Images below –
Homage to Magritte,
in a way…
CSSE 377 Software Architecture and Design 2
Steve Chenoweth, Rose-Hulman Institute
Week 2, Day 3, September 15, 2011
Architectural Styles 1
And this is not a filter!
Today


Right – The real Magritte.
What does “This is not a
pipe” mean, Rene?
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.
2
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 class
3. Performance scenarios
6. Architecture methodology


See next slide 
We want to be at “models and theories”
3
Cirque du Soleil aerial contortionist is Isabelle Chasse,
from http://www.montrealenespanol.com/quidam.htm
“Here’s a great war story
from the 1B project -Impossible, but true!”
Top: The real person shown here is Denzil Biter,
City Engineer for Clarksville, TN. See
www.cityofclarksville.com/ cityengineer/.
Astronomer documenting
observations. From
www.astro.caltech.edu/
observatories/palomar/
faq/answers.htm.
Folklore
Boulder climber from www.gtonline.net
/community/ gmc/boulder/pecqueries.htm
Ad hoc solutions
New problems
The “Novelty Circle” of
engineering; or, “Hey,
where’s my needle in
this haystack?”
Codification
“Yikes!”
Image from www.
sturdykidstuff.com/ tables.html
(novelty and nefarious outside forces)
Models & theories
Background is Claude Monet’s Meule, Soleil Couchant, 1891; Museum of Fine Arts, Boston Figure derived
Am I architecting
software?
from Software
Architecture:
Perspectives on an Emerging Discipline, by Mary Shaw and David Garlan.
Why am I architecting software?
Prentice Hall, 1996, ISBN 0-13-182957-2.
Improved
practice
Image from www.reg.uflSlide 4
.edu/ suppapp-splash.html.
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/.
5
What is an Architectural Style?
6
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
7
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?
8
Today’s styles (more tomorrow)
Garlan & Shaw’s TOC:
Call and Return
 Data Abstraction
 Implicit Invocation
 Pipe and Filter
 Layered

(Slides 31+)
… with examples!
9
Call and Return





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.
Main
One
Two
Foo
Three
Bar
10
Call and Return Properties



Components are subroutines
Connectors are invocations
Heuristic: Cycles (recursion) are discouraged


Heuristic: Multitasking is discouraged




Clearest when subroutines (or whatever the boxes mean)
are called once to do “something”
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.
11
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.
12
Data Abstraction (Object Oriented)
13
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

14
Data Abstraction
Advantages
May change representation without
affecting clients
 Facilitates decomposition of problems
 Objects that mirror real-world entities will
evolve slowly

15
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
– a change in the number of
parameters you need to pass
 Example
16
Implicit Invocation (Callback)
Register interest in events
Event 1
Event 2
Event 3
...
Foo
Notification of events
Bar
Announcement of events
17
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
18
Implicit Invocation
Advantages
Reuse: any component can be introduced
by registering it for appropriate events
 Evolution: components may be replaced
without affecting other interfaces

19
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

20
Implicit Invocation Example
Publisher-Subscriber systems – used for event management, and more:
DB
Web
Client
Another
System
Connection
to Broker
S1
P1
Subscriptions
S2
Connection
to Broker
Subscriptions
P2
Subscriptions
Managed
System
Subscription
Broker
Architecture
S3
Connection
to Broker
Connection
to Broker
Services
Provided
Subscription
Distribution

Services
Provided
Connection
to Broker
Managed
System
S’s = subscribers to information. P’s = providers of information.
Think RSS Feed, or Object Request Brokers like CORBA.
21
Pipe and Filter
22
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

23
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
24
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!.

25
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?
26
Layered
U
p
C
v
27
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

28
Layered Advantages
Abstraction: separation of concerns
 Evolution: changes to one layer only affect
neighboring layers
 Reuse: layers have strong interfaces

29
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

30
More Complex
Examples…
 See Garlan & Shaw’s
article (Case Studies) for
more info on each of these.
KWIC – Call and Return
32
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.
33
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
34
KWIC – Data Abstraction
35
KWIC – Implicit Invocation
36
KWIC – Pipe and Filter
37
Oscilloscope



Sample electrical
signals
Display pictures
(traces)
Perform
measurements
38
Oscilloscope – Data Abstraction
39
Oscilloscope – Layered
40
Oscilloscope – Pipe and Filter
41
Oscilloscope – Modified Pipe and Filter
42