Distributed Systems

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Distributed Systems

• Instructor Carlo Ferrari
• a.y. 2007/2008

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Failure handling

• Ability to continue computation in the presence
  of failures.
• detect failures
• mask failures
• repair failures
• tolerate failures
• Failure is a kind of standard situation

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Concurrency

• Processes execute simultaneously and share
  resources.
• process/object synchronisation
• inter-process communication

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Transparency

• Concealment from the user and the application
  programmer of the separation of components in a
  Distributed Systems, so that the system is
  perceived as a whole rather than as a collection
  of independent component

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Transparencies

• Access transparency enables local and remote
  resources to be accessed using identical
  operations.
• Location transparency enables resources to be
  accessed without knowledge of their physical or
  network location (for example, which building or
  IP address).
• Concurrency transparency enables several
  processes to operate concurrently using shared
  resources without interference between them.
• Replication transparency enables multiple
  instances of resources to be used to increase
  reliability and performance without knowledge of
  the replicas by users or application programmers.

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• Failure transparency enables the concealment of
  faults, allowing users and application programs
  to complete their tasks despite the failure of
  hardware or software components.
• Mobility transparency allows the movement of
  resources and clients within a system without
  affecting the operation of users or programs.
• Performance transparency allows the system to be
  reconfigured to improve performance as loads
  vary.
• Scaling transparency allows the system and
  applications to expand in scale without change to
  the system structure or the application
  algorithms.

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Transparency
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Challenges

• Heterogeneity ... Middleware
• Openness ... Key software interfaces
• Security ... encryption and knowledge of identity
• Scalability

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• Failure handling ... Recovery
• Concurrency ... synchronisation and communication
• Transparency ... A single system?

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Models ...

• Shared properties and common design problems can
  be represented in the form of descriptive models
• Each model is intended to provide an abstract,
  simplified but consistent description of a
  relevant aspect of the design
• Remember ... No global time
• message-based communication

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Difficulties and threats

• Different modes of use
• Different infrastructures
• Non syncronized clocks
• Conflicting data updates
• Many modes of failures
• Attack on data integrity
• Denial of service

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two types of models

• Architectural models are concerned with
• the placement of parts and the relationship
  between them
• The mapping onto the underlying network of
  computers
• Fundamentals models
• interaction
• failure
• security

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Architecture

• Concerned with
• the placement of parts and the relationship
  between them
• The mapping onto the underlying network of
  computers
• Processes and objects

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Architectural Styles

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

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layered
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object-based
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event-based
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shared data-space
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Software Layers
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Software layers

• Service layers
• Higher-level access services at lower layers
• Services can be located on different computers

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Important layers

• Platform
• lowest-level hardwaresoftware
• common programming interface, yet
• different implementations of operating system
• facilities for co-ordination communication
• Middleware
• programming support for distributed computing

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...middleware provides...

• support for distributed processes/objects
• suitable for applications programming
• communication via
• remote method invocation (Java RMI), or
• remote procedure call (Sun RPC)
• services infrastructure for application programs
• naming, security, transactions, event
  notification,

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• Support a higher level of abstraction for
• Communication between group of processes
• Notification of events
• Replication of shared data
• Multimedia data transmission in real time

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The layered view...

• though appropriate for simple types of resource
  data sharing
• e.g. databases of names/addresses/exam grades
• too restrictive for more complex functions?
• reliability, security, fault-tolerance, etc,
  need access to applications data
• see end-to-end argument Saltzer, Reed
• Clarke

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• Some communiction-related functions can be
  completely and reliably implemented only with the
  knowledge and help of the application standing at
  the end points of the communication systems
• Saltzer, J.H., Read, D.P. and Clarke, D. (1984).
  End-to-End Arguements in System Deisgn. ACM
  Transactions on Computer Systems Vol. 2, No 4,
  pp. 277-288

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• Checks, error-correction mechanism and security
  are at many levels ...
• Checking the correctness within the communication
  systems could not be enough

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Architectural models

• Define
• software components (processes, objects)
• ways in which components interact
• mapping of components onto the
  underlying network
• Why needed?
• to handle varying environments and usage
• to guarantee performance

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Client server
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Centralized Architectures
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(No Transcript)
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Application Layering
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Multitiered Architectures
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Multitiered Architectures (3)

• Figure 2-6. An example of a server acting as
  client.

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Distributed Systems lecture 4 - 9/10/07

• note al corso
• Architectures

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...obiettivi formativi

• Presentare in forma sistematica le problematiche
  fondamentali riguardanti il progetto di sistemi
  distribuiti con particolare riferimento alle
  questioni relative
• alla gestione della loro eterogeneità,
• alla scalabilità,
• alla condivisione di risorse,
• alla sicurezza,
• alla tolleranza ai guasti,
• al controllo della concorrenza

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argomenti

• Modelli e architetture
• Oggetti distribuiti e invocazione remota
• Affidabilità e fault tolerance
• Algoritmi distribuiti
• Transazioni distribuite
• e inoltre ...
• Modelli e strumenti per la programmazione

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ambiti applicativi ...

• Distributed Supercomputing
• On-demand Computing
• Data-Intensive Computing
• Collaborative Computing
• Multimedia

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Sistemi Distribuiti

• A.S. Tanenbaum, M. Van Steen, Distributed
  Systems Principles and Paradigm, Prentice-Hall,
  II edition, 2007
• ? Anche in ? italiano ?
• La prima edizione ... Abbastanza datata

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Sistemi Distribuiti

• George Coulouris, Jean Dollimore, Tim Kindberg,
• Distributed Systems concepts and design,
  fourth edition, Addison-Wesley, 2005
• M.L. Liu, Distributed Computing, principles and
  applications, Pearson 2004
• J. Graba, An introduction to Network Programming
  with Java, Addison-Wesley, 2003

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Sistemi Distribuiti

• 7 CFU, 54 ore complessive in 9 settimane
• Lezioni
• Martedì, 8.40-10.15 Aula Le
• Mercoledì, 8.40-10.15, Aula Be
• Venerdì, 16.15-17.50, Aula Be
• Ricevimento studenti
• Mercoledì, 10.30-12.30, DEI-G stanza 313
• per appuntamento
• carlo_at_dei.unipd.it, 049-827.7729, a lezione

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Sistemi Distribuiti

• Prerequisiti
• ................................. TUTTO
  ..................................
• Reti di calcolatori,
• Basi di Dati,
• Sistemi Operativi

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• Probability and Computing
• Intelligenza Artificiale
• Informatica Musicale
• Sistemi in Tempo Reale
• Robotica
• Sistemi Wireless

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Sistemi Distribuiti Una piccola formalità

• Iscrizione OBBLIGATORIA al corso da effettuarsi
  tramite bacheche elettroniche, ovvero
  sis.dei.unipd.it) seguire il link Informazioni
  delle strutture-Dipartimento di Ingegneria
  dellInformazione-Liste di iscrizione ad appelli
  o corsi-Iscrizione a un appello o corso e poi
  selezionare dalla lista Sistemi Distribuiti
• Iscrizioni aperte fino al 12 ottobre

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Sistemi Distribuiti modalità desame(aspetti
generali)

• 5 appelli nel corso della.a.
• 2 appelli, al termine del primo trimestre
  (trimestre di erogazione del corso-sessione
  autunnale)
• 2 appelli nella sessione di recupero
  (agosto-settembre)
• 1 appello nella sessione primaverile, al termine
  del secondo trimestre (n.b. nessun appello
  durante la sessione estiva)

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Sistemi Distribuiti modalità desame standard

• Lesame si conclude con una prova orale
• Ogni studente può sempre scegliere la modalità
  standard
• nessun vincolo alla partecipazione ai 5 appelli

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Sistemi Distribuiti modalità desame

• Elaborato/progetto
• Presentazioni/approfondimenti
• (dipende dal numero e dalla tipologia di
  studenti iscritti ...) ma comunque ci saranno dei
  cambiamenti rispetto allo scorso anno.
• Prova scritta, eventualmente in itinere
• aspetti particolari (ancora in via di
  definizione)
• Prove svincolate dalle date degli appelli

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Sistemi Distribuiti

• Avvisi

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Sistemi Distribuiti

• Sito del corso
• http//www.dei.unipd.it/carlo/DS0708

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Sistemi Distribuiti

• Siti collegati
• www.cdk4.net oppure
• www.pearsoned.co.uk/coulouris

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Sistemi Distribuiti

• Attività di laboratorio
• ... Non prevista .... ?

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Sistemi Distribuiti

• Tesi di laurea specialistica in ingegneria
  informatica
• 20 crediti .... 29 crediti .... 35 crediti
• 9 crediti .... 6 crediti liberi ... 0 crediti
• 6 crediti liberi

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Sistemi Distributi

• Attenzione alle tempistiche ...
• 20 crediti ... 4 mesi/uomo
• 29 crediti ... 6 mesi/uomo
• 35 crediti ... 7 mesi/uomo

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Sistemi Distributi

• Laboratorio di Sistemi Distribuiti
• Laboratorio di Bioinformatica Strutturale
• ... ma anche
• Laboratori di Robotica, Telecomunicazioni,
  Sistemi in tempo reale, Musica al calcolatore,
  Basi di Dati, Sistemi Informatici, Intelligenza
  Artificiale....

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• Bioinformatica
• Protein Folding
• Protein Docking
• Sistemi Distribuiti
• Grid Computing
• Agents
• Biometrics

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(No Transcript)
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Multiple servers
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Proxy servers
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Peer processes
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A distributed application based on peer processes
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Client server and mobility

• Mobile code
• downloaded from server, runs on locally
• e.g. web applets
• Mobile agent (code data stato)
• travels from computer to another
• collects information, returning to origin.

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Web applets
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• Network computers
• Thin clients ... See X-11
• Mobile devices forming spontaneous networks

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Spontaneous networks ...

• Easy connection to a LAN
• Easy integration with local services
• Limited connectivity
• Security and privacy
• Discovery serviceregistrationlookup

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• Vertical distribution
• Horizontal distribution
• Peer-to-peer systems

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Distributed Systems

• End of lecture 4