Distributed System: The Overall Architecture

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Distributed SystemThe Overall Architecture
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Previously Internet---Topology and Reliability

• Internet is a scale-free network
• A small number of nodes have a large number of
  links while the majority of nodes only have a
  small number of links
• Internet is robust to random failures, but
  vulnerable to targeted attacks

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Previously IT Architecture VS. IT Infrastructure

• An IT architecture is a blueprint showing how the
  parts will interact and interrelate.
• System, information, departments...
• Multiplicity of structures and views
• An IT infrastructure is the implementation of an
  architecture.
• processors, software, databases, electronic
  links, data centers, standards, skills,
  electronic processes...
• We now tend to divide computing into applications
  and infrastructures

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Distributed Computing
The Importance of the IT Infrastructure
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Host-Based Hierarchy

• Master-slave relationship
• A hierarchy in which the host is in control
• Workload is shared among different levels of
  processors
• Terminals are only access device
• Data storage

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Decentralized Stand-Alone Systems (1)
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Decentralized Stand-Alone Systems (2)

• They are decentralized, not distributed
• Islands of computation problems for stand-alone
  systems
• Ad hoc data flow to the corporate host
• ERP

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Peer-to-Peer LAN-Based Systems (1)
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Peer-to-Peer LAN-Based Systems (2)

• No hierarchy
• Peer-to-peer communications
• Interconnecting LANs rather than hierarchical
  communications through a central hub
• No superior computer

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Hybrid Enterprise-wide Systems (1)
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Hybrid Enterprise-wide Systems (2)

• The structure of choice for many years
• Allows company to link "information islands" and
  automate business processes
• Systems integration

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Client-Server Systems (1)

• Arose to take advantage of the processing
  capabilities of both host machines and PCs in the
  same system
• The computational flow of pure client-server
  systems is asymmetric.
• Constraints on the use of the client-server style
• Limit the number of clients that can be connected
  to a server
• Impose a restriction that servers cannot interact
  with other servers.
• N-tiered client-server model
• DB, application, presentation

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Client-Server Systems (2) ---Distribution of
Processing
Distributed Presentation
Remote Presentation
Distributed Function
Remote Data Management
Distributed Database
Data Management
Data Management
Data Management
Data Management
Data Management
Application Function
Application Function
Application Function
Data Management
Server
Presentation
Network
Application Function
Application Function
Application Function
Client
Presentation
Presentation
Presentation
Presentation
Presentation
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Client-Server Systems (3)
Remote Presentation

• Most of the Web-based content systems today
• DB sitting at the back
• Web server(s) provide contents
• Browsers display contents
• Traditional GIS
• Spatial data in DB
• GIS server processing data and rendering image
• Clients display maps

Data Management
Application Function
Server
Network
Client
Presentation
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Client-Server Systems (4)
Remote Data Management

• Traditional C/S DB applications (fat client)
• DBMS stores and retrieves data
• Client applications process and display data
• Client maintenance may be challenging

Data Management
Server
Network
Client
Application Function
Presentation
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Client-Server Systems (5)
Distributed Function

• Some new Web-GIS systems
• Image rendering moved to the client
• Reduce network flow
• Load balancing
• Most of the online-gaming systems

Data Management
Application Function
Server
Network
Client
Application Function
Presentation
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Client-Server Systems (6)
Distributed Database

• Mobile computing
• Less dynamic data stored locally (e.g. product
  category)
• Data sysc. when connected
• Some applications in retail chains
• Local data copy to improve service availability
• E.g. Microsoft access and SQL Server

Data Management
Server
Network
Client
Data Management
Application Function
Presentation
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Client-Server Systems (7) ---Three-tier
Client-Server Style
Server (usually DB server) connected to the
network via one or more servers, and sometimes
directly as well
Multiple specialized servers, some possibly
dedicated to middleware (application servers)
Internet or LANs
Clients, some of which may be portable
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Client-Server Systems (8) ---Benefits and
Drawbacks

• Better access to information
• Shift the focus of computing to user and empower
  employees
• Increases organizational flexibility
• Allows new technology to be added more easily
  without affecting rest of system
• Streamlines work flow between functional areas
• Encourages people to work together via networks
• Supports new organizational structures via its
  connectivity
• Drawbacks
• Not lower in cost
• Easier for users, far more complex for IS
• What looked like simple connections have turned
  into large, often fragile, complex systems

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Internet-Based Computing (1)

• In the late 1990s, the client-server trend was
  augmented by the Internet
• The tenets of client-server remain
• With Internet sitting at the heart
• Ubiquitous computing
• Various computing platforms servers, PCs,
  handhelds
• A new computing environment
• Irregular request arrival patterns
• Unpredictable user numbers
• Thin client and fat client
• Network computers Java

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Internet-Based Computing (2) ---Server-based
Computing

• Some services are by nature centralized
• Web search
• Corporate info query
• Handheld device
• Services reside on corporate servers
• Applications can be securely accessed by any
  device, they can be updated directly on the
  server, and they do not have to be tailored to
  run on specific machines

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P2P Computing---Definition (1)

• Peer-to-peer systems are distributed systems
  consisting of interconnected nodes able to
  self-organize into network topologies with the
  purpose of sharing resources such as content, CPU
  cycles, storage and bandwidth, capable of
  adapting to failures and accommodating transient
  populations of nodes while maintaining acceptable
  connectivity and performance, without requiring
  the intermediation or support of a globally
  centralized server or authority.

Androutsellis-Theotokis S,Spinellis D. A Survey
of Peer-to-Peer Content Distribution
Technologies, J.ACM Computing
Surveys,2004,36(4)335-371.
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Peer-to-Peer Computing (2)

• Components directly interact as peers by
  exchanging services.
• Request/reply interaction
• May involve complex bidirectional protocols of
  interactions
• Two-way communication existing between two or
  more p2p components
• P2P VS. C/S
• Load distribution
• The topology of the system changes at runtime as
  users enter the system or change their connections

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Peer-to-Peer Computing (3) --- P2P Applications

• Communication and Collaboration
• Chat/Irc, Instant Messaging (MSN, Yahoo!
  Messenger), Chat/IRC
• Distributed Computation
• Seti_at_home Sullivan III et al. 1997SetiAtHome
  2003
• genome_at_home Larson et al. 2003 GenomeAtHome
  2003
• Internet Service Support
• Peer-to-peer multicast systems VanRenesse et al.
  2003, Internet indirection infrastructures
  Stoica et al. 2002
• Database Systems
• LRM Bernstein et al. 2002
• Content Distribution
• Sharing of digital media and other data between
  users
• Napster, Gnutella, BitTorrent, eMule...
• P2P Video PPLive, TVAnts

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P2P Computing--- Content Distribution (4)

• Creating a distributed storage medium that allows
  for the publishing, searching, and retrieval of
  files by members of its network.
• May also include nonfunctional features like
• Provisions for security, anonymity, fairness
• Increased scalability and performance
• Resource management and organization capabilities

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Web Services (1)

• Software modules invoked via URLs
• permit widespread computer-to-computer use of the
  Internet. One computer program or Web Service
  makes a request of another Web Service to perform
  its task (or set of tasks) and pass back the
  answer
• Hot topic about Web service
• Next generation of distributed systems
• Allows the building of large, complex systems by
  linking and integrating various modules
• Makes the Internet the hub of computing

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Web Services (2)

• Permits flexible systems not possible before
• Dynamic binding of Web services at execution time
• Releases companies from building and maintaining
  systems in house
• Will draw on existing systems
• Wrapping encapsulate functionality from an
  existing application in an XML envelope
• Exposing for use by others

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Web Services (3)

• WSDL
• Service description, metadata describing the
  service
• e.g. interface, location etc.
• SOAP
• A standard for XML-based information exchange
  between distributed applications
• UDDI
• A specification for distributed registries of web
  services

Find
Publish
Bind
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Web Services (4)
Web Service Consumer
UDDI
Find a Service http//www.uddi.org Link to WSDL
document
Web Service
How do we talk? (WSDL) http//yourserviice.com//?W
SDL XML with service descriptions
Let me talk to you (SOAP) http//yourservice.com/s
vc1 XML/SOAP BODY
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Web Services (5)

• Foundations for Web Services

Service directory UDDI
Service description WSDL
Service interaction SOAP
Format description XML Schema
Data format XML
Communication Protocol HTTP
Communication Network Internet
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Web Services (6)

• Significance of Web Services
• Web Services offers an IT architecture based on
  the openness of the Internet.
• This modularity permits handling a huge variety
  of possibilities by mixing and matching, and
  allows easier cross-company system linking
• Companies thus only pay for the functionality
  they use when they use it, which reduces the
  number of IT assets companies need to house and
  maintain

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Service-Oriented Architecture (1)

• Move component interactions from hard-coded to
  dynamically discovered and invoked
• By exposing the data and functions in a way that
  other systems can easily use
• Service-Oriented Architecture (SOA)
• An architecture is a formal description of a
  system, defining its components, their
  interrelationships, and the principles and
  guidelines governing their design and evolution
  over time
• A service is a software component that can be
  accessed via a network to provide functionality
  to a service requester
• SOA is a style of building reliable distributed
  systems that deliver functionality as services,
  with the additional emphasis on loose coupling
  between interacting services

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Service-Oriented Architecture (2)

• SOA refers to the design of a system, not to its
  implementation.
• Service interaction in a service-oriented
  environment

Registry
Advertise
?
?
Discover
Service
?
Client
Interact
?
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Service-Oriented Architecture (3)

• SOA features
• Loosely-coupled
• Coarse-grained
• Standards-based
• SOA benefits
• Greater interoperability
• Integration of legacy systems
• Increased reuse
• More agile business processes
• Reduced maintenance costs

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Service-Oriented Architecture (4)

• Loose-coupling
• The interacting software components minimize
  their in-built knowledge of each other
• For example, having learned about a services
  existence, a client can discover its
  capabilities, its policies, its location, its
  interfaces and its supported protocols
• The benefits of loose coupling include
• Flexibility
• Scalability
• Replaceability
• Fault tolerance

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Service-Oriented Architecture (5)

• Achieving loose coupling between services
• Web Service protocols
• Publish-and-subscribe
• Components interact via announced events
• Messaging-oriented middleware (MOM)
• Asynchronous message-passing with ability to
  store, route or transform messages in the process
  of delivery

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Grid Computing (1)

• A computational grid is a hardware and software
  infrastructure that provides dependable,
  consistent, pervasive, and inexpensive access to
  high-end computational capabilities
• Grid is a generalized network computing system
  that is supposed to scale to Internet levels and
  handle data and computation seamlessly

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Grid Computing (2)
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Grid Computing (3)

• Resource management in Grid systems involves
  managing the basic elements
• Grid elements
• Processing elements
• Compute servers (double each 18 months)
• Handhelds
• File servers
• Networks (double each 9 months)
• Playstations, cell phones etc
• Storage elements
• Network elements

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Grid Computing (4)

• Grid systems can be classified depending on their
  usage

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Grid Computing (5)

• Computational Grid
• Denotes a system that has a higher aggregate
  capacity than any of its constituent machine
• Data Grid
• Systems that provide an infrastructure for
  synthesizing new information from data
  repositories such as digital libraries or data
  warehouses
• Applications for these systems would be special
  purpose data mining that correlates information
  from multiple different high volume data sources
• Service Grid
• Systems that provide services that are not
  provided by any single machine

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Grid Computing (6)

• Grid Computing Benefits
• Exploit Underutilized resources
• CPU Scavenging, Hotspot leveling
• Resource Balancing
• Virtualize resources across an enterprise
• Data Grids, Compute Grids
• Enable collaboration for virtual organizations

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Distributed Computing
The Importance of the IT Infrastructure
2
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IT Infrastructure---An Overview

• The shared and reliable services that provide the
  foundation for the enterprise IT portfolio
• Enabling business applications sitting above
• Difficult to cost-justify
• Technologists and business people have different
  views of infrastructure
• Peter Weill et al presented a model that meshes
  different views together
• Gives technologists and business users a common
  language

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The Structure of IT Infrastructures (1)
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The Structure of IT Infrastructures (2)

• Four layers of IT infrastructure
• IT components
• Technology components, such as computers, DBMS
  packages, OS etc.
• Generally not understood by business people
• Human IT infrastructure
• Translation of the IT components layer into
  services
• Experts' knowledge, skills, experience and
  standards
• Shared IT services
• The infrastructure as a set of services that
  users can draw upon and share to conduct
  business.
• Shared and standard applications
• Stable applications such as accounting and HR

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Public Infrastructure

• Weills model in turn sits on top of the public
  infrastructure
• E.g. the Internet, vendors, telecomm companies.
• Similarity between IT infrastructure and public
  infrastructure such as roads, hospital, schools
  etc.
• Everyone wants but no-one wants to pay
• Provided by a "central authority"
• Government or IT Department
• Delicate and "difficult" investment balance

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Three Views of Infrastructure

• Economies of scale (Utility)
• Infrastructure cost as an administrative expense
• Minimize expense
• Outsourcing may be viewed favorably because the
  IT infrastructure is not seen as strategic
• Support for business programs (Dependent)
• Infrastructure treated as business expense
• Measured by short-term business benefits
• Infrastructure planning in current business plan
• Flexibility to meet changes in the marketplace
  (Enabling)
• Primary benefit long-term flexibility
• Intended to provide the foundation for changing
  direction in the future
• IT cost seen as business investment