COMP28112 Lecture 2

COMP28112 Lecture 2

•Architectures of distributed systems

•Fundamental Models

19 Apr 2023 COMP28112 Lecture 3 2

Architectures of Distributed Systems

• Tightly coupled– Highly integrated machines that may look as a single

computer.

• Loosely coupled (share nothing)– Client-Server– Peer-to-Peer– A Key differentiation based on the Programming Interface:

• Distributed Objects

• Web Services• Read “Web Services are not Distributed Objects”, Internet Computing, Nov-Dec. 2003,

http://www.allthingsdistributed.com/historical/archives/000343.html


Tightly coupled systems• Distributed shared memory (DSM)provides the

illusion of a single shared memory: it spares the programmer the concerns of message passing.

I n st r u c to r ’ s G u id e fo r C o u lo u r i s , D o l l i m o r e a n d K i n d b e r g D i st r ib u te d S y ste m s : C o n c e p t s a n d D e si g n E d n . 3 © A d d i so n - W e sle y P u b l i s h e r s 2 0 0 0

F ig u r e 1 8 .1T h e d is t r ib u te d s h a r e d m e m o r y a b s t r a c t io n

Phys icalmemory

Processaccess ing DSM

DSM appears asmemory in address

space of process

Phys icalmemory

Phys icalmemory

Dis tributed shared memory


Issues with Distributed Shared Memory• Machines are still connected by a network:

– Minimize network traffic– Reduce the latency between request and completion

• How to keep track of the location of shared data• How to overcome delays when accessing remote data• How to make shared data concurrently accessible• Replicate shared data on multiple machines:

– Need memory coherence

• Lots of research in the context of building parallel computers – see Chapter 18 in Coulouris et al book.


Architectural Styles(talking about loosely-coupled systems)

Based on the logical organization of the components of distributed systems:

• Layered architectures• Object-based architectures• Data-centered architectures• Event-based architectures


The layered architectural style

See Figure 2-1 in Tanenbaum and Van Steen book


The object-based architectural style

See Figure 2-1 in Tanenbaum and Van Steen book


The Event-Based architectural style

• See Figure 2-2 in Tanenbaum and Van Steen book


Shared-Data Space Architectural Style

• See Figure 2-2 in Tanenbaum and Van Steen book


System Architectures of Distributed Systems

In str uc to r ’ s G uid e fo r C o ulo ur i s, D o ll im o re and K ind b er g D i str ib u ted S y ste m s: C o nc ep t s a nd D e si g n E d n . 4 © P ea r so n E d uc a tio n 2 0 0 5

F ig u re 2 .1S o ftw a re a n d h a rd w a re s e rv ic e la y e rs in d is tr ib u te d s ys te m s

Applications, services

Computer and network hardware

Platform

Operating system

Middleware


Middleware

• Middleware: a software layer that provides a programming abstraction to mask the heterogeneity of the underlying platforms (networks, languages, hardware, …)– E.g., CORBA, Java RMI

• But… the end-to-end argument implies that some aspects of communication support cannot always be abstracted away from applications.


The end-to-end argument

“The function in question can completely and correctly be implemented only with the knowledge and help of the application standing at the end points of the communication system. Therefore, providing that questioned function as a feature of the communication system itself is not possible. (Sometimes an incomplete version of the function provided by the communication system may be useful as a performance enhancement.)”

• See Coulouris pages 33-34 and http://www.reed.com/dpr/locus/Papers/EndtoEnd.html


Client-Server Model

I n s t r u c t o r ’ s G u i d e f o r C o u l o u r i s , D o l l i m o r e a n d K i n d b e r g D i s t r i b u t e d S y s t e m s : C o n c e p t s a n d D e s i g n E d n . 4 © P e a r s o n E d u c a t i o n 2 0 0 5

F i g u r e 2 . 2C l i e n t s i n v o k e i n d i v i d u a l s e r v e r s

S erver

Client

Client

invocation

result

S erverinvocation

result

P rocess:K ey:

Computer:


Client-Server Model• Characteristics of a server:

– Passive (slave)

– Waits for requests

– Upon receipts of requests, it processes them and sends replies

– Can be stateless (does not keep any information between requests) or stateful (remembers information between requests)

• Characteristics of a client:– Active (master)

– Sends requests

– Waits for and receives server replies


Peer-to-Peer (P2P)

Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Figure 2.3A distributed application based on peer processes

Application

Application

Application

Peer 1

Peer 2

Peer 3

Peers 5 .... N

Sharableobjects

Application

Peer 4

Earlier Example:•Usenet NewsOther examples:•Napster, Gnutella, Freenet, …


Client-Server vs P2P

• Client-Server– Widely Used

– Functional Specialisation

– Asymmetrical

– Tends to be centralised

– Tends to scale poorly

• P2P– Symmetrical, computers

have same “rights”– Truly Distributed

– Share / exploit resources with a large number of participants

– Resource discovery is a challenge


Variations on a theme…

• Sometimes we need to consider:– The use of multiple servers to increase performance

and resilience– The use of mobile code– Users’ need for low-cost computers– Requirements to add and remove mobile devices


Multiple Servers


Figure 2.4A service provided by multiple servers

Server

Server

Server

Service

Client

Client


Proxies and Caches


F i g u r e 2 . 5W e b p r o x y s e r v e r

Client

Proxy

Web

server

Web

server

serverClient


Mobile Code…


F ig u r e 2 . 6W e b a p p le t s

a) c lient request results in the downloading of applet code

Web server

ClientWeb

serverApplet

Applet code

Client

b) c lient interacts with the applet


Thin Clients


Figure 2.7Thin clients and compute servers

ThinClient

ApplicationProcess

Network computer or PCCompute server

network


Design Requirements• Performance

– Responsiveness– Throughput– Load balancing

• Quality of Service• Caching and Replication• Dependability

– Correctness– Security – Fault-Tolerance


Fundamental Models• Models include all the essential ingredients that we

need to consider in order to understand and reason about some aspects of a system’s behaviour.

• A way of abstracting from reality.• Aspects that we need to capture include:

– Process Interaction – Failure– Security

Managing concurrency and failure underlies many of the problems we face!


Factors affecting process interaction

• Performance of communication channels

• Computer Clocks and Timing Events

• Two variants of the process interaction model:– Synchronous systems:

• Process Execution Speeds: has known lower and upper bound

• Message transmitted delay: bounded

• Clock drift rates: drift rate from real time has a known bound

– Asynchronous systems:• All the above may take an arbitrarily long time.


Event Ordering


F ig u r e 2 . 8R e a l - t im e o r d e r in g o f e v e n t s

send

receive

send

receive

m 1 m 2

2

1

3

4X

Y

Z

Phys ical time

Am 3

receive receive

send

receive receive receivet1 t2 t3

receive

receive

m 2

m 1


Failures in distributed systems

• What can go wrong?– (see Figure 2.10 in Coulouris)

• How to mask failures?– Read about checksums in network protocols


In Summary...• Different Architectures have different solutions

• Managing Concurrency, Failures, and Security underlies many of the problems we face.

• Reading:– Coulouris, Chapter 2; Tanenbaum, Sec. 2.1, 2.2 (skim through the rest

of Chapter 2)

– Read the text associated with the figures from Coulouris et al and Tanenbaum et al textbooks that are shown in this handout.

• Next time: Remote procedure calls.

COMP28112 Lecture 2

Documents

single shared memory

communication system

location of shared datahow

architectural stylestalking

data space architectural

programming abstraction

questioned function

programming interface