Architectural Styles 1

CSSE 377 Software Architecture and Design 2Steve Chenoweth, Rose-Hulman InstituteWeek 2, Day 3, September 15, 2011

And this is not a filter!

Images below – Homage to Magritte, in a way…

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Today First biweekly quiz from Tuesday – go over Architectural styles, Part 1 – this

Relates strongly to software reliability/availability, and the other QA’s We’ll talk about these thru slide 28 today. More details – read the article case studies! Tonight – HW 2

Project 2 – Progress reports: We’ll discuss in class the issues you’ve come up

with, trying to test your system’s availability over since Tuesday. Time to work on Project 2 in class Friday, 9/16 (tomorrow), 11:55 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.

Right – The real Magritte. What does “This is not a pipe” mean, Rene?

First biweekly quiz - feedback

Look at the ones you missed! “Key” is now out on course website, under Quizzes

Grading scale (“out of” 100, could get 111): Counted # 3 as 2 pts each part (poss 12) Counted all the others as 11 if full credit ()

Let’s go over a couple in class3. Performance scenarios

6. Architecture methodology See next slide We want to be at “models and theories”

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Slide 4Am I architecting software? Why am I architecting software?

The “Novelty Circle” of The “Novelty Circle” of engineering; or, “Hey, engineering; or, “Hey, where’s my needle in where’s my needle in

this haystack?”this haystack?”

Top: The real person shown here is Denzil Biter, City Engineer for Clarksville, TN. See www.cityofclarksville.com/ cityengineer/.

FolkloreAd hoc solutions

New problems (novelty and nefarious outside forces)

Codification

Models & theories

Background is Claude Monet’s Meule, Soleil Couchant, 1891; Museum of Fine Arts, Boston Figure derived from Software Architecture: Perspectives on an Emerging Discipline, by Mary Shaw and David Garlan. Prentice Hall, 1996, ISBN 0-13-182957-2.

Image from www.reg.ufl.edu/ suppapp-splash.html.

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Architectural Styles

Acknowledgement: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/project/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 designComponents: computational elementsConnectors: interactions between

componentsConstraints: 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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Today’s styles (more tomorrow)

Call and Return Data Abstraction Implicit Invocation Pipe and Filter Layered

… with examples!

Garlan & Shaw’s TOC:

(Slides 31+)

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Call and Return

Main

One Two Three

Foo Bar

Basically, this is application of a “divide and conquer” systems theory.

“I’ll design and code Main and One, you want to take Two and Three? Then we’ll 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” Usually synchronous “call and return” message style

Heuristic: Try to follow an organized pattern of calling subroutines (like hierarchical).

Invariants? Not a lot of guarantees about how data is manipulated as it moves through the system.

Note: “Invariants” = architectural principles that are true for this style,especially in regard to maintaining integrity of data.

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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 (method) invocations Invariant 1: object maintains integrity of its

representation Invariant 2: representation is hidden from

other objects

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Data Abstraction Advantages May change representation without

affecting clients Facilitates decomposition of problems Objects that mirror real-world entities will

evolve slowly

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Data Abstraction Disadvantages Must know identity of object in order to

interact with itOur 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

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Implicit Invocation (Callback)

Event 1Event 2Event 3...

Foo

Bar

Register interest in events

Notification of events

Announcement of events

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Implicit Invocation Properties Components are modules with:

proceduresevents that are announcedevents of interest

Invariant: announcers of events do not know which components are registered with those events

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Implicit Invocation Advantages Reuse: any component can be introduced

by registering it for appropriate events Evolution: components may be replaced

without affecting other interfaces

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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

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Implicit Invocation Example Publisher-Subscriber systems – used for event management, and more:

DB

S’s = subscribers to information. P’s = providers of information.Think RSS Feed, or Object Request Brokers like CORBA.

S1 P1

P2S3

S2WebClient

AnotherSystem

ManagedSystem

ManagedSystem

Connectionto Broker

Connectionto Broker

Connectionto Broker

Connectionto Broker

Connectionto Broker

Sub

scrip

tion

Dis

trib

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SubscriptionBroker

Architecture

ServicesProvided

ServicesProvided

Subscriptions

Subscriptions

Subscriptions

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Pipe and Filter

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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

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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) Work flow manager – has a supervisory layer above the

pipes and filters controls the flows of data 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/F’s, 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). Two years ago in 377, teams built pipe and filter systems and had a speed

contest. The winner converted a megabyte spreadsheet into XML in two seconds!.

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Pipe and Filter Disadvantages Often lead to batch processing

What’s that? Programs in each stage run to completion before the next one starts.

Note that “batch” is legitimately how server maintenance is done – like backups and DB updating after hours, with checkpoints in the processes between steps

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 there’s an error?

Can you debug the problem? Can you backtrack for recovery?

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LayeredUp

vC

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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

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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

More Complex Examples…

See Garlan & Shaw’s article (Case Studies) for more info on each of these.

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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. He’s still around – see his profile at http://sigsoft.org/SEN/parnas.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