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Distributed Systems Alexis Delis ad@di.uoa.gr Monday 6:00-9:00 Spring 2002 www.di.uoa.gr/~ad/MDE519.html Grades - Evaluation Final Examination: 35% In-class Presentations & Homeworks: 30% Project(s): 35% Book: A.S. Tanenbaum, M. van Steen, Distributed Systems , Prentice Hall, 2002.

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distributed systems

Distributed Systems

Alexis Delis


Monday 6:00-9:00

Spring 2002



Grades - Evaluation

Final Examination: 35%

In-class Presentations & Homeworks: 30%

Project(s): 35%


A.S. Tanenbaum, M. van Steen, Distributed Systems, Prentice Hall, 2002.


Papers (available from the course’s WWW site)

definition of a distributed system
Definition of a Distributed System
  • A distributed system is:
  • A collection of independent computers that appears to its users as a single coherent system.
definition of a distributed system4
Definition of a Distributed System


A distributed system is organized as middleware.Note that the middleware layer may extend over multiple machines.

Examples: Workflow Systems, WWW-services (URL)

goals of distributed systems
Goals of Distributed Systems
  • connecting users with resources
  • transparency
  • openess
  • scalability
transparency in a distributed system
Transparency in a Distributed System

Different forms of transparency in a distributed system.

  • Open DS: a system that offer services
  • according to Standard Rules.
  • Such rules are described in terms of protocols
  • IDLs help in the description of such protocols.
  • Interoperability?
  • Portability?
  • Difference between the two?
  • Numerous “definitions” of scalability
  • system can be scalable in terms of size
    • Add more users/resources into the system
  • Geographically Scalable system
    • Users/resources may lie far apart
  • Administratively Scalable
    • Easy to manage even if it spans multiple orgs.

GOLDEN RULE: as system becomes scalable in one of these

three dimension exhibits loss in performance.

scalability problems
Scalability Problems

Examples of scalability limitations.

scaling techniques
Scaling Techniques
  • asynchronous communication
  • filling (database) forms.
  • distribution (take a component, break into

smaller pieces, spread these components

across the system

Example: DNS – Domain Name Service.

  • Division in (non-overlapping) zones – domains
scaling techniques11
Scaling Techniques


  • The difference between letting:
  • a server (character/string-based communication) or
  • a client check forms as they are being filled (bulk uploading).
scaling techniques12
Scaling Techniques


An example of dividing the DNS name space into zones.

scalability caching
Scalability - Caching
  • Generally Good Idea: Replicate (data/services)
  • Replication increases availability (of systems)
  • Caching (difference from replication?)
  • Main problem with caching/replication?
  • Is this a serious problem?
  • CDNs (how do they do it?)
hardware concepts
Hardware Concepts


Different basic organizations and memories in distributed computer systems



  • A bus-based multiprocessor.


  • A crossbar switch (n*n crosspoint switches needed)
  • An omega switching network (low latency problem)
  • How to avoid latency?

use hierarchical schemes: NUMA (NonUniform Memory Access)

homogeneous multicomputer systems building is easy problem interconnection network
Homogeneous Multicomputer Systems(building is “easy”-problem:interconnection network)

Homogeneous Multi-computers: System Area Networks (SANs)


  • Bus-based (FDDI/ Fast-Ethernet /Gigabit network )
  • Grid (switched based; messages are routed via the interconnection network)
  • Hypercube(switched based; four dimensional (b); two ordinary cubes with 8 vertices and 12 edges)
switched multi computers
Switched Multi-computers
  • Can vary significantly…
  • From…
  • Massively Parallel Processors ($$ MPP – CM5, IBM SP2 etc.)
  • To..
  • Clusters/Networks of Workstations (cheap$ - COWs or NOWs)
software concepts
Software Concepts
  • An overview between
  • DOS (Distributed Operating Systems)
  • NOS (Network Operating Systems)
  • Middleware
uniprocessor operating systems
Uniprocessor Operating Systems


  • Separating applications from operating system code through a microkernel.
multiprocessor operating systems
Multiprocessor Operating Systems

monitor Counter {


int count = 0;


int value() { return count;}

void incr () { count = count + 1;}

void decr() { count = count – 1;}


  • A monitor to protect an integer against concurrent access.
multiprocessor operating systems 2
Multiprocessor Operating Systems (2)

monitor Counter {


int count = 0;

int blocked_procs = 0;

condition unblocked;


int value () { return count;}

void incr () {

if (blocked_procs == 0)

count = count + 1;


signal (unblocked);


void decr() {

if (count ==0) {

blocked_procs = blocked_procs + 1;

wait (unblocked);

blocked_procs = blocked_procs – 1;



count = count – 1;



  • A monitor to protect an integer against concurrent access, but blocking a process.
distributed shared memory systems dsm
Distributed Shared Memory Systems (DSM)

Trick: replicate pages that are referenced frequently!

Another: replicate all pages (??) – problems? Solutions?

  • Pages of address space distributed among four machines
  • Situation after CPU 1 references page 10
  • Situation if page 10 is read only and replication is used
distributed shared memory systems
Distributed Shared Memory Systems
  • Always an issue: size of a page in DSM – why?
  • Large sized page are potentially good but they may
  • entail false sharing
  • Having data belonging to two independent processes in the
  • same page…
network operating system
Network Operating System


  • General structure of a network operating system.
network operating system26
Network Operating System


  • Two clients and a server in a network operating system.
  • Services allowed to users…
  • rlogin machineA
  • rcp machineA:file1 machineB:file2
  • ftp, http, sftp, ssh, etc.
network operating system27
Network Operating System
  • Issue of FS mount-ing


  • Different clients may mount the servers in different places.
positioning middleware
Positioning Middleware


  • General structure of a distributed system as middleware.
middleware and openness
Middleware and Openness


  • In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications.
comparison between systems
Comparison between Systems
  • A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems.
clients and servers
Clients and Servers


  • General interaction between a client and a server.
an example client and server
An Example Client and Server
  • The header.h file used by the client and server.
an example the server
An Example: The Server
  • A sample server.
an example the client
An Example: The Client

1-27 b

  • A client using the server to copy a file.
three processing levels
Three Processing Levels


  • The general organization of an Internet Search Engine into three different layers
multitiered architectures
Multitiered Architectures


  • Alternative client-server organizations (a) – (e).
multitiered architectures37
Multitiered Architectures


  • An example of a server acting as a client.
modern architectures
Modern Architectures


  • An example of horizontal distribution of a Web service.
ongoing research work
Ongoing (Research) Work
  • DSs based on Horizontal Distribution(of data/services)
  • DSs based on Horizontal and/or Vertical Distribution (of data/services)
  • Peer-to-Peer
    • Distributions of data
    • Services (indexing, querying, TP processing)
    • Self-organizing systems