Distributed Programming in Java

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Distributed Programming in Java

• Web Services (6)

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Business Process
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• Business activity unit of work
• Business process
• set up a sequence of business activities to
  process the order
• set up data into the data pool

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Product Order Process
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setup

• Creates the sequence of activities

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setData

• Bootstraps the common data pool

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createProductOrder

• Creates a process order, and kicks off a sequence
  of business activities

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processOrder

• Processes an order by executing business
  activities in sequence as defined

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Publish-Subscribe

• Stand-alone event service that receives and
  distributes events to other services
• For example, if we use an asynchronous business
  process we would want to notify a Customer on
  completing their order
• Service forwards published events to subscribers
  without requiring subscribers to be aware of
  publishers (as in Observer)

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Motivation

• A single event mechanism instead of writing many
  specific ones
• A situation where we cannot anticipate all
  publishers of events
• Addresses needs by centralizing event
  distribution in a generic mechanism
• Mechanism can be thoroughly tested

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Topic-Based Subscription

• Topics are names of events
• Events identified by topics do not need to follow
  a particular interface
• We dont even need to know originators

Topic hierarchy
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Publish-Subscribe
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Participants (1)

• Subscriber
• Interface that event service calls to notify a
  client about an event it subscribed for
• May register for multiple types of events
• SubscriberImpl
• Subscriber web service with a distinct URL for
  sending update messages

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Participants (2)

• EventService
• Subscribers can subscribe to events
• Publishers can publish events, and events and
  data will be forwarded to subs
• EventServiceImpl
• Maintains subscriber lists and event types for
  which a subscriber registers
• Determines which subscribes to notify

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Participants (3)

• PublisherImpl
• Assembles information that subscribers expect to
  see, which is an implicit contract between
  subscribers and publishers
• Delivers event type and associated data to the
  event service for distribution
• Loosely coupled when subscribers register for
  event types they do not need to care how
  publishers deliver events

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Dynamics
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Design Considerations

• Subscription lists should be persistent
• Event service can become a bottleneck between
  publishers and subscribers
• Event data structure and topic should be
  considered part of the contract
• between publishers and subscribers
• so subscribers can be programmed against stable
  assumptions

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EventService
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EventService
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service.getSubscriber

• service.getSubscriber() returns a stub to the
  Subscriber service we want to notify
• Can specify a different URL than the default by
  invoking getSubscriber(URL) directly

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Subscriber

• Create a subscriber web service, register it
  for the ProductOrderUpdate topic

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Subscriber

• It uses the EventServiceLocator which was
  generated from the WSDL file

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Generating the WSDL

• These commands generate the WSDL files

!/bin/sh java org.apache.axis.wsdl.Java2WSDL
\ -o "com/servicefoundry/books/webservices/eventse
rvice/Subscriber.wsdl" \ -l "http//localhost9090
/axis/services/Subscriber" \ -n "urnPubSub"
\ -p"com.servicefoundry.books.webservices.eventser
vice.stubs" "urnPubSub" \ com.servicefoundry.book
s.webservices.eventservice.Subscriber java
org.apache.axis.wsdl.Java2WSDL \ -o
"com/servicefoundry/books/webservices/eventservice
/EventService.wsdl" \ -l "http//localhost9090/ax
is/services/EventService" \ -n "urnPubSub"
\ -p"com.servicefoundry.books.webservices.eventser
vice.stubs" "urnPubSub" \ com.servicefoundry.book
s.webservices.eventservice.EventService
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Stub Generation

• Generate the stubs from the WSDL
• These commands create the EventService stub in
  the ....stubs package

!/bin/sh java org.apache.axis.wsdl.WSDL2Java -o
. \ -NurnPubSub com.servicefoundry.books.webservi
ces.eventservice.stubs \ com/servicefoundry/books/
webservices/eventservice/Subscriber.wsdl java
org.apache.axis.wsdl.WSDL2Java -o .
\ -NurnPubSub com.servicefoundry.books.webservice
s.eventservice.stubs \ com/servicefoundry/books/we
bservices/eventservice/EventService.wsdl
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Publisher

• Publisher does not implement a service

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Physical Tiers

• Subscriber should not execute lengthy business
  logic in the Axis container
• Inefficient use of connection pooling,
  persistence layer, etc.
• Implement the logic in another physical tier and
  talk to it using a Connector
• Example connector is RMI

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Problem with Publisher/Subscriber

• Client must provide a web service itself to be
  able to receive notification from another web
  service
• Standalone application is not bundled with an
  integrated full-blown web service engine
• How can a standalone client application receive
  notification?

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Faux Implementation

• Requiring subscribers to run an Axis engine is
  too heavy-weight a solution
• Instead, clients can use a simple solution to
  receive callbacks, which amounts to implementing
  your own SOAP processing
• However, you dont want to affect the service
  invoker (eg EventService) in any way
• Thus, you make it look like the real thing
• Use a class that executes in a separate thread
  and is waiting for incoming socket connection
  with SOAP messages

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Faux Implementation
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References

• In todays lecture we covered in detail
• Chapter 12, Publish-Subscribe
• And briefly discussed
• Chapter 13, Physical Tiers
• Chapter 14, Faux Implementation
• This gives us an overview of most of the patterns
  documented in PM