Computer Science 328 Distributed Systems

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Computer Science 328Distributed Systems

• Lecture 2
• Models of Distributed Systems

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What is a Distributed System Model?

• A DS model addresses
• What are the main entities?
• How do they interact?
• What characteristics affect the individual
  collective behavior?
• The purpose of the DS model is to
• Make explicit all assumptions about the system
• Enable generalizations about the system, based on
  these assumptions.

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Software and Hardware Service Layers
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Software Layers

• Software architecture the structure of software
  as layers or modules in terms of services offered
  and requested between processes located in the
  same or different computers.
• Platform the lowest-level hardware and software
  layers, i.e., Intel x86/Windows, Sun Sparc/SunOS,
  PowerPC/MacOS, Intel x86/Linux.
• Middleware a layer of software whose purpose is
  to mask heterogeneity and to provide a
  programming model to applications, CORBA, Java
  remote object invocation (RMI), Microsoft
  Distributed Component Object Model (DCOM).

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System Architecture Client-Server Model

• Dual roles
• A web server is often a client of a local file
  server.
• Web servers and other Internet servers are
  clients of the DNS server which translates
  Internet
• domain names to network addresses.
• A search engine is a server, but it runs
  programs called web crawlers that access web
  servers
• throughout the Internet for information
  inquiries.

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Service Provided by Multiple Servers

• Servers may
• partition the set of
• objects
• maintain replicated
• copies of the entire
• set of objects on
• several hosts.

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Distributed System Architectures

• Client-Server Architecture

Clients
Clients
Service
Server
invocation
invocation
Results
Results


invocation
invocation
Results
Results
Single-Server
Multiple-Server
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Web Proxy Server
Web proxy servers provide a shared cache for
client machines at a site or across several
sites.
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Peer Process Architecture
All the processes play similar roles, interacting
cooperatively as peers to perform tasks. For
example, a distributed whiteboard application
allows users to view and interactively modify a
picture.
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Mobile Code -- Web Applets
Web applets usually give good interactive
response since the effect of network delays and
variable bandwidth wont come into play.
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Mobile Agents

• A mobile agent is a running program (both code
  and data) that travels from one machine to
  another, carrying out a task (such as collecting
  information and returning the result)
• A mobile agent may utilize local resources, e.g.,
  access individual database entries.
• Both mobile code and mobile agents are potential
  security threat to local resources.
• Machines receiving a mobile agent should
  determine on which local resource the agent is
  allowed to use.

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Thin Clients and Compute Servers

• Thin client a software layer that supports a
  window-based user interface
• on a local computer while executing application
  programs on a remote
• machine.
• Advantage low management and hardware costs at
  the client
• Disadvantage Delays are increased because of
  transfer of image and
• data between the thin client and the
  application process.

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Example of Thin Clients X-11 Window Systems

• X-11 server provides an extensive library of
  procedures for displaying and modifying graphical
  objects.
• X-11 client is the application program a user is
  interacting with.
• The client program communicates with the server
  by invoking procedures using the RPC mechanisms.

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Fundamental Models

• Fundamental Models address important aspects of a
  DS, in abstraction.
• Interaction Model
• Addresses communication and coordination between
  processes
• Failure Model
• Defines and classifies faults and methods of
  recovery or tolerance
• Security Model
• Defines security threats and mechanisms for
  resisting them

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Interaction Model

• Communication Performance
• Latency
• Propagation delay the time it takes for the
  first bit of a message to reach the destination.
• Transmission delay the time interval between
  transmission of the first bit and the last bit of
  the message.
• Processing delay the time it takes for the OS to
  process/send/ receive the message.
• Queueing delay the time it takes a message to be
  queued either at end hosts or intermediate nodes
  waiting for transmission.
• Bandwidth the total amount of information that
  can be transmitted over a given time.
• Jitter the time difference between the delays
  incurred by different messages.

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Interaction Model

• Clocks Timing events
• No global notion of time
• Clock drift rates the relative rate at which a
  computer clock drifts away from a perfect
  reference clock.
• Clock synchronization
• Global positioning system (GPS) a few computers
  may use radio receivers to get time readings from
  GPS with an accuracy of 1 microsecond. They then
  send timing messages to other computers in their
  respective networks.
• Logical clocks messages are time stamped with a
  sequence number that reflects their logical
  ordering.

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Real-time Ordering of Events
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Interaction Model

• Synchronous Distributed System
• Each step in a process takes lb lt time lt ub
• Each message is received within bounded time
• Each local clocks drift has a known bound
• Asynchronous Distributed System
• No bounds on process execution
• No bounds on message transmission delays
• The drift rate of a clock is arbitrary
• Internet is an asynchronous distributed system

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Failure Model

• Omission failures
• Process omission failure
• Crash a process halts and does not execute any
  further operations can be detected by timeout in
  synchronous DS
• In asynchronous systems, a timeout may be a
  consequence of process crash, extremely large
  message delays, or message losses.
• Fail-stop a process crash is called fail-stop
  if other processes can detect certainly that the
  process has crashed.

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Failure Model

• Omission failures
• Communication omission failures
• Send-omission loss of messages between the
  sending process and the outgoing message buffer
• Channel omission loss of message in the
  communication channel.
• Receive-omission loss of messages between the
  incoming message buffer and the receiving process

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Processes and Channels
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Failure Model (contd)

• Arbitrary failures
• Arbitrary process failure arbitrarily omits
  intended processing steps or takes unintended
  processing steps.
• Arbitrary channel failures messages may be
  corrupted, duplicated, delivered out of order,
  incurs extremely large delays or non-existent
  messages may be delivered.
• Timing failures
• Clock failure the processs local clock exceeds
  the bound on its drift rate from real time.
• Process-time failure process exceeds the bound
  on the interval between two steps.
• Channel-time failure message transmission time
  exceed the budgeted bound.

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Omission and Arbitrary Failures
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Timing Failures
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Security Model

• A DS can be secured by securing processes,
  channels objects
• Process security
• Channel security
• Denial of access
• Mobile code security
• Security threats can be defeated by
• Authentication
• Cryptography

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Objects and Principals

• Principal the authority that is associated with
  each invocation and each
• result.
• Authentication
• Access rights rules that specify who is allowed
  to perform the operations
• of an object.

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Channel Security

• An enemy can copy, alter or inject messages as
  the travel.
• An enemy can save copies of messages and replay
  them at a later time.
• Cryptography Messages are scrambled in such a
  way as to hid its
• contents.
• Authentication A portion of a message is
  encrypted that contains enough
• information to guarantee its authenticity.

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Denial of Service

• Excessive and pointless invocations on services
  or message transmissions are made, resulting in
  overloading of physical resources.
• Can be dealt with by using packet filters and
  sniffers that detect abnormal activities.