Transcript CS 501: Software Engineering Fall 2000 Lecture 15 System Architecture II Distributed and Real Time Systems.

CS 501: Software Engineering
Fall 2000
Lecture 15
System Architecture II
Distributed and Real Time Systems
Administration
Assignment 2: Requirements
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Grades -- presentation, report, individual
Comments at presentation
Comments from teaching assistant
Assignment 3: Design
Comments on Requirements Report
Audience
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Client and design team
Will be updated over time
Content
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Level of detail -- will be used to validate the implementation
Requirements, not design
Precise, but not legalistic
Sequence Diagram: Notation
libMem:
LibraryMember
BookBorrower
theBook:Book
theCopy:Copy
borrow(theCopy)
okToBorrow
borrow
borrow
dotted line
shows object
lifetime
rectangle
shows focus
of control
Sequence Diagram: Branching
libMem:
LibraryMember
theBook:Book
theCopy:Copy
BookBorrower
1:borrow(theCopy)
2:okToBorrow
[not ok]3:noborrow
[ok]3:borrow
4:borrow
branch
Example: Distributed Database
two copies of the
same data
Distributed Data and Replication
Distributed Data
Data is held on several computer systems. A transaction may need
to assemble data from several sources.
Replication
Several copies of the data are held in different locations.
Mirror: Complete data set is replicated
Cache: Dynamic set of data is replicated (e.g., most recently used)
With replicated data, the biggest problem is consistency.
Example: Broadcast Search
User
User interface
server
Databases
Example: UseNet
Stateless Protocol v. Stateful
Stateless protocol
Example: http
Open connection
Send message
Return reply
Close connection
State in http must be sent with every message
(e.g., as parameter string or in a cookie)
Stateless Protocol v. Stateful
Stateful (session) protocol
Example: Z39.50
Open connection
Begin session
Interactive session
End session
Close connection
Server remembers the results of previous transactions
(e.g., authentication, partial results) until session is
closed.
Firewall
Private
network
Public
network
Firewall
A firewall is a computer at the junction of two network
segments that:
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Inspects every packet that attempts to cross the boundary
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Rejects any packet that does not satisfy certain criteria, e.g.,
an incoming request to open a TCP connection
an unknown packet type
The Domain Name System
First attempt to resolve
www.cs.cornell.edu
.edu
server
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cornell.edu
server
cs.cornell.edu
server
Discussion of the First Attempt
Problems?
The Domain Name System
Better method
local DNS
server
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almaden.ibm.com
cornell.edu
Local ece.cmu.edu
cache ibm.com
acm.org
.edu
.edu
server
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cornell.edu
server
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cs.cornell.edu
server
Real Time System
A real time system is a software system whose correct
functioning depends upon the results produced and
the time at which they are produced.
• A soft real time system is degraded if the results
are not produced within required time constraints
• A hard real time system fails if the results are not
produced within required time constraints
Example: Web Server
http message
daemon
TCP port 80
spawned processes
The daemon listens at port 80.
When a message arrives it:
spawns a processes to handle the message
returns to listening at port 80
Embedded Systems
Software and hardware are combined to provide an
integrated unit, usually dedicated to a specific task:
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Digital telephone
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Automobile engine control
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GPS
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Scientific instruments
The software may be embedded in the device in a manner
that can not be altered after manufacture.
Example: Autonomous Land Vehicle
GPS
Steer
Sonar
Model
Laser
Control
signals
Throttle
Controls
Sensors
Signal
processing
Other Applications
Response critical
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Network router
Telephone switch
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Seat bag controller
Shared systems
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Multi-user data processing
Time sharing
Techniques
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Special purpose hardware
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Multi-threading and multi-tasking
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Parallel processing
=> digital signal processing
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Interrupts
=> levels and priorities
Multi-Threading
Several similar threads operating concurrently:
• Re-entrant code -- separation of pure code from
data for each thread
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Testing -- single thread and multi thread
May be real time (e.g., telephone switch) or nontime critical
Real Time Executive
Schedules and dispatches tasks in a real time system
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Real time clock
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Interrupt handler
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Scheduler
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Resource manager
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Dispatcher
Must be extremely reliable
Timing
Timing mechanisms
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Synchronous (clocked) -- periodic stimuli
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Asynchronous -- wait for next signal
Example: Communications protocols may be
synchronous or asynchronous
Hardware v. Software
Design of embedded systems requires close understanding
of hardware characteristics
• Special purpose hardware requires special tools and
expertise.
• Some functions may be implemented in either
hardware of software (e.g., floating point unit)
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Design requires separation of functions
Distinction between hardware and software may be
blurred.
Example: Dartmouth Time Shared
System
master
processor
Communications
processor
I/O
Mulitplexor
Central
processor
Central
processor
Communications
processor
Central
processor
Software Considerations
Resource considerations may dictate software design and
implementation:
• Low level language (e.g., C) where programmer has close
link to machine
• Inter-process communication may be too slow (e.g., C
fork).
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May implement special buffering, etc., to control timings
Example: CD Controller
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Input
block
7
3
2
5
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Circular buffer
1
Output
block
Continuous Operation
Many systems must operate continuously
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Software update while operating
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Hardware monitoring and repair
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Alternative power supplies, networks, etc.
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Remote operation
These functions must be designed into the fundamental
architecture.
Routers and Other Network Computing
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Interoperation with third party devices
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Support for several versions of protocols
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Restart after total failure
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Defensive programming -- must survive
=> erroneous or malicious messages
=> extreme loads
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Time outs, dropped packets, etc.
Evolution of network systems