Objektorienteret Middleware

1
Objektorienteret Middleware

• Presentation 2
• Distributed Systems A brush up, and relations
  to Middleware, Heterogeneity Transparency

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Motivation

• We need to establish some common understanding of
  concepts
• We will use these concepts to measure and compare
  the different middleware technologies

3
Agenda

• What is a Distributed System?
• We need to have the same definition in mind
• Distributed Systems Characteristics
• Examples of Distributed Systems
• We will extract requirements from these
• Distributed System Common Requirements Concepts
• We shall use these requirements to point out
  relevant concepts
• Transparency in Distributed System

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What is a Distributed System

• Plenum What is your definition?

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What is a Distributed System?

• You know you have one when the crash of a
  computer youve never heard of stops you from
  getting any work done. - Leslie Lamport, 1987

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What is a Distributed System?

• A distributed system is a collection of
  autonomous hosts (computers) that are connected
  through a computer network. Each host executes
  components and operates a distribution
  middleware, which enables the components to
  coordinate their activities in such a way that
  users perceive the system as a single, integrated
  computing facility.
• Wolfgang Emmerich (2000)

We will use this definition in OOMI
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Common Requirements

• What are we trying to achieve when we construct a
  distributed system?
• Certain requirements are common to many
  distributed systems (Emmerich, 2000)
• Resource Sharing
• Openness Heterogeneity
• Concurrency
• Scalability
• Fault Tolerance
• Transparency

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Resource Sharing

• Ability to use any hardware, software or data
  anywhere in the system
• Resource manager controls access, provides naming
  scheme and controls concurrency
• Resource sharing model (e.g. client/ server or
  object-based) describing how
• resources are provided,
• they are used and
• provider and user interact with each other.

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Openness Heterogeneity

• Openness is concerned with extensions and
  improvements of distributed systems
• Most projects requires systems to be easily
  extendible and maintainable, this means the
  architecture must be open
• OO Paradigm is renowned for its openness as is
  component models you are already experts
• OO middleware thus strives to support openness by
  birth
• Heterogeneity is about supporting several
  hardware and software platforms
• Some middleware strives to support heterogeneity,
  but not in all areas
• Differences in data representation of interface
  types on different processors (of different
  vendors) have to be resolved (e.g. UNIX vs
  Windows)
• Heterogeneity is a key concept to remember

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Openness Heterogeneity

• The vision everything works with everything
• Reality highly improbable scenario

C
Java
C
Ada
Web service
CORBA
Mac OS/X
UNIX
Java
Perl
C
Delphi
.NET Remoting
Java RMI
Windows NT
LINUX
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Concurrency

• Components in distributed systems are executed in
  concurrent processes
• Components access and update shared resources
  (e.g. variables, databases, device drivers)
• Integrity of the system may be violated if
  concurrent updates are not coordinated
• This also holds true in non-distributed systems
  (using threads processes), but here we often
  have more simulations users
• gt greater risk of concurrency problems

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Scalability

• Adaption of distributed systems to
• accomodate more users
• respond faster
• Usually done in centralized systems by adding
  more and/or faster processors
• May be done by allowing several nodes to run the
  same components and share the load
• Load balancing
• Components should not need to be changed when
  scale of a system increases
• Design components to be scalable!

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Fault Tolerance

• Hardware, software and networks fail!
• Distributed systems must maintain availability
  even at low levels of hardware/software/network
  reliability
• Fault tolerance is achieved by
• recovery
• redundancy

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Transparency in Distributed Systems

• Distributed systems should be perceived by users
  and application programmers as a whole rather
  than as a collection of cooperating components
• Transparency has different dimensions that were
  identified by the ANSA project (now part of an
  ISO standard)
• International Standard on Open Distributed
  Processing (OPD) (ISO, 1996).
• These represent various properties that
  distributed systems aim at achieving
• but only partly implement (a level-of-transpare
  ncy)
• We shall use these to characterize the various
  available middleware products

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Distribution Transparency
Concurrency Transparency
Access Transparency
Location Transparency
We may measure the characteristics of middleware
technologies on their support for the different
dimensions of transparency as shown above.
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Access Transparency

• Enables local and remote information objects to
  be accessed using identical operations.
• Example File system operations in NFS.
• Example Navigation in the Web.
• Example SQL Queries
• Example Distributed Objects
• Makes life easier for the developers
• True when making objects distributed
• There are pitfalls

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Location Transparency

• Enables information objects to be accessed
  without knowledge of their location.
• Example File system operations in NFS
• Example Pages in the Web
• Example Tables in distributed databases
• Example Objects being moved between servers
• Integral part of replication/migration/scal.
  transparency

Server 1 (with object)
Server 1 (with object)
Client Program
Client Program
Hard-coded Reference (e.g. IP)
Hard-coded Reference (e.g. IP)
Server 2
Server 2
Name Server
Not easy to move the object to other server
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Group Work

• At each table 10 minutes
• Discuss the remaining ANSA transparencies
• What are the purpose of each
• When and where are they relevant
• How may they be implemented
• Would you do the implementation yourself in a
  project?

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Concurrency Transparency

• Enables several processes to operate concurrently
  using shared information objects without
  interference between them
• We would like it to be handled by the middleware
• Example NFS
• Example Automatic teller machine network
• Example Database management system

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Replication Transparency

• Enables multiple instances of information objects
  to be used to increase reliability and
  performance without knowledge of the replicas by
  users or application programs
• Example Distributed DBMS
• Example Mirroring Web Pages
• Relies on location transparency

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Failure Transparency

• Enables the concealment of faults
• Allows users and applications to complete their
  tasks despite the failure of other components.
• Example Database Management System
• If system automatically recovers from faulty
  server by supplying the object from another
  server, this is failure transparency

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Migration Transparency

• Allows the movement of information objects within
  a system without affecting the operations of
  users or application programs
• Example NFS
• Example Web Pages
• Also relies on location transparency

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Performance Transparency

• Allows the system to be reconfigured to improve
  performance as loads vary.
• May be achieved through by employing activation,
  caching and similar technologies, thus
  automatically boosting performance on critical
  resources

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Scaling Transparency

• Allows the system and applications to expand in
  scale without change to the system structure or
  the application algorithms.
• May rely on replication transparency and thus
  location transparency for achieving this