Architectural Models

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Chapter 2

• Architectural Models

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Keywords

• Middleware
• Interface vs. implementation
• Client-server models
• OOP

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Process Classification in DS

• Classified by responsibilities
• Server processes
• E.g. SQL server, Web server
• Client processes
• Web browser, SQL query analyzer
• Peer processes
• chat

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Software layers
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Platform

• Lowest-level hardware and software
• E.g. Intel X86, Sun SPARC
• E.g. Windows, SunOS
• Q which one is better multiple standards or
  single standard?

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Middleware

• Mask heterogeneity/implementation
• Transparency
• Provide convenient programming model
• Commonly implemented over internet protocols
  TCP/IP
• Examples JVM, CORBA, DCOM, RMI

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Middleware Services

• Communication facilities
• Naming
• Distributed transactions
• Persistent storage
• Stores persistent objects (in marshaled form),
  which live between activations by servers
• Persistent Java, CORBA persistent object
  service.
• Security

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Middleware And Openness

• Protocols used by each middleware layer should be
  same Similarly for interfaces they offer to
  applications.

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Middleware Limitations

• Platform independence? Middleware dependence
• Some communication-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 system
  Saltzer, Reed and Clarke, 1984

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Examples

• Transferring large files
• TCP not enough
• Irrigating farm-land
• Rule requiring help from higher-level
• Q Can mankind comprehend itself?

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Client-Server Model
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Relationship

• Client/server is a process.
• M-to-M
• Servers may in turn be clients of other servers.
• E.g. web server, search engines

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Improvements/Variations

• Multiple servers
• Partition jobs
• Maintain replicated copies
• Caches
• Peer processes
• Reduce the delay of access
• Eliminate the bottleneck
• Group communication
• Mobile code, like web applets

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Multiple-threads Server

• Warm-up threads vs. processes
• Usually a server (which is a process) has
  multiple threads for concurrency reason.
• E.g. web server
• Same principle applies to client.
• E.g. web browser

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OO Implementation

• Object-oriented programming
• class and instance
• encapsulation (information hiding), inheritance
  (with overriding) and polymorphism
• OO approach
• Everything is an object
• Not true in C
• Modular
• Interaction by messages

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OO benefits

• Encapsulation
• Protection by controlled access (values and
  synchronization)
• Information hiding (internal operations can be
  modified as long as interface is kept same)
• Inheritance
• Overriding (can also add new methods and
  variables)
• Efficient software development
• Polymorphism

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Method-overloading

• Different methods in the same class can have same
  name, provided number and/or types of arguments
  are different for each method. Why do we need it?
• class Example
• public static int doubleIt (int x) return 2x
• public static String doubleIt (String x)
• return x x

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Polymorphism

• Same method name means different things,
  depending on objects position in hierarchy. In
  Java, objects implementing same interface or
  extending same superclass can override method to
  do different things.
• Separation of interface and implementation

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Example Device Driver

• Define interface with generic methods, e.g., read
  and write. Individual device drivers override
  those abstract methods.
• Same message sent to variety of objects would
  then take on different forms.

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OO benefits --- Really?

• Difficult to achieve all the goals
• Careful design, future prediction, code reuse
• Design patterns, refactoring techniques
• Managers tend to overlook them.
• Does it improve coding efficiency?
• Said by a survey not much compared to
  memory-management-hiding techniques

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Fundamental Models of DS

• Interaction model
• Failure model
• Security model

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Interaction Model

• Multiple processes
• Rate and timing of message passing cannot in
  general be predicted.
• Two significant factors
• Communication channel
• Latency, bandwidth, jitter
• Computer clocks and timing events
• Clock drift

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Two Variants

• Synchronous There is upper-bound T (sec) on time
  from sending request till receiving reply On
  timeout one can conclude there is failure.
• Asynchronous No finite upper-bound, but
  guaranteed to be finite under no failure (e.g.,
  Internet) cannot tell if network is being slow
  or has failed.
• Comparisons.

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Event Ordering

• User X sends a message with the subject Meeting
• Users Y and Z reply with the subject Re Meeting

Seen by user A
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Failure Model

• Omission failures
• Fail to perform actions
• Arbitrary failures
• Timing failures
• How to mask failures?
• Re-transmit message
• Checksum Hamming code

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Security Model

• Protect objects
• Access rights
• Protect processes and their interactions
• Threats to servers, clients

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Defeating Threats

• Cryptograph and shared secrets
• Using large numbers as secret keys
• Authentication
• User names passwords
• Secure channels