Middleware emergence
 Distributed computing evolved from single-tier
(terminal/mainframe) to 2-tier (client-server)
to n-tier architecture
 To support heterogeneous environments open
networking standards were introduced (i.e. OSI)
 How do we build/connect applications over a network?
 Answer: middleware
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N-tier architecture
 More scalable architecture that separates
processing to different servers
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What is Middleware?
 Middleware enables applications running across
multiple platforms to communicate with each other .
 Middleware shields the developer from dependencies
on Network Protocols, OS and hardware platforms.
 Middleware is a software layer that lies between the
operating system and the applications on each site of
the system.
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ODBC – primitive middleware?
 Example of primitive middleware: ODBC
 Client application is shielded from the details of the
database implementation
 Not true middleware: defines client side software only
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Types of Middleware
 Homegrown Middleware Solutions
 RPC (Remote Procedure Calls)
 Object-Oriented MW - ORB (Object Request
Brokers)
 MOM (Message Oriented Middleware)
 Transaction Processing Monitors
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Homegrown Middleware
 Customized to meet specific needs
 Needs substantial investments
Pros
 Can meet specific needs
Cons
 Not cost effective
 Lacks scalability
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Remote Procedure calls
 Procedure/Function Oriented
 Follows the Request/Reply Model
 Mode of Communication is synchronous
 Provides Location/Platform transparency
 Technologies: ONC RPC, DCE/RPC
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Object Oriented
 Language Independent
 Provides Location and Access transparency
 Mode of Communication is Synchronous
 Competing Standards
(CORBA, DCOM, RMI)
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Message Oriented
 Data is transferred between applications as a message
 Mode Of Communication :- Asynchronous
 Basic Components of MOM
1.Message (Information that needs to be transferred)
2.Queue (A container used to store and distribute
messages)
 Types Of MOM
1. Message Queuing (Store and Forward)
2. Publish / Subscribe
 Products: IBM WebSphere MQ, TIBCO Rendezvous
App-1
Queue
App-2
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Transaction Processing Monitors
 Transaction Processing Monitors are used for
coordinating and monitoring the efforts of various
application in a distributed environment.
 Products: Tuxedo, IBM CICS, Microsoft Transaction
Server
App1
Client App
App2
TPM
App3
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Property of Transactions
 Transaction is a collection of operations that should
be treated as a single logical operation.
 To ensure reliable transactions ACID properties are
needed
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Property of Transactions
ACID properties:
 A: Atomicity: either the entire set of operations
happens or none of it does.
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Property of Transactions
ACID properties:
 A: Atomicity: either the entire set of operations
happens or none of it does.
 C: Consistency: the set of operations taken together
should move the system from one consistent state to
another.
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Property of Transactions
ACID properties:
 A: Atomicity: either the entire set of operations
happens or none of it does.
 C: Consistency: the set of operations taken together
should move the system from one consistent state to
another.
 I: Isolation: even though multiple transactions may
operate concurrently, there is a total order on all
transactions. Stated another way: each transaction
perceives the system as if no other transactions were
running concurrently.
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Property of Transactions
 Transaction is a collection of operations that should be treated
as a single logical operation.
ACID properties:
 A: Atomicity: either the entire set of operations happens or
none of it does.
 C: Consistency: the set of operations taken together should
move the system from one consistent state to another.
 I: Isolation: even though multiple transactions may operate
concurrently, there is a total order on all transactions. Stated
another way: each transaction perceives the system as if no
other transactions were running concurrently.
 D: Durability: even the face of a crash, once the system has said
that a transaction completed, the results of the transaction
must be permanent.
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Why Web Services?
 Lack of standardization in middleware
 Web services basic platform is available everywhere
(XML, HTTP)
 Can serve as an integration tool for different
business applications across organization boundaries
without regards to what each business uses as the
internal platform
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Why Web Services?
Can serve as an integration tool for different
business applications across organization boundaries
without regards to what each business uses as the
internal platform
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What are Web Services?
 Web services provide a standard means of
interoperating between different software
applications, running on a variety of platforms.
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Web Services
 Web services provide a standard means of
interoperating between different software
applications, running on a variety of platforms.
 W3C Working Group definition:
A Web service is a software system designed to support
interoperable machine-to-machine interaction over a
network…
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Web Services
 Web services provide a standard means of
interoperating between different software
applications, running on a variety of platforms.
 W3C Working Group definition:
A Web service is a software system designed to support
interoperable machine-to-machine interaction over a network.
It has an interface described in a machine-processable
format (specifically WSDL).
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Web Services
 Web services provide a standard means of
interoperating between different software
applications, running on a variety of platforms.
 W3C Working Group definition:
A Web service is a software system designed to support
interoperable machine-to-machine interaction over a network.
It has an interface described in a machine-processable
format (specifically WSDL). Other systems interact with the
Web service in a manner prescribed by its description using
SOAP messages, typically conveyed using HTTP with an XML
serialization in conjunction with other Web-related standards
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Web Services
 Specifications for Web Services are defined as
modules:



SOAP
WSDL
UDDI
 Basic platform is XML over HTTP
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Web Services
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XML
 Simple markup, data description language
(based on SGML)
 Human-readable
 Customizable schema can be used to define atributes
and elements
 XML can be used to created custom XML-based
languages and services
 XML-based messages transported over HTTP serve as
the basic platform for Web Services
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SOAP
 XML-based protocol defining message format
 One-way asynchronous technology
 Can use a variety of message passing styles: RPC,
publish/subscribe
 Primary underlying protocol is HTTP, but others can
be used (SMTP)
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WSDL
 XML-based language
 Defines/describes Web services interfaces, data and
message types, interaction patterns and protocol
mappings
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UDDI
 Web Services Registry (of WSDL documents)
 Protocol for discovering and publishing Web Services
 UDDI registry is accessed by XML-based SOAP
messages
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