?1? ???????

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?1? ??????? 1.1 ????????1.2 ???????1.3
?????????
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1.1 ????????

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

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Figure 1-1. A distributed system organized as
middleware. The middleware layer extends over
multiple machines, and offers each application
the same interface.
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??????????

• Making resources accessible
• Distribution transparency
• Openness
• Scalability

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Making Resources Accessible

• Making it easy for users and applications to
  access remote resources
• Share remote resources in a controlled and
  efficient manner
• Benefits
• Better economics by sharing expensive resources
• Easier to collaborate and exchange information
• Create virtual organizations where geographically
  dispersed people can work together using
  groupware
• Enables electronic commerce
• Problems
• Eavesdropping or intrusion on communication
• Tracking of communication to build a profile

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Transparency in a Distributed System
Figure 1-2. Different forms of transparency in a
distributed system (ISO, 1995).
Transparency is NOT always good! Cost in
performance!
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Openness

• In form of services
• Services are described via interfaces
• Interface Definition Language (IDL)
• Interoperability, Portability, Extensibility

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Scalability

• Scalability can be measured against three
  dimensions
• Size be able to easily add more users and
  resources to a system
• Geography be able to handle users and resources
  that are far apart
• Administrative be able to manage even if it
  spans independent administrative organizations

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Centralized versus distributed

• Centralized approach
• Distributed approach
• No global clock
• No complete/global information
• Decisions based only on local information
• Failure locality

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Scaling Techniques

• Hiding communication latency
• Code at client e.g. Javascript to check a form
• Distributing components
• Divide a component into smaller parts
• E.g. dividing the DNS name space into zones
• Replicating components
• Caching is a special form of replication

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1.2 ???????

• Distributed Computing Systems
• Cluster Computing Systems
• Grid Computing Systems
• Cloud Computing Systems
• Distributed Information Systems
• Transaction Processing Systems
• Enterprise Application Integration
• Distributed Pervasive Systems
• Home Systems
• Electronic Health Care Systems
• Sensor Networks

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Cluster Computing Systems

• Figure 1-6. An example of a cluster computing
  system.

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Grid Computing Systems

• Figure 1-7. A layered architecture for grid
  computing systems.

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Cloud Computing Systems
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Transaction Processing Systems

• Figure 1-9. A nested transaction.

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Transaction Processing Systems

• Figure 1-10. The role of a TP monitor in
  distributed systems.

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Enterprise Application Integration

• Figure 1-11. Middleware as a communication
  facilitator in enterprise application
  integration. RPC, RMI and MOM are examples.

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Distributed Pervasive Systems

• Embrace contextual changes.
• Encourage ad hoc composition.
• Recognize sharing as the default.

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Electronic Health Care Systems

• Figure 1-12. Monitoring a person in a pervasive
  electronic health care system, using
• (a) a local hub or (b) a continuous wireless
  connection.

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Wireless Sensor Networks

• Centralized storage and process.
• Distributed storage and process.

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1.3 ?????????

• Software architectures
• how various software components are organized and
  how they interact
• System architecture
• An instance of a software architecture after
  deciding on the software components, their
  interaction and their placement.

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Architectural Styles
Object-based architectures
Layered architectures
Event-based architectures
Data-shared architectures
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System Architecture

• Centralized architectures
• Application layering
• Multitiered architectures
• Decentralized architectures
• Structured peer-to-peer
• Unstructured peer-to-peer
• Hybrid architectures
• Edge-server systems
• Collaborative distributed systems

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Centralized Architectures
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Application Layering

• Figure 2-4. The simplified organization of an
  Internet search engine

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Application Layering

• Figure 2-5. Alternative client-server
  organizations (a)(e).

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Multitiered Architectures

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

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Decentralized Architecture

• Structured peer-to-peer
• Distributed Hash Table
• Content-Addressable Network
• Unstructured peer-to-peer

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Structured Peer-to-Peer Architectures
Data mapping in Chord and CAN.
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Unstructured Peer-to-Peer Architectures

• The overlay network resembles a random graph.
  Each node maintains a list of c neighbors, where,
  ideally, each of theses neighbors represents a
  randomly chosen live node from the current set of
  nodes. The list of neighbors is referred to as a
  partial view.
• How to maintain a partial view?
• Nodes are in push or pull mode. Using only one
  mode leads to isolated sub-networks so most nodes
  will do both (exchange mode)
• To add to the group, simply contact any node. To
  leave, simply leave.

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Topology Management of Overlay Networks
A two-layered approach for constructing and
maintaining specific overlay topologies
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Network with Superpeers
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Hybrid Architectures edge-server systems
Figure 2-13. Viewing the Internet as consisting
of a collection of edge servers.
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Hybrid Architectures collaborative sys.

• Figure 2-14. The principal working of BitTorrent

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Summary

• ???????
• ????????
• Making resources accessible
• Distribution transparency
• Openness
• Scalability
• ????????
• Distributed computing system
• Distributed information system
• Distributed pervasive system
• ?????????
• Centralized
• Decentralized
• Hybrid

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Homework Questions

• 1.
• 2.
• 3.
• 4.