Transcript Document 7696603

System Models
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Outline
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Introduction
Architectural models
Fundamental models
Guideline
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Introduction
• An architectural model defines
– the way in which the components of systems
interact with one another and
– the way in which they are mapped onto an
underlying network of computers
• There are three fundamental models that
help to reveal key problems for the
designers of distributed system.
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Difficulties and threats for
distributed systems
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Difficulties and threats for
distributed systems
• Widely varying modes of use
• Wide range of system environments
• Internal problems:
– non-synchronized clocks
– conflicting data updates
– many modes of hardware and software failure
involving the individual components of a
system
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Architectural models
• An architectural model of a distributed
system
– first simplifies and abstracts the functions of the
individual components of a DS and
– then it considers:
• The placement of the components across a network
of computers
• The interrelationships between the components.
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Architectural models (cont.)
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Software layers
System architectures
Variations on the client-server model
Interfaces and objects
Design requirements for distributed
architectures
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Software layers
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Applications, services
Middleware
Operating system
Computer and network hardware
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System architectures
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Client-server model
Services provided by multiple servers
Proxy srvers and caches
Peer processes
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Variations on the client-server
model
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Mobile code
Mobile agents
Network computers
Thin client
Mobile devices and spontaneous
networking
• The X-11 window system
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Design requirements for
distributed architectures
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Design requirements for
distributed architectures
• Performance issues
• Use of caching and replication
• Dependability issues
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Performance issues
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Performance issues
• Responsiveness
– Users of interactive aplication require a fast and
consistent response to interaction.
• Throughput
– The rate at which computational work is done.
• Quality of services
– The ability to meet the deadlines of users need.
• Balancing computer loads
– In some case load balancing may involve moving
partially-completed work as the loads on hosts changes.
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Use of caching and replication
• The performance issues often appear to be
major obstacles to the successful
deployment of DS, but much progress has
been made in the design of systems that
overcome them by the use of data
replication and caching.
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Dependability issues
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Dependability issues
• The dependability of computer systems as
correctness, security and fault tolerance
• Fault tolerance: reliability is achieved through
redundancy.
• Security: the architectural impact of the
requirement for security concerns the need to
locate sensitive data and other resources only in
computers that can be effectively secured against
attack
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Fundamental Models
• A) Interaction model
• B) Failure model
• C) Security model
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A) Interaction model
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Performance of communication channels
Computer clocks and timing events
Two variants of the interaction model
Agreement in pepperland
Event ordering
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Performance of communication
channels
• Communication performance is often a
limiting characteristic.
• The delay between the sending of a message
by one process and its receipt by another is
referred to as latency.
• Bandwidth
• Jitter is the variation in the time taken to
deliver a series of messages.
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Computer clock and timing event
• It is impossible to maintain a single global
notion of time.
• There are several approaches to correcting
the times on computer clocks. (from GPS)
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Two variants of the interaction
model
• Synchronous distributed system
– The time to execute each step of a process has known
lower and uper bounds.
– Each message transmitted over a channel is received
within a known bounded time
– Each process has a local clock whose drift rate from
real time has a known bound.
• Asynchronous distributed system
– No bound on process executiong speeds
– No bound on message transmisson delays
– No bound on clock drift rates.
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Agreement in pepperland
• The pepperland divisions need to agree on which of them
will lead the charge against the Blue Meanies, and when
the charge will take place.
• In asynchronous pepperland, the messengers are very
variable in their speed.
• The divisions know some useful constraints: every
message takes at least min. minutes and at most max
minutes to arive.
• The leading division sends a message ’charge!’, then waits
for min minutes, then it charges.
• The other division’s charge is guaranteed to be after the
leading division’s, but no more than (max-min) after it.
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Event ordering
• In many cases, we are interested in knowing
whether an event (sending or receiving a
message) at one process occurred before,
after or concurrently with another event at
another process.
• The execution of a system can be described
in terms of events and their ordering despite
the lack of accurate clocks.
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B) Failure model
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Omission failures
Arbitrary failures
Failure detection
Impossibility of reaching agreement in the
presence of failure
• Masking failure
• Reliability of one to one communication
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C) Security model
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Protecting objects
Securing processes and their interactions
The enemy
Defeating security threats
Other possible threats from the enemy
The uses of security models
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Guideline
• Most DS are arranged according to one of a
variety of architectural models.
• The fundamental models – interaction,
failure, and security
– identify the common characteristics of the basic
components from which distributed systems are
constructed.
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