539D 25107

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539D 25/1/07

• AspectJ tutorial, exercise 4
• Three types of Middleware
• Project proposal
• Paper discussions

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Exercise 4a

• aspect Answer4a
• Rectangle around() execution(Rectangle
  Group.getBounds())
• return FigureElement.MAX_BOUNDS

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Exercise 4b

• aspect Answer4b
• private Rectangle Group.cache null
• Rectangle around(Group g)
• execution(Rectangle
  Group.getBounds())
• this(g)
• if (g.cache null)
• g.cache proceed(g)
• return g.cache

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Exercise 4c
aspect Answer4c before(Group g)
call(void move(int, int))
target(g) g.cache null
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Exercise 4d

• aspect Answer4d
• private Rectangle Group.cache null
• private Group Point.enclosingGroup null
• before(Point p, Group g)
• execution(void add(FigureElement))
  args(p) target(g)
• p.enclosingGroup g
• Rectangle around(Group g)
• before(Point p) set( Point.) target(p)
  
• if (p.enclosingGroup ! null)
• p.enclosingGroup.cache null

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Exercise 4e

• aspect Answer4e
• private Rectangle Group.cache null
• private Group FigureElement.enclosingGroup
  null
• before(FigureElement p, Group g)
• execution(void add(FigureElement))
  args(p) target(g)
• p.enclosingGroup g
• Rectangle around(Group g)
• before(Point p) set( Point.) target(p)
  
• FigureElement fe p
• while (fe.enclosingGroup ! null)
• fe.enclosingGroup.cache null
• fe fe.enclosingGroup

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Middleware

• RPC and Object Middleware
• CORBA
• Message-Oriented Middleware
• JMS
• Web services
• WSDL/SOAP

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Managing Dependencies between Components

• Coupling To join together two units
• Tight coupling considered harmful
• Loose coupling is the holy-grail of SOA
• Basic strategy for loose-coupling
  Add another layer of indirection
• Sounds easy, but tradeoffs are hard to measure
• Some coupling is necessary and can be useful
• Take advantage of invariants that will not change
  often

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Two kinds of coupling Interfaces

• Referential coupling
• Need to know the name of recipient
• Reduce by using content-based routing
• Temporal coupling
• Need to synchronize with recipient
• Reduce by using message persistence
• Interfaces add coupling also but since they are
  mostly necessary we usually dont apply that term

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Remote Procedure Call (RPC)

• Masks remote function calls as being local
• Client/server model
• Request/reply paradigm usually implemented with
  message passing
• Marshalling of function parameters and return
  value
• Marshalling code is generated from IDL

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IDL Approach
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IDL
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Text and Graphics from Prof. Wolfgang Emmerich,
University College, London
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RPC Approach

• IDL is compiled into generated stubs

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CORBA

• Adds interfaces to RPC
• Collection of methods implemented by a class
• Adds instances to RPC
• Easier to correlate state
• Easier to migrate state
• Adds object groups to RPC
• Replication
• Load balancing

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CORBA

• Referential coupling
• CORBA objects usually refer to each other by a
  name given as a string
• Names are converted into network address by
  centralized Naming Server
• Optionally a Trading Service can be used
• Works like a Yellow Pages
• Lookup objects by their interface instead of
  their name
• Big Problem Distributed garbage collection does
  not scale
• How should the server know when the client is
  done using an object?

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CORBA

• Temporal Coupling
• RPC is synchronous
• Failures are passed back to client as exceptions
• CORBA also adds asynchronous void methods
• Fire-and-forget
• Client infrastructure will keep trying for a
  certain
• period but application will not be notified
• CORBA also adds deferred synchronous
• Application can poll infrastructure to find out
  when a procedure call has completed

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CORBA Interfaces

• Server objects implement IDL interface
• IDL has tight match to major OO languages
• Java, C, Python, etc..
• Easy to translate Java interface to IDL
• IDL has awkward mapping to other languages
• Most notably C

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Message-Oriented Middleware

• Asynchronous Messaging in Distributed Systems
• Loosely coupled architectures
• Components come and go as they please
• Two major kinds
• Publish-Subscribe
• Point-to-Point

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Publish-Subscribe

• Multiple applications need to receive the same
  message (broadcast)
• Publishers and Subscriber and loosely coupled
• Delivery is based on event subscriptions
• Content-Based Routing
• Event topics (types) organized hierarchically
• Components
• Publisher/Subscriber
• Connector
• Event Router

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Topic Hierarchy

• Organized in a tree
• Similar to XML
• Contains main topics and topic values

• Stocks
• NASDAQ
• AMD
• int price int change
• IBM
• int price int change
• NYSE

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Subscriptions

• Subscribers select which events they are
  interested in
• Register subscriptions with local event router
• Make use of
• Logical operators and, or, not
• Arithmetic operators ,
• Wildcard () matches any event type or data
• Ex All US stock that have fallen more than two
  dollars
• /Stocks/NASDAQ/, 2 /Stocks/NYSE/, 2

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Event Routers

• Can make intelligent use of subscription
  information

IBM AMD
Subscriber
Canada Router
England Router
IBM
Subscriber
IBM
Publisher
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Event Routers

• Can make intelligent use of subscription
  information

IBM AMD
Subscriber
Canada Router
England Router
IBM
Subscriber
AMD
Publisher
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Point-to-Point Middleware

• Messages are sent to a unique receiver
• Provides temporal decoupling of sending and
  receiving messages

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Message-Oriented Middleware

• Referential Coupling
• Managed by event router infrastructure
• Subscribers dont know the identity of Publishers
• Publishers dont know the identity of Subscribers
• Content-based routing using subscription
  languages
• Publishers/Subscribers need only attach to an
  event-routing network

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Message-Oriented Middleware

• Temporal Coupling
• Traditional Pub/Sub drops messages once they are
  delivered to all active subscribers
• Point-to-Point saves messages for pickup by
  receivers
• Same behavior as e-mail
• Pub/Sub Persistent Subscriptions
• Event routers act also as message queues for
  subscribers

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Message-Oriented Middleware

• Interfaces
• Publishers must make topic structure known to
  subscribers
• Changes to topic structure can cause subscribers
  to break
• Topic languages mitigate problems to some extent
• Same motivation as other query languages such as
  AspectJ pointcut language

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Web Services

• Get some of the benefits of CORBA but take
  advantage of existing WWW infrastructure
• XML message format instead of binary encoding
• Tradeoff Give up performance for ease of
  programming and maintainance
• Hijack HTTP
• Let existing web servers do the grunt work
• Naming, location, connection and buffer handling
• XML Schema interface (WSDL) instead of IDL
• More powerful than IDL

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XML Schema

• SimpleType primitives such as int, string, date
• ComplexType defines a regular expression of
  Element sequences
• Element is either
• SimpleType
• ComplexType
• Notice the recursion between ComplexType and
  Element
• Allows us to define patterns of trees instead of
  patterns of strings

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Where are Web Services?

• C2B (Customer to Business)
• Provide clients access to large-scale data
  resources through programmable interface
• Browser-based
• Use JavaScript APIs to compose services
• Mash-ups
• Dependent on JavaScript
• Google Maps
• SOAP/WSDL
• eBay
• Amazon

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Where are Web Services?

• B2B (Business to Business)
• Manage another business data on their behalf
• Supply Chain Management
• Manage product ordering, inventory, shipping,
  tracking etc..
• Customer Relations Management
• Analysis of customer behavior to aid product and
  service decision making (e.g. pricing, new
  product development, etc)
• Saleforce.com

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Web Services Referential Coupling

• Makes use of URLs
• Take advantage of DNS
• No centralized Naming Server is required
• Trading Service realized as UDDI
• Not used in practice
• Hard to find two services that implement the same
  interface
• Cant trust services unless you know the company
  that implements them, then you dont need the
  Trading Service anyway
• Can Artificial Intelligence help?
• Work on Semantic Web is growing (slowly)

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Web Services

• Temporal Coupling
• Similar to CORBA
• Some work on integrating Web services and Pub/Sub
• Nothing mainstream yet
• Interfaces
• Provided by WSDL
• More powerful than OO type systems
• Harder to translate between Java interface to
  WSDL or vice versa

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Conclusion

• CORBA (Distributed Objects)
• Tight coupling provides powerful platform and
  services
• Useful for embedded systems, LAN, and fixed
  architectures
• Message-Oriented Middleware
• Content-based routing provides loose coupling but
  requires complex infrastructure
• Web Services
• Cuts out features of CORBA that do not scale
• Integrates naturally with WWW infrastructure and
  XML databases

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Course Project

• 2 page (single column) proposal due in two week
• 4 page (two-column) paper due one week before end
  of term
• You may choose to do a large implementation
  project rather than a traditional research
  project
• Refactor an open-source application using AspectJ
• Logging, Monitoring, or anything like Logging
  does not count!
• Build a complete, useable, Web application by
  composing two or more Web services
• Dont just throw up some pins on Google Maps!

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Class Discussions

• Remember discussion is worth 10 of your mark
• Be prepared each class to address the four
  questions I have listed
• For the first few discussions, we will start by
  asking for volunteer answers to each question

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Questions

• What is the problem addressed by this paper?
• What is the approach to solve this problem?
• How do the authors validate (prove/show/argue)
  that their approach solves the problem?
• What is one part of the approach or validation
  that you think can be improved or extended?