Distributed Systems

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Distributed Systems

• Alexis Delis
• ad_at_di.uoa.gr
• Monday 600-900
• Spring 2002
• www.di.uoa.gr/ad/MDE519.html

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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. Other Papers
(available from the courses WWW site)
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Definition of a Distributed System

• A distributed system is
• A collection of independent computers that
  appears to its users as a single coherent system.

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Definition of a Distributed System
1.1
A distributed system is organized as
middleware.Note that the middleware layer may
extend over multiple machines. Examples Workflow
Systems, WWW-services (URL)
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Goals of Distributed Systems

• connecting users with resources
• transparency
• openess
• scalability

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Transparency in a Distributed System
Different forms of transparency in a distributed
system.
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Openness

• 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?

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Scalability

• 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.

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Scalability Problems
Examples of scalability limitations.
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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

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Scaling Techniques
1.4

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

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Scaling Techniques
1.5
An example of dividing the DNS name space into
zones.
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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?)

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Hardware Concepts
1.6
Different basic organizations and memories in
distributed computer systems
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Multiprocessors
1.7

• A bus-based multiprocessor.

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Multiprocessors
1.8

• A crossbar switch (nn crosspoint switches
  needed)
• An omega switching network (low latency problem)
• How to avoid latency?
• use hierarchical schemes NUMA (NonUniform
  Memory Access)

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Homogeneous Multicomputer Systems(building is
easy-probleminterconnection network)
Homogeneous Multi-computers System Area Networks
(SANs)
1-9

• 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)

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Switched Multi-computers

• Can vary significantly
• From
• Massively Parallel Processors ( MPP CM5, IBM
  SP2 etc.)
• To..
• Clusters/Networks of Workstations (cheap -
  COWs or NOWs)

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Software Concepts

• An overview between
• DOS (Distributed Operating Systems)
• NOS (Network Operating Systems)
• Middleware

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Uniprocessor Operating Systems
1.11

• Separating applications from operating system
  code through a microkernel.

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Multiprocessor Operating Systems
monitor Counter private int count
0 public int value() return count void
incr () count count 1 void decr()
count count 1

• A monitor to protect an integer against
  concurrent access.

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Multiprocessor Operating Systems (2)
monitor Counter private int count 0 int
blocked_procs 0 condition unblocked public
int value () return count void incr ()
if (blocked_procs 0) count
count 1 else signal
(unblocked)
void decr() if (count 0) blocked_procs
blocked_procs 1 wait (unblocked)
blocked_procs blocked_procs 1 else
count count 1

• A monitor to protect an integer against
  concurrent access, but blocking a process.

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Distributed Shared Memory Systems (DSM)

• 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

Trick replicate pages that are referenced
frequently! Another replicate all pages (??)
problems? Solutions?
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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

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Network Operating System
1-19

• General structure of a network operating system.

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Network Operating System
1-20

• Two clients and a server in a network operating
  system.
• Services allowed to users
• rlogin machineA
• rcp machineAfile1 machineBfile2
• ftp, http, sftp, ssh, etc.

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Network Operating System

• Issue of FS mount-ing

1.21

• Different clients may mount the servers in
  different places.

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Positioning Middleware
1-22

• General structure of a distributed system as
  middleware.

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Middleware and Openness
1.23

• 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.

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Comparison between Systems

• A comparison between multiprocessor operating
  systems, multicomputer operating systems, network
  operating systems, and middleware based
  distributed systems.

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Clients and Servers
1.25

• General interaction between a client and a server.

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An Example Client and Server

• The header.h file used by the client and server.

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An Example The Server

• A sample server.

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An Example The Client
1-27 b

• A client using the server to copy a file.

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Three Processing Levels
1-28

• The general organization of an Internet Search
  Engine into three different layers

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Multitiered Architectures
1-29

• Alternative client-server organizations (a) (e).

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Multitiered Architectures
1-30

• An example of a server acting as a client.

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Modern Architectures
1-31

• An example of horizontal distribution of a Web
  service.

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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