Middleware Basics

1
Middleware Basics

• http//wwwis.win.tue.nl/houben/isa/isa-ls-middlew
  are.pdf
• http//www.granneman.com/downloads/databaseovervie
  w.pdf
• Any type of software that facilitates
  communication between two or more software
  systems
• A general term for any programming that serves
  to glue together or mediate between two separate
  and often existing programs
• An extension of the operating system which
  provides a transparent communication layer to
  applications
• A software layer that serves to shield the
  application heterogeneity of the underlying
  computer platforms and networks

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Middleware Scenario

• Web server passes the request for data from a
  database to the middleware
• Middleware renders it into a form meaningful as a
  query to the database
• Database responds with data to the middleware
• Middleware formats into html and sends it to the
  Web server
• Web server sends the page to the user making the
  request

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Middleware Models

• Point-to-Point Middleware Linking one
  application to another, sending a procedure call
  or message without engaging in middle-tier
  processing in terms of logic or mapping
• Many-to-many Linking each application to many
  other applications making use of integration
  servers and other variations

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Communication Modes

• Synchronous client waits for the result,
  coupling the applications tightly, and making
  unnecessarily wasteful use of the communication
  medium. A somewhat better alternative would be
  deferred Synchronous mode where the client asks
  for result by polling. Either way the nature of
  communication is direct
• Asynchronous Client send the request and
  continues with its work. The server initiates
  contact with the client when ready with results
• Queued In most cases the messages for the
  server may be queued and processed one-by-one in
  some order of priority

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(Continued )

• Publish/Subscribe sending request to a pub/sub
  engine or broker. The broker then forwards it to
  interested (subscribing) applications. Allows
  for anonymity, flexibility, and generality among
  participating applications
• Fire/Forget broadcasting messages to multiple
  applications JAXM (Java API for XML Messaging)
  supports synchronous/asynchronous calls with
  acknowledgements and the fire/forget messaging
  option

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Middleware Categories

• Transaction Oriented offers a tuple abstraction
  (SQL) and distributed transaction processing
• Examples
• IBM CICS, Encina
• BEA Tuxedo
• Microsoft Transaction Server
• LDAP - Lightweight Directory Access Protocol, is
  an Internet protocol that email programs use to
  look up contact information from a server, such
  as ClickMail Central Directory, a simple protocol
  to access remote directories, and not
  transactions, LDAP servers index all the data in
  their entries, and "filters" may be used to
  select just the person or group you want, and
  return just the information you want

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(Continued )

• Message-oriented Middleware (MOM) offers a
  mailbox abstraction, asynchronous mode, generally
  queuing software, not requiring direct
  connection, there are some implementations that
  rely on a broadcast or multicast messaging system
• Examples
• Microsoft Message Queue (MQ)
• TIBCO Rendezvous
• IBM Websphere MQ (MQSeries MQSeries)
• Sonic Software's SonicMQ
• Java Message Queue
• Coridan MantaRay (http//www.mantamq.org) (open
  source)
• ActiveMQ (open source)
• XmlBlaster (http//www.xmlblaster.org) an
  open-source Java MOM implementation

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(Continued )

• Procedural Middleware (RPC) offers a procedural
  abstraction, synchronous client/server
  interaction, RPC allows server components to be
  defined using an IDL, compiling client server
  stubs, suffer from scalability problems
• Examples
• Sun Open Network Computing (ONC)
• Unix/Linux/DCE RPC
• OSF DCE
• Netbula JavaRPC
• PowerRPC

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(Continued )

• Object-Oriented Middleware offers an object
  abstraction, synchronous/ asynchronous
  client/server interaction, Like RPC (Remote
  Procedure Call), OOM offers synchronous, typed
  communication between components of a distributed
  program, system-wide through the Object Directory
  (Interface Trader, Yellow Pages)
• Examples
• OMG CORBA
• Sun Java/RMI
• SOAP (RPC on top of HTTP, XML RPC)
• Microsoft COM/DCOM
• Sun Enterprise Java Beans
• .NET (Virtual Machine like Java)

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(Continued ..)

• Transaction Monitors runtime software that
  monitors a transaction as it passes from one
  stage in a process to another, ensuring that the
  transaction processes completely or, if an error
  occurs, to take appropriate actions, TP monitors
  are especially important in three-tier
  architectures that employ load balancing because
  a transaction may be forwarded to any of several
  servers based on their availability, TP monitor
  technology provides the distributed client/server
  environment the capacity to efficiently and
  reliably develop, run, and manage transaction
  applications.

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(continued )

• Application Servers generally operate as web
  enabled middleware, processing actions from web
  enabled applications, providing application logic
  processing and sharing, connecting to backend
  resources

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(continued )

• Integration Servers can account for differences
  in cooperating application semantics and
  platforms, transforming message schemas and alter
  the contents as needed

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Other Middleware Examples

• Closed Source MS ASP, Macromedia Cold Fusion
• Closed/Open UserLand Frontier/Manila
• Open Source PHP/MySQl, JSP, Perl, Python, Zope
  (built with Python)

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PHP Scenario

• A web browser issues a request for a file with
  .php extension
• Web Server takes this file and sends it as an
  input request to the PHP parser
• The PHP parser finds PHP specific code in the
  file, processes it, and places the output back in
  the file in place of the PHP code
• The new output file is sent back to the web
  server
• Web server sends the file to the requesting
  browser
• The browser displays the output