machine•location transparency!!

(almost)

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Warning: passing parameters and results

can be tricky

Question

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• Parameter passing mechanisms• By value• By reference• By value-results• …

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Passing Value Parameters (1)

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Passing Value Parameters (2)

a) Original message (JILL, 5) on the Pentium (little endian)b) The message after receipt on the SPARC (big endian)c) The message after being inverted. The little numbers in

boxes indicate the address of each byte

Number is backwards String is backwards

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Conclusion from the byte order problem

• You have to explicitly define your interfaces.

• This problem is one of the fundamental motivators for the use of Interface Definition Languages (IDL)– WSDL, OMG IDL, …

• Passing value parameters is a key component of access transparency.

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Interface definition

Interface definitions are critical factors in

• Openness• Flexibility• Access transparency• Enabling proxy construction in

middleware

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Activity – what can go wrong?

(with an RPC)Brainstorm things that can go wrong

when a program on machine A tries to make an RPC to a server on machine B

A BNetwork

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Pick the top three• The instructor will pick some of the

failures you brainstormed• Multi-vote on the ones in whose

solutions you are most interested• You will use the top three vote-

getters in the next activity

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Activity – what could you do about it?

Brainstorm approaches to dealing with the top three failure modes

A BNetwork

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Communication patterns -reminder

Parameters:• Query-response or producer-consumer• Blocking or non-blocking• Connection-based or connection-less• Persistent or transientExamples:• blocking transient query-response == RPC• non-blocking persistent producer/consumer

== message queuing

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Finding the server or peer• In all cases, the process that initiates a

communication has to find the process it wants to communicate with

• The address could be in– Source code (simplest, least flexible)– Configuration file– A network directory service (requires client

to know a name for a server) (e.g. DNS)– A series of directories (e.g. LDAP, then RMI

Registry)

• Interaction with the directory is usually blocking request/response

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Communication essentials

• Communication patterns• Communication structure: OSI reference

model and TCP/IP coverage• Major middleware communication

services– RPC– RMI– http– I hate to wait …– Message passing

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Focus on Remote Objects

• Here, object ==state + methods + interface

• Usually, only the interface is distributed

• “Remote objects” refers to approaches in which, for each object there is some server where all of it state resides

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Distributed Objects

• Common organization of a remote object with client-side proxy.

2-16

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Basic Java RMI

rmiregistry**

Server

Client

1. Server registers its name and remote object

2. Client gets reference (a stub)  to remote object from rmiregistry

3. Client calls server

Interface.class_Stub.class *

1.

2.

3.

Interface.class_Skel.class *

* = not needed if Client and Server are both Java 1.5 or newer

** = RMI Registry can be a separate process or an object inside the Server process. If separate, must be on same machine.

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Passing objects around …• In RMI, parameters and return

values can be object references– If the parameter refers to a local

object, do you copy the object or pass only a reference?

– If the parameter refers to a remote object, what do you pass?

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Local, remote objects in Java RMI

• Local objects are passed by value• Remote objects by reference• Passing a remote object by

reference means passing a proxy by value

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Parameter Passing in RMI