Introduction to Middleware for Software Engineering

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Introduction to Middleware for Software
Engineering

• Eric Wohlstadter
• 539D

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What is Middleware?

• Mediates heterogeneities (differences)
• Defines standard protocols for distributed
  computing
• Abstracts details of using protocols
• Programmers use abstractions
• Abstractions implement protocols
• Provided as off-the-shelf software
• Not traditionally integrated in the operating
  system

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Problems with Distribution
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Some Middleware Categories

• Remote Procedure
• Object-oriented
• Component Containers
• Message-oriented

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Procedural Middleware

• Problems Addressed
• Tedious to build distributed applications with
  sockets
• Heterogeneous data types
• Memory management (buffers)
• Network failures
• Approach
• Compiler generated socket code
• Socket code becomes the assembly language
• Examples
• Sun RPC
• SOAP

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RPC Approach

• Define function signatures using an interface
  definition language (IDL)
• IDL ensures compatibility across platforms

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RPC Approach
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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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Object Middleware

• Problem Addressed
• Tedious to build distributed applications with
  RPC
• Extensibility is hard
• Difficult to manage stateful resources
• Approach
• Distributed Objects
• Examples
• CORBA
• Java RMI
• DCOM

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Object Approach

• Interface inheritance allows server to add new
  features without invalidating clients
• Client can create and destroy instances of
  objects which live on server
• Client maintains distinct stub instances for each
  distributed object
• Objects can be migrated and/or replicated

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Containers

• Problem Addressed
• Hard to manage crosscutting concerns
• Approach
• Declarative Services
• O/R Mapping
• Examples
• Enterprise Java Beans
• COM (.NET)
• Spring Framework

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CORBA-style method
transfer(int amount,int account1, int account2)
/ Apply access control / / Signal
beginning of transaction / / Update Database
for account1 / / Update Database for account2
/ / Signal Commit or Rollback of Transaction
/ Programmer must handle persistence,
transactions, security
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EJB-style method

• _at_TransactionAttribute(REQUIRED)
• _at_ Interceptor(AccountSecurity.class)
• transfer (int amount, Account account1, Account
  account2) / Update Account 1 /
• / Update Account 2 /
• Many details taken care of by container

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How the container intervenes
EJBContainer
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Message-Oriented Middleware

• Problems Addressed
• High-coupling between client and server
• Approach
• Message Queuing
• Temporal Decoupling
• Pub-Sub
• Referential Decoupling
• Can be combined
• Examples
• MSMQ
• MQSeries

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Point-to-Point
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Pub-Sub

• Messages delivered through router infrastructure
• Deliver is based on topic subscriptions

NEC AMD
Subscriber
Router
Router
IBM
Subscriber
IBM
Publisher
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Conclusion

• Evolution of RPC to Containers and MOM
• We will look at
• Using AOP to implement middleware
• Applying AOP to distributed application
  programming
• Distributed AOP
• Designing new kinds of middleware (mobile, P2P,
  service-oriented)