Download Middleware

Chapter 5
Middleware
Middleware
• Software: in context of smart environment
• Software to provide services to facilitate
– Rapid development
– Ease of integration
– Improved reliability
– Increased scalability
• Lies between applications software & platform
• Connectivity software that joins applications
thru communication mechanisms creating
transparency, scalability, & interoperability
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Middleware
• Defined by API it provides to applications
that use it & by protocols it supports
• Should reduce complexities
– Of networks, OS, applications
• Should provide cross-platform
infrastructure
• Should improve the system
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Overview of Middleware
1. Understanding of Middleware
2. Desirable characteristics
3. Different forms
– Advantages/disadvantages
4. Technologies
5. Standards
6. Benefits
* Provide a working knowledge base *
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Perceive-Reason-Act
AI Approach
• Use to define characteristics of Middleware
• Perceive: sensors
– Software to read, store, calibrate, etc.
• Reason: AI applications
– Software to reason about the environment &
provide actions
• Act: controllers
– To change environment
See Figure 5.1, pg. 104
What are the communications requirements?
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Needs of System (Figure 5.1)
• Interoperability: 2+ entities communicate
• Reliability: guarantee delivery
• Efficiency: minimize consumption &
delivery time
• Throughput: lots of data, no bottlenecks
• Distributed network: applications not on
single computer
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Wants of System
• Provide for future extensions &
•
•
•
•
•
contingencies
Scalable
Hot - swappable - don't shut down to
change component or application
Secure
Highly available - running, accessible
Fault - tolerant
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Desirable Characteristics of
System
• Simplicity and power: for developers & for API
• Natural & seamless extension of development
•
•
•
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environment: for implementer - so focus stays
on application
Flexibility for easy modifications of software separation of interface from applications
Maintainability
Reusability
Portability
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Middleware Architecture
• Communication is the focus
• Client - server model (Figure 5.3, pg. 107)
– 3-tier for much Middleware
– Middle tier addition
• Increase in number of clients
• Better flexibility, maintainability, reusability,
scalability
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Forms of Middleware
• Middleware sits between OS & application
• Huge number of products
• Vary in "form"
– Role, terminology, composition
• Overview of key forms
– Transaction
-- Object
– RPC
-- Agent
– Message
-- Database
– Web
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Transaction Processing
• Middle tier of processing routines between
system that provides transaction-based services
& clients
– e.g.: ATM - deposit, withdraw, check balance
– See Figure 5.4, pg. 109
• Advantages
– Independence of layers, database
– Easily customized on all components
– Transparency
– Mature & well-tested efficient, reliable
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Transaction Processing
• Disadvantages
– Limited scalability - each client (ATM) adds
overhead
– Older - uses low-level languages
• Examples: IBM's CISTP, BEA TUXEDO
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Message-Oriented (MOM)
• Provides communication between
applications on one or more machines;
different platforms (Figure 5.5, pg. 109)
• Generally asynchronous
• Message queuing, persistence (delays),
delivery
• Peer-to-peer connectivity;
agreed upon protocols
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Message-Oriented (MOM)
• Advantages
– Simplifies cross-platform issues; portability
– Increased operability, flexibility
– Good for event-driven systems
– Mature (1980's)
• Disadvantages
– Asynchronous allows for network overload
– Not implemented for some platforms
• Example: Oracle, Advanced Queuing, Arjuna
Messaging, IBM MQ Series, MS MSMQ
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Object-Oriented
aka Object-Oriented Brokers (ORB)
• Transparent extensions of object-oriented
development environment,
– Figure 5.6, pg. 110
• Support: interface definition language,
O communication mechanisms,
O activation/location mechanisms
• Facilitate: locating objects & establishing
communication - similar to MOM
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Object-Oriented
aka Object-Oriented Brokers (ORB)
• Advantages - Same as MOM
– Familiarity with object-oriented environment
– Provide for rapid integration
– Preserves separation between implementation &
interface
• Disadvantages
– Different ORB's support different levels of service,
platforms, certain object-oriented languages
– May be difficult to find one that supports all needs
• Examples: OMG's CORBA, MS COM/DCOM, Sun's JAVA
RMI
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Database
• Can be complex
• Need API access to standard database interfaces
• Development connectivity tools & language
extensions to facilitate applications to database
• Advantages
– Standardization, simple
• Disadvantages
– Not always cross platform
– May not support advanced database features
– May provide blocking, synchronous connections
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Remote Procedure Call (RPC)
• Stubs embedded in client-server applications at
compile, facilitate calls between client-server
(Figure
5.7, pg. 111)
• Advantages
– Provide consistency of procedure calls locally & remotely
– Network transparency of client-server location
• Disadvantages
– Most are synchronous - forces call-wait scenario, possible
blockages
– Asynchronous mechanisms add complexity to development
– Synchronous - not good for object-oriented or peer-to-peer
• Example: Open Group's Dist. Computing Environment
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Web Services
• Popular - bridge interface gap between
application hidden by network security
(e.g. firewalls)
• Leverages WWW technologies & protocols
(Figure 5.8, pg. 112)
– XML interface
– HTTP communications
• New technologies
– SOAP - Simple Object Access Protocol
– WSDL - Web Services Definition Language
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Web Services
• Advantages
– Ubiquitous nature of web servers/API's
– Available software to facilitate development
– ASCII-based messages improve
troubleshooting
– Most corporate systems allow such traffic
– Easy transition
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Web Services
• Disadvantages
– Potential security holes (HTTP)
– Conversion of data structures to SML is
computationally expensive; data structure is
larger
• Slower
• Needs more bandwidth
• Examples: Apple's Web Objects, IBM Websphere,
MS.NET, Sun's Open Net Environment
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Agent-Oriented
• Applications: financial management,
military logistics, personal information
management
• Newest: Intelligent software agents
• Autonomous, intelligent software entities
with ability to perceive environment,
reason, act (to accomplish goals)
• Tend to be implemented as frameworks
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Agent-Oriented
• Provide for
– Interagent communication
– Load balancing
– Mobility (move agents between machines)
• May include MOM or Object-Oriented
Middleware
• Examples: HIVE, CMU's RETSINA
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Frameworks
• Similar to but different from Middleware
• Targeted at specific domain
• Provide API, user interface, tools for
development & management
• May provide own middleware services or
utilize common ones
• Examples: Lotus Notes, MS Office, Transarc's
Encina, Cognos, HP's OpenView
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Frameworks - Comments
• Framework vs. Middleware: not standard,
debated
• Author distinguishes:
– Middleware: invisible, no interface
– Framework - provides interfaces
• Ubicomp - ubiquitous computing
– Numerous initiatives
– e.g., Universal plug-and-play
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Middleware Standards
• Help with functionality, interoperability
among implementers
• Consortiums (IEEE) - compromise,
voluntary
• Corporations - de facto standards; market
share, influence
• e.g., IBM PC; MS Windows
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Standards - Examples
• COM/DCOM - MS - de facto
– Distributed Component Object Model
– Communication protocol between objects
– IDL - Interface Description Language
• CORBA - Object Management Group -
consortium
– Common Object Request Broker Architecture
– Powerful, Widely-used
– Uses IDL-to-program language mapping for many
object-oriented language; generate skeleton & stub
code
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Standards - Examples
• DCE - Open Group
– Distributed Computing Environment
– Popular, forms basis of many middleware
layers
– Set of integrated system service specs.;
OS, platform, network independent
– Provide: RPC, distributed file system, diskless
workstation support
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Java Middleware
Technologies - Sun
Numerous middleware initiatives & support
• Supports CORBA
• J. Remote Method Invocation (RMI) - CORBA like
• J. Message Service (JMS) - provides MOM
• J. Web Services Developer Pack (WSDP) - to
integrate webservices into J. applications
• J. Servlet & J. Server Pages (JSP) - extend server
functionality, dynamic content support
• J. Jini - adaptive network-centric applications
• Disadvantage: One source, one language
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Web Service Standards
World Wide Web Consortium (WSC) - a leader
• HTML, HTTP, SML, SOAP/SMLP, WSDL, others
Organization for Advancement of Structured
Information Standards (OASIS)
• DocBook (documentation), DSML (directory
services), ebXML (eBusiness), SAML (security
assertion), UDDI (universal description,
discover, & integration of web services)
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Database Standards
• SQL (Structured Query Language) - Oracle
– Based on IBM's SEQUEL of 1970's
– de facto standard
– Are non-standard extensions
• ODBC - Open Database Connectivity - MS
– middleware database driver: database applications communication
– Vendors: ODBC-compliant database
– Sun's JDBC for Javas is ODBC - comp.
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Middleware Design
Considerations
• Complement project, easier to design,
develop, & maintain
• Interoperability, reliability, efficiency
throughput
• Secure, dynamic, adaptable, scalable,
available, fault-tolerant
• Flexible, portable, maintainable, reusable
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Middleware Issues
• May add unwanted infrastructure to project
• Risky when using proprietary middleware
(if sole source)
• Extensions: Open Source vs. Proprietary
• Middleware defined by API & protocols
– Be wary of proprietary extensions
• Shift from OS/Platform dependence to
middleware dependence
– Projects look to middleware for services
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Middleware Conclusion
• See 5.6, pg. 119+ for examples
• Middleware provides variety of services
• Applicable to Mobile Computing and Smart
Environments
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