Architectural Styles 1

1
Architectural Styles 1
Images below Homage to Magritte, in a way

• CSSE 377 Software Architecture and Design 2Steve
  Chenoweth, Rose-Hulman InstituteWeek 3, Day 1,
  March 23, 2009

And this is not a filter!
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Today
Right The real Magritte. What does This is
not a pipe mean, Rene?

• First biweekly quiz from Friday go over
• Architectural styles, Part 1 this
• Relates strongly to software reliability, and the
  other QAs
• Well talk about these thru slide 28 today.
• More details read the article case studies!
• Tonight turn in your teams case study choices
  (see details, HW 9)
• Project 2
• Progress reports Well discuss in class the
  issues youve come up with, trying to test your
  systems availability over the weekend.
• Time to work on Project 2 in class
• Tuesday, 3/24, 1155 PM File a spreadsheet
  showing the availability of the part of the
  system you stressed, and how much you predict you
  can improve it. Also turn in your journal with a
  discussion of that spreadsheet how you decided
  on the numbers, especially.
• Policy clarification
• Late significant points off, unless previously
  negotiated.

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Acknowledgements

• Some of the material in these slides is taken
  from An Introduction to Software Architecture
  by Garlan and Shaw, http//www.cs.cmu.edu/afs/cs/p
  roject/vit/ftp/pdf/intro_softarch.pdf.

David Garlan and Mary Shaw, from their home pages
http//www.cs.cmu.edu/garlan/ and
http//spoke.compose.cs.cmu.edu/shaweb/.
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What is an Architectural Style?
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What is a Software Architectural Style?

• Architectural Style Components Connectors
  Constraints
• ? Primarily, the dynamics of the design
• Components computational elements
• Connectors interactions between components
• Constraints how components and connectors may be
  combined

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Questions About Architectural Styles

• What is the design vocabulary?
• What are the allowable patterns?
• What is the underlying computational model?
• What are the essential invariants?
• What are common examples?
• What are advantages and disadvantages?
• What are common specializations?

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Todays styles (more tomorrow)
Garlan Shaws TOC

• Call and Return
• Data Abstraction
• Implicit Invocation
• Pipe and Filter
• Layered
• with examples!

(Slides 29)
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Call and Return

• Basically, this is application of a divide and
  conquer systems theory.
• Ill design and code Main and One, you want to
  take Two and Three? Then well have another look
  at Foo and Bar
• Most common style?
• Fits with organizational considerations!
• Like calling a sequence of functions.

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Call and Return Properties

• Components are subroutines
• Connectors are invocations
• Heuristic Cycles (recursion) are discouraged
• Clearest when subroutines (or whatever the boxes
  mean) are called once to do something
• Heuristic Multitasking is discouraged
• Easier to follow what happens next
• Heuristic Try to follow an organized patter of
  calling subroutines (like hierarchical).

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Call and Return Example

• Commonly seen as a way to organize large systems
  like this.
• Gives a basic way to control coupling and
  cohesion.
• Down in the details, usually you need layers or
  abstraction (OO) to keep complexity down.

Example is LDAP (Lightweight Directory Access
Protocol) system, UC Berkeley. From
http//softwareengineeringnotes.blogspot.com/2007/
07/call-and-return-architecture.html.
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Data Abstraction (Object Oriented)
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Data Abstraction Properties

• Components are objects - act as managers of
  resources
• Connectors are function invocations
• Invariant 1 object maintains integrity of its
  representation
• Invariant 2 representation is hidden from other
  objects

13
Data Abstraction Advantages

• May change representation without affecting
  clients
• Facilitates decomposition of problems
• Objects that mirror real-world entities will
  evolve slowly

14
Data Abstraction Disadvantages

• Must know identity of object in order to interact
  with it
• Our next style fixes that!
• Side effects changes caused by method calls may
  propagate to clients
• Example a change in the number of parameters
  you need to pass

15
Implicit Invocation (Callback)
Announcement of events
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Implicit Invocation Properties

• Components are modules with
• procedures
• events that are announced
• events of interest
• Invariant announcers of events do not know which
  components are registered with those events

17
Implicit Invocation Advantages

• Reuse any component can be introduced by
  registering it for appropriate events
• Evolution components may be replaced without
  affecting other interfaces

18
Implicit Invocation Disadvantages

• Control no way to know what will happen after
  event is announced
• Data may lead to performance problems
• Correctness meaning of procedure is context
  dependent

19
Implicit Invocation Example

• Publisher-Subscriber systems used for event
  management, and more

Managed System
S1
P1
DB
Connection to Broker
Connection to Broker
Services Provided
Subscriptions
Web Client
Subscription Broker Architecture
S2
Subscription Distribution
Connection to Broker
Subscriptions
Managed System
Another System
P2
S3
Connection to Broker
Connection to Broker
Services Provided
Subscriptions
Ss subscribers to information. Ps
providers of information. Think RSS Feed, or
Object Request Brokers like CORBA.
20
Pipe and Filter
21
Pipe and Filter Properties

• Components apply local transformation to their
  inputs - filters
• Connectors act as conduits - pipes
• Invariant 1 filters must be independent
• Invariant 2 filters do not know their neighbors

22
Pipe and Filter Examples

• Common Examples
• Unix shell scripts
• Compilers
• How you translate the data coming going from an
  application (often using 3rd party products)
• Specializations
• Pipelines linear structures
• Bounded pipes restrict amount of data
• Typed pipes data must be of certain type

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Pipe and Filter Advantages

• Behavior is just composition of filters
• Support reuse -- just reconnect
• Using standard I/Fs, can build out of
  off-the-shelf pieces
• Easy to maintain and enhance
• Permit analysis - throughput, deadlock
• Support concurrency
• The filters can hand-off individual records, vs.
  whole files. Or better
• This is roughly how router connections on the
  Internet work, sometimes with only 1-bit delays
  per box (like for ATM).
• Last year in 377, teams built pipe and filter
  systems and had a speed contest.

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Pipe and Filter Disadvantages

• Often lead to batch processing
• May be hard to coordinate streams
• May force lowest common denominator for
  transmission
• On one computer a memory hog(how would you fix
  that?)
• What do you do when theres an error?
• Can you debug the problem?
• Can you backtrack for recovery?

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Layered
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Layered Properties

• Components often implement a virtual machine for
  upper layers
• Connectors defined by protocols between layers
• Quasi-invariant Layers often only interact with
  neighbors

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Layered Advantages

• Abstraction separation of concerns
• Evolution changes to one layer only affect
  neighboring layers
• Reuse layers have strong interfaces

28
Layered Disadvantages

• Some systems are hard to partition this way
• Performance may require closer coupling between
  upper and lower layers
• Hard to find the right levels of abstraction for
  all features

29
More Complex Examples

• ? See Garlan Shaws article (Case Studies) for
  more info on each of these.

30
KWIC Call and Return
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Key Word in Context (KWIC)

• The KWIC index system accepts an ordered set of
  lines, each line is an ordered set of words, and
  each word is an ordered set of characters. Any
  line may be "circularly shifted'' by repeatedly
  removing the first word and appending it at the
  end of the line. The KWIC index system outputs a
  listing of all circular shifts of all lines in
  alphabetical order.
• -- David Parnas, 1972. Hes still around see
  his profile at http//sigsoft.org/SEN/parna
  s.html.

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KWIC Example

• Input
• descent of man
• the ascent of man
• the old man and the sea

• Output
• the ASCENT of man
• DESCENT of man
• descent of MAN
• the ascent of MAN
• the old MAN and the sea
• the OLD man and the sea
• the old man and the SEA

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KWIC Data Abstraction
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KWIC Implicit Invocation
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KWIC Pipe and Filter
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Oscilloscope

• Sample electrical signals
• Display pictures (traces)
• Perform measurements

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Oscilloscope Data Abstraction
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Oscilloscope Layered
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Oscilloscope Pipe and Filter
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Oscilloscope Modified Pipe and Filter