Software Construction & Design 1
The University of Sydney Page 1
Software Construction and
Design 2
SOFT3202 / COMP9202
Enterprise & GoF Design
Patterns
School of Information Technologies
Dr. Basem Suleiman
The University of Sydney Page 2
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Agenda
– GoF Design Patterns
– State
– Service Oriented Architecture
– Web Services
– SOAP-based Services
– Representation State Transfer (REST)
– RESTful Services
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State Design Pattern
GoF
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State Design Pattern
– Object behavioral
– Object relationships which can be changed dynamically at run-time
– How a group of objects can collaborate to perform a task that no single
object can do alone
– Intent
– Allow an object to change its behavior when its internal state changes
– Think finite-state-machine
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Motivating Scenario
– TCP network connection
– States: Established, Listening and Closed
– TCP connection responds based on its current state
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State Pattern – When to Use
– The state of n object drives its behavior and change its behavior
dynamically at run-time
– Operations have large, multipart conditional statements that depend on the
object’s state
– Repeated conditional structure
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State Pattern – Structure
By State_Design_Pattern_UML_Class_Diagram.png: JoaoTrindade (talk)Original uploader was JoaoTrindade at en.wikipediaderivative work: Ertwroc (talk) – State_Design_Pattern_UML_Class_Diagram.png, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=10234908
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State Pattern – Participants
– Context (TCPConnection)
– Defines the interface of interest to clients
– Maintains an instance of a ConcreteState subclass that defines the current state
– State (TCPState)
– Defines an interface for encapsulating the behavior associated with a certain
state of the Context
– ConcreteState subclasses (TCPEstablished, TCPListen, TCPClosed)
– Each subclass implements a behavior associated with a state of the Context
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State Pattern – Collaborations
– Context delegates state-specific requests to the current ConcreteState
object
– A context may pass itself as an argument to the State object handling the
request, so the State object access the context if necessary
– Context is the primary interface for clients
– Clients can configure a context with State objects, so its clients don’t have to
deal with the State objects directly
– Either Context or the ConcreteState subclasses can decide which state
succeeds another and under what circumstances
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State Pattern – Consequences
– Localizes state-specific behavior for different states
– Using data values and context operations make code maintenance difficult
– State distribution across different sub-classes useful when there are many states
– Better code structure for state-specific code (better than monolithic)
– It makes state transition explicit
– State transitions as variable assignments
– State objects can protect the context from inconsistent state
– State objects can be shared
– When the state they represent is encoded entirely in their type
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State Pattern – Implementation (1)
– Defining the state transitions
– Let the state subclasses specify their successor state to make the transition
(decentralized)
– Achieves flexibility – easy to modify and extend the logic
– Introduces implementation dependencies between subclasses
– Table-based state transitions
– Look-up table that maps every possible input to a succeeding state
– Easy to modify (transition data not the program code) but:
• Less efficient than a functional call
• Harder to understand the logic (transition criteria is less explicit)
• Difficult to add actions to accompany the state transitions
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State Pattern – Implementation (2)
– When to create and destroy state objects?
– Pre-create them and never destroy them
• Useful for frequent state changes (save costs of re-creating states)
• Context must keep reference to all states
– Only when they are needed and destroyed them thereafter
• States are not known at run-time and context change states frequently
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State Pattern – Implementation (3)
– Using dynamic inheritance
– Changing the object’s class at run-time is not possible in most OO languages
– Delegation-based languages allow changing the delegation target at run-time
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State Pattern – Code Example
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State Pattern – Code Example
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State Pattern – Code Example
– What is the output of StateDemo?
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State Pattern – Related Patterns
– Flyweight
– Explains when and how state objects can be shared
– Singleton
– State object is often implemented as Singleton
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Service-Oriented
Architecture (SOA)
The University of Sydney Page 22
Service-Oriented Architecture (SOA)
“a style of software design where services are provided to the other components by
application components, through a communication protocol over a network” –
Wikipedia (May 2019)
– Paradigm, approach or style
– Can help to architect applications so that certain quality attributes are satisfied
– Services are the core of this approach
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What Is A Service?
– A unit of abstracted functionality that can be accessed remotely
and consumed through abstract interface
– Logically represents a business activity/functionality with a specified
outcome
– Self-contained
– Abstracts implementation details
– May compose of one or more services
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A Service-based Application
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SOA – A Service-based Application
(MVS/CICS)
Bank
(Win2K3/
COM+)
Credit Card
processor
Linux/
EJB
Supplier 2
(Win2K3
.NET)
Supplier 1
(Solaris/
CORBA)
Shipper 2
Smart client app
Browser-based client
On-line retail
application
(J2EE/IIOP/JMS) Web Server
Reseller
Win2K3
EJB
User
authentication
(Win2K/
MQ)
Shipper 1
– Building large-scale integrated
applications from services
– Specific enough to have clear focus
– Integration and communication
– Platform independence
– Abstracted from end users
– Reliability and robustness
– Avoid rewriting legacy code
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SOA Principles
– Boundaries are explicit
– Services are external, crossing boundaries has costs and implications
– Services are autonomous
– May be outside your control, can change
– Share schema and contract
– Not classes and implementations
– Policy-based compatibility
– Requirements stated in policies, not in code
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Web Services
SOAP Services
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Web Services
– Relatively standardized mechanism by which one piece of software
application can connect and communicate with another using web protocols
– It provides a simple and open way of integrating functions or data from
various systems
– It can be used within an organization and/or across the public Internet
– At least two implementations: SOAP-based vs. RESTful services
– Original design of Web Services is very application-centric in contrast to
the resource-centric Web and REST style
COMP5347 Web Application Development
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Web Service Specifications
– Simple Object Access Protocol (SOAP): a messaging protocol for transferring
information (XML format)
– Web Service Description Language (WSDL): A standard for describing Web
services (XML format)
– Interfaces, methods, parameters, request, response
– Generated and consumed by tools
– Universal Description, Discovery and Integration (UDDI): a registry and protocol
for publishing and discovering web services
– Like white, Yellow and Green ‘pages’
– Not really used
– Other specifications include Security, Privacy, Reliable Messaging, Transaction,
Interoperability
– Web service specifications are occasionally referred to collectively as WS-*
COMP5347 Web Application Development
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Web Service – SOAP-based Services
– A component that offers services that can be accessed through messages that
follow some particular XML-based standards
– XML messages in the protocol
– XML definitions of services
– Pass around XML documents
– Building on other XML standards
– All ‘human readable’ (for some subset of ‘humans’)
– These messages may be carried over HTTP…
– Standards are managed through W3C
– Standardise only what goes on the wire
– No programming models or APIs
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Terminology
– Sender
– Receiver
– Intermediary
– A receiver that then sends the message onwards
– Ultimate Receiver
– The receiver that actually provides the service
– Does not send the message on any further
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Web Services Consumption
– Find a service
– Find out how to call it
– What information it needs
– Where it is located
– Ask it to do something for you
– Invoke a service method
– And add a few useful services
– Transactions? Security? Reliability?
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Calling A Service – SOAP
Envelope (Mandatory) –
Marks the start and end of a
message
Body (Mandatory) –
Data for the actual message
or document being sent
Header (Optional) –
General information about
message – e.g. authentication
and transaction management
– SOAP – Simple Object Access Protocol!
– Originally simple remote procedure calls using XML-
formatted protocol
– Envelope, header, body
– Extensible protocol
• Add new header elements for new semantics
– Very widely accepted standard
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WSDL Structure
– WSDL Ports
– Describe the operations that can be performed and messages that
are involved
– WSDL Messages
– Describes the parts of a messages. A part can be seen as parameters
of a message
– WSDL Types
– Defines the data types of used by the web service
– WSDL Bindings
– Describes the message format and protocol details for each port
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SOAP Example – Stock Quote
GET /StockQuote HTTP/1.1
Host: www.stockquoteserver.com
Content-Type: text/xml
Content-Length: nnnn
SOAPMethodName: Stock-Namespace-URI#GetLastTradePrice
HTTP/1.1 200 OK
Content-Type: text/xml
Content-Length: nnnn
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WSDL Structure
definition of types……..
definition of a message….
definition of a port…….
definition of a binding….
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Publishing and Consuming a Service
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SOAP-based Services – Workflow
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SOAP Services – Performance Factors
– Processor time for XML encoding/decoding
– Is XML slower than binary alternatives?
– Number and size of msgs passed around
– Does verbosity cost performance?
– Proc time taken by transport protocols
– TCP/IP and HTTP
– Network delays
– Switches, routers, …
– Others …
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Web Services Stack
Connected Applications
Messaging
XML
Transports
Secure Reliable Transacted
M
e
ta
d
a
ta
Management
Business
Process …Devices Mobile
P2P EAI B2B Grid
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Representation State
Transfer (REST), RESTful
Services
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Representation State Transfer (REST)
– Defined by Roy Fielding in his 2000 thesis
– REpresentational State Transfer (REST, “RESTful”)
– Web and agile developer community has been working on ways to make a
‘Web’ for programs rather than people
– Adopt ideas that make WWW successful, but suited to consumption by programs
rather than human readers
– “Web-friendly”: use web technologies in ways that match what the Web expects
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REST – Architectural Style
– REST is an architectural style rather than a strict protocol
– REST-style architectures consist of clients and servers
– Requests and responses are built around the transfer of representations of resources
– A resource can be essentially any coherent and meaningful concept that
may be addressed
– A representation of a resource is typically a document that captures the
current or intended state of a resource
Based on Roy Fielding’s doctoral dissertation, rephrased by Wikipedia http://en.wikipedia.org/wiki/Representational_State_Transfer
COMP5347 Web Application Development
http://en.wikipedia.org/wiki/Client_(computing)
http://en.wikipedia.org/wiki/Server_(computing)
http://en.wikipedia.org/wiki/Resource_(Web)
http://en.wikipedia.org/wiki/Representation_(systemics)
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REST – The Basic Idea
HTTP Client
(e.g. web browser)
Web Server Database
GET /book/222
POST /order
301 location: /order/612
PUT /order/612
SELECT *
FROM Books
WHERE isbn=222
INSERT
INTO Orders …
UPDATE Orders
WHERE id=612
SET…
[adapted from Cesare Pautasso, http://www.pautasso.info, 2008]
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REST – Stateless vs. Stateful
http://www.ibm.com/developerworks/webservices/library/ws-restful/
COMP5347 Web Application Development
http://www.ibm.com/developerworks/webservices/library/ws-restful/
The University of Sydney Page 48
REST – Design Principles
1. Resource Identification through URI
2. Uniform Interface for all resources(HTTP verbs)
– GET (Query the state)
– PUT (Modify (or create)
– POST (Create a resource, with system choosing the identifier)
– DELETE (Delete a resource)
3. “Self-Descriptive” Messages through Meta-Data
4. Hyperlinks to define the application state
– Address the resources explicitly in the request message
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REST – Tenets
– Resource-based rather than service-oriented
– Addressability: interesting things (resources) should have names
– Statelessness: no stateful conversations with a resource
– Representations: a resource has state representation
– Links: resources can also contain links to other resources
– Uniform Interface: HTTP methods that do the same thing
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REST – Resources
– A resource is something “interesting” in your system
– Anything like Blog posting, printer, a transaction, others
– Requests are sent to URIs (“nouns”)
– Choosing the resources and their URIs is the central design decision for making a
RESTful service
– The operations (“verbs”) are always the same simple HTTP ones (“get”,
“post”, “put”) and they act as expected
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REST – Resource Representations
– We deal with representations of resources
– Not the resources themselves
• “Pass-by-value” semantics
– Representation can be in any format
• Representations like JSON or XML are good for Web-based
services
– Each resource has one or more representations
– Each resource implements the uniform HTTP interface
– Resources URIs
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REST – Resource Architecture
Physical Resources
Logical Resources
Uniform Interface
(Web Server)
Resource
Representation
(e.g. XML document)
Consumer
(Web Client)
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REST – Uniform Resource Identifier (URI)
– Internet Standard for resource naming and identification
– Examples:
http://tools.ietf.org/html/rfc3986
https://www.google.com/search?q=rest&start=10
– REST advocates the use of “nice” URIs…
– In most HTTP stacks URIs cannot have arbitrary length (4Kb)
URI Scheme Authority Path
Query
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REST – Resource URIs
– URIs should be descriptive?
– Convey some ideas about how the underlying resources are arranged
• http://spreadsheet/cells/a2,a9
• http://jim.webber.name/2007/06.aspx
– URIs should be opaque?
– Convey no semantics, can’t infer anything from them
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REST – Links
– Connectedness is good in Web-based systems
– Resource representations can contain other URIs
– Resources contain links (or URI templates) to other resources
– Links act as state transitions
– Think of resources as states in a state machine
– And links as state transitions
– Application (conversation) state is captured in terms of these states
– Server state is captured in the resources themselves, and their
underlying data stores
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REST – The HTTP Verbs
– Retrieve a representation of a resource: GET
– Get metadata about an existing resource: HEAD
– Create a new resource: PUT to a new URI,
or POST and the resource will get a new system-chosen URI
– Modify an existing resource: PUT to an
existing URI
– Delete an existing resource: DELETE
– See which of the verbs the resource
understands: OPTIONS
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Resource Types
– Most of the time we can differentiate between collection type of resources and
individual resource
– Revisions and revision
– Articles and article
– This can be nested and developers/architect often decide the nesting direction
– /movies/ForrestGump/actors/TomHanks
– /directors/AngLee/movies/LifeOfPi
COMP5347 Web Application Development
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REST – Request URLs and Methods
Action URL path Parameters Example
Create new revision /revisions http://localhost:3000/revisions
Get all revisions /revisions http://localhost:3000/revisions
Get a revision /revisions revision_id http://localhost:3000/revisions/123
Update a revision /revisions revision_id http://localhost:3000/revisions/123
Delete a revision /revisions revision_id http://localhost:3000/revisions/123
COMP5347 Web Application Development
Request Method Use case Response
POST Add new data in a collection New data created
GET Read data from data source Data objects
PUT Update existing data Updated object
DELETE Delete an object NULL
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Uniform Interface Principle (CRUD Example)
CRUD REST
CREATE PUT (user chooses the URI) or POST
(system chooses the URI)
Initialize the state of a new resource
READ GET Retrieve the current state of a resource
UPDATE PUT Modify the state of a resource
DELETE DELETE Clear a resource; afterwards the URI is no longer
valid
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GET Semantics
– GET retrieves the representation of a resource
– Should not affect the resource state (idempotent)
– Shared understanding of GET semantics
– Don’t violate that understanding!
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POST Semantics
– POST creates a new resource
– But the server decides on that resource’s URI
– Common human Web example: posting to a blog
–Server decides URI of posting and any comments made on that post
– Programmatic Web example: creating a new employee record
–And subsequently adding to it
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POST Request and Response
– Request
POST / HTTP/1.1
Content-Type: application/xml
Host: localhost:8888
Content-Length: ….
Verb, path, and HTTP
version
Content type (XML)
Content (again XML)
– Response
201 CREATED
Location: /orders/jwebber/ABCD/2007-07-08-13-50-53
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PUT Semantics
– PUT creates a new resource but the client decides on the URI
– Providing the server logic allows it
– Also used to update existing resources by overwriting them in-place
– Don’t use POST here
– Because PUT is idempotent!
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PUT Request
PUT /orders/jwebber/ABCD/2007-07-08-13-50-53 HTTP/1.1
Content-Type: application/xml
Host: localhost:8888
Content-Length: ….
Verb, path and HTTP
version
Updated content
(higher buy price)
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PUT Response
200 OK
Location: /orders/jwebber/ABCD/2007-07-080-13-50-53
Content-Type: application/xml
Minimalist response might
contain only status and location
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DELETE Semantics
– Stop the resource from being accessible
– Logical delete, not necessarily physical
– Request
DELETE /user/jwebber HTTP/1.1
Host: example.org
– Response
200 OK
Content-Type: application/xml
jwebber
This is important for
decoupling implementation
details from resources
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HEAD Semantics
– HEAD is like GET, except it only retrieves metadata
– Request
HEAD /user/jwebber HTTP/1.1
Host: example.org
– Response
200 OK
Content-Type: application/xml
Last-Modified: 2007-07-08T15:00:34Z
ETag: aabd653b-65d0-74da-bc63-4bca-ba3ef3f50432
Useful for caching,
performance
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OPTIONS Semantics
– Asks which methods are supported by a resource
– Easy to spot read-only resources for example
– Request
OPTIONS /user/jwebber HTTP/1.1
Host: example.org
– Response
200 OK
Allowed: GET,HEAD,POST
You can only read and
add to this resource
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HTTP Status Codes
– The HTTP status codes provide metadata about the state of resources
– They are part of what makes the Web a rich platform for building
distributed systems
– They cover five broad categories
–1xx – Metadata
–2xx – Everything’s fine
–3xx – Redirection
–4xx – Client did something wrong
–5xx – Server did a bad thing
– There are a handful of these codes that we need to know in more detail
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REST Strengths
– Simplicity
– Uniform interface is immutable (harder to break clients)
– HTTP/XML is ubiquitous (goes through firewalls)
– Stateless/synchronous interaction
– More fault-tolerant
– Proven scalability
– “after all the Web works”, caching, clustered server farms for QoS
– Perceived ease of adoption (light infrastructure)
– just need a browser to get started -no need to buy WS-* middleware
– easy to use with any popular rapid-dev languages/frameworks
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REST Weaknesses
– Mapping REST-style synchronous semantics on top of back end systems creates
design mismatches (when they are based on asynchronous messaging or event
driven interaction)
– Cannot yet deliver all enterprise-style “-ilities” that WS-* can
– SOAP services/WS-* provides extensive WS framework and extensions
– E.g., Security and transactions are well-supported in WS-*
– Challenging to identify and locate resources appropriately in all applications
– Semantics/Syntax description very informal (user/human oriented)
– Lack of tool support for rapid construction of clients
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SOAP or RESTful Services
– Service-oriented Architectures can be implemented in different ways
– Focus should be on whatever architecture gets the job done and recognize the significant
value of open standards
– avoid being religious about any specific architectures or technologies
– SOAP and the family of WS-* specifications have their strengths and weaknesses and
will be highly suitable to some applications and positively terrible for others. Likewise
with REST.
– The decision of which to use depends entirely on the circumstances of the application.
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References
– Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. 1995. Design
Patterns: Elements of Reusable Object-Oriented Software. Pearson.
– Wikipedia, State Design Pattern, https://en.wikipedia.org/wiki/State_pattern
– “REST in Practice”, by Jim Webber, Savas Parastatidis and Ian Robinson; O’Reilly
2010
– “Architectural Styles and the Design of Network-based Software Architectures” by R.
Fielding (PhD thesis, 2000).
http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm
– Enterprise-Scale Software Architecture (COMP5348) slides
– Web Application Development (COMP5347) slides
– Wikipedia, List of Web Services Specifications.
https://en.wikipedia.org/wiki/List_of_web_service_specifications
https://en.wikipedia.org/wiki/State_pattern
https://en.wikipedia.org/wiki/List_of_web_service_specifications
The University of Sydney Page 75
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