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Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Distributed Systems Corso di Laurea Specialistica in Ingegneria Informatica AA.

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Presentation on theme: "Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Distributed Systems Corso di Laurea Specialistica in Ingegneria Informatica AA."— Presentation transcript:

1 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Distributed Systems Corso di Laurea Specialistica in Ingegneria Informatica AA 2006/2007 Introduction Prof. Roberto Baldoni Ing. Alessia Milani Ing. Leonardo Querzoni Ing. Silvia Bonomi

2 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica A definition A distributed system is a set of spatially separate entities, each of these with a certain computational power that are able to communicate and to coordinate among themselves for reaching a common goal

3 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Primary Goal: sharing data/resources Problems Synchronization Coordination

4 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Coordination has to be implemented taking into account the following conditions that deviates from centralized systems: 1.Temporal and spatial concurrency 2.No global Clock 3.Failures 4.Unpredictable latencies These limitations restrict, for example, the set of coordination problems we can be solve in a distributed setting

5 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Distributed Systems Examples internet intranet sistema mobile But also..... Service Oriented Architectures Overlay Networks Grid P2P Pervasive Systems&Ubiquitous Computing

6 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Pervasive Systems Internet everywhere One-persons Many computers Mobility ……….. How can I keep consistent my mailbox???

7 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Characteristics..and Challenges Heterogeneity Openess Security Scalability Fault Tolerance Concurrency Transparency

8 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Heterogeneity Networks Hardware Operating Systems Programming Languages Implementations from different Developers Soluzioni Middleware (from RPC to Service oriented Architectures) Mobile code and Virtual Machine

9 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Openess Capability of a system to be extended and re-implemented Necessary condition, set of documents with software interfaces Interface Definition Language (it describes the syntax and the semantic of a service/component, available functions/services, input parameters, exceptions, etc) Problem: semantic description of a service. A specification of a service/componet is well-formed if it is : –Complete. A specification is complete if everything related to the implementation has been specified. If something has not been specified, the designer needs to add implementation dependent details. –Neutral. A specification is neutral is it does not offer any detail on a possible implementation

10 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Openess (ii) Interoperability. The capability of two systems to cooperate by using services/components specified by a common standard Portability. The capability if a service/component implemented on a distributed system A to work on a system B without doing any modification Flexibility. The capacity of a system to configure/ orchestrate components developed by various programmers Add-on Features. The capacity of a distributed system of adding components/services and be integrating in a running system

11 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Openess (iii) Other recent capabilities: –Evolvability. The capacity of a system to evolve in time for example leaving active two different version of the same service. –Self-* (self organization, self management, self healing etc). The capacity of a system to reconfigure, to manage itself without uman intervention The number of independent software developers make very complex the development of a distributed platform Examples: –RFC for internet –JBoss for J2EE platform

12 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Security Confidenziality (protection against the interception of data from unauthorized users) Integrity (protection against data alteration) Availability (protection against the interference in the access to a resource)

13 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Scalability A system is scalable if it remains running with adequate performance even if the number of users grow up of orders of magnitude Centralization is against scalability : –Service (single service for all users) –Data (single data structure for all processes) Date Computers Web servers 1979, Dec , July130, , July56,218,0005,560,866 Computers connected to the internet

14 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Scalability (ii) It becomes necessary using: –Service Replication Coordination Problems –Data Replication Consistency Problems –Distributed Algorithms No node has the current state of the whole system Nodes base their decisions on data they own A failure of a node should not compromize the goal of the algorithm Geographical Scalability

15 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Internet-scale Applications Enterprise Data Centers Scalable Consistency- based Applications First Open Workshop Budapest What is a large-scale distributed system? Cooperative Information Systems Scalable QoS based Applications eGov envelo p

16 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Failure Management Failure detection –Example: Checksum detects a corrupted packet Failure masking –Example: message retransmission Tolerating Failures –Example: intrusion tolerant systems Failure Recovery –Example: completing long running computation Redoundancy –Example: DNS

17 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Concurrency Multiple access to shared resources –If clients invokes concurrently read and write methods on a shared variable –Which value returns each read? Coordination Synchronization

18 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Transparency Access: allow to access remote and local resources with the same operations Location: allows to access resources without knowing their physical location Concurrency: allows a set of processes to run concurrently on shared resources without interfering among themselves Failures: allow to mask failures in order that users can complete remaining requested operations Mobility: allows to move resources and users without influencing operation issued by users Performance: allow system reconfiguration changing the load Performance of a solution based on a distributed system not always improve with respect to a solution based on a centralized system.

19 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Layering hw and sw

20 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Interaction Models client/server peer-to-peer

21 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Interaction Models Interaction models impact on scalability, availability, cost, security performances Eg. client/server with replicated services: availability, scalability performance: replication imposes an extra work for maintaining consistency despite replica crashes

22 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Web proxy server

23 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Web applets

24 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Thin clients and compute servers Thin Client Application Process Network computer or PC Compute server network

25 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Spontaneous networking in a hotel Internet gateway PDA service Music service Discovery Alarm Camera Guests devices Laptop TV/PC Hotel wireless network

26 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Real-time ordering of events

27 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Processes and channels

28 Università di Roma La Sapienza Dipartimento di Informatica e Sistemistica Middleware : problems to face Heterogeneity: OS, clock speeds, data representation, memory, architecture HW Local Asynchrony: load on a noad, diffeent OW, Interrupts Lack of global knowledge: knowledge propagates through messages whose messages whose propagation times will be much slower than time taken by the execution of an internal event Network Asyncrony: propagation times of message can be unpredictable. Failures of nodes or network partitions. Lack of a global order of events This limits the set of problems that can be solved through deterministic algorithms on some distributed systems


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