Distributed Programming in Java

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Distributed Programming in Java

• Review (2)

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Topics

• Concurrency
• Networking and event handling
• Baseline architecture
• Remote objects
• Web services
• Space-based computing

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Remote Objects

• What problems?
• location transparency
• What mechanisms and Java constructs?
• RMI
• What patterns?
• broker, client-server-dispatcher, proxy, and
  active object

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Broker

• Separate communication infrastructure from
  application functionality via a federation of
  brokers that mediates all the communication
  between the components of the system

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Broker

• Structure of the Broker pattern

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Separation of Behavior and Implementation

• Remote object behavior defined in an interface
  (Service)
• Remote object behavior implemented in a server
  class (ServiceImpl)
• Proxy sends request to remote object

Also see Broker pattern
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Print Server

• Remote server with callbacks to clients

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Active Object

• Objectify service invocations and let server
  (active object) schedule the execution

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Applying the Pattern
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PrinterImpl
public void submit(PrinterListener listener,
PrintJob job) throws RemoteException try
job.submittedBy listener jobs.put(job)
catch (InterruptedException e) public
void run() while (true) try PrintJob
job (PrintJob) jobs.take() print(job)
// process next job job.submittedBy. //
get callback client completed(job) //
notify the client catch (Exception e)
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Web Services

• What problems?
• interoperability, deployment
• What mechanisms and Java constructs?
• SOAP, WSDL, Axis
• What patterns?
• service-oriented architecture, architecture
  adapter, and business objects
• business process, and publish-subscribe
• physical tiers, faux implementation

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Web Services

• What are web services? In essence, a technology
  for application integration based on open
  standards (HTTP, XML)
• In what sense are they related to the Web
  (capital W to refer to the WWW)?
• In fact, deploying web services over the web is
  more of an artefact than a necessity ...
• What we care about is the web of services

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Web of Services

• Typical (recursive) composition of web services
  as a web of services

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Service-Oriented Architecture

• Servers describe their services in a WSDL
  document, and publish them (eg UDDI)
• Clients bind to services (create proxies from
  WSDL), and invoke using SOAP

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Architecture Adapter

• WS frameworks such as Axis allow you to be fully
  WS-agnostic to access WS from Java use method
  calls via architecture adapters

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Business Objects

• Objects are marginalized in the SOA
• Web services require a flat object model
• If objects are passed as parameters, they need to
  be implemented as beans
• In Axis, beans can be defined through
  ltbeanmappinggt tags in WSDD
• Also applied to flow logic by moving it out from
  the programming language (BPEL !)

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Using Axis Step by Step

• Provide a Java interface or class that describes
  the service interface
• Create WSDL using Java2WSDL tool
• Create bindings through WSDL2Java tool
• Implement the service interface
• Deploy the service
• Implement clients using generated stubs

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Retailer
public void submitOrder(OrderHolder orderHolder)
throws NoSuchProductException,
NoItemCanBeShipped ... SupplierService
service new SupplierServiceLocator() //
contact the first supplier Supplier port1
service.getSupplier( new URL("http//localhost9
091/axis/services/Supplier")) port1.placeOrder(o
rderHolder) // check if order has been
satisfied if (!orderSatisfied(orderHolder.valu
e)) Supplier port2 service.getSupplier( n
ew URL("http//localhost9092/axis/services/Suppli
er")) port2.placeOrder(orderHolder) ...
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Deployment
ltdeployment xmlns"http//xml.apache.org/axis/wsdd
/" xmlnsjava"http//xml.apache.org/axis/wsdd/p
roviders/java"gt ltservice name"Retailer"
provider"javaRPC"gt ltparameter
name"className" value"a3.retailer.RetailerImpl"/
gt ltparameter name"scope" value"Application"/gt
ltparameter name"allowedMethods"
value"submitOrder"/gt ltbeanMapping
qname"nsOrder" xmlnsns"urnRetailer"
languageSpecificType"javaa3.util.Order"/gt
ltoperation name"submitOrder"gt ltparameter
name"order" mode"INOUT"/gt lt/operationgt
ltbeanMapping qname"nsNoSuchProductException"
xmlnsns"urnRetailer" languageSpecificType"jav
aa3.retailer.NoSuchProductException"/gt
ltbeanMapping qname"nsNoItemCanBeShipped"
xmlnsns"urnRetailer" languageSpecificType"jav
aa3.retailer.NoItemCanBeShipped"/gt lt/servicegt
lt/deploymentgt
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Business Process
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Participants

• BusinessProcess
• Interface to the business process
• BusinessProcessImpl
• Logic of the business process, which can be a
  simple ActivitySequence
• BusinessActivity
• Unit of work in the business process
• Data
• Captures side effects of business activities (in
  this pattern a common pool)

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Publish-Subscribe
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Participants

• Subscriber
• Interface that event service calls to notify a
  client about an event it subscribed for
• SubscriberImpl register with distinct URLs for
  receiving update messages
• EventService
• Subscribers can subscribe to events
• Publishers can publish events, and events and
  data will be forwarded to subs

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Space-Based Computing

• What problems?
• coordination
• What mechanisms and Java constructs?
• tuple spaces
• What patterns?
• supervisor-worker

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Space-Based Computing

• A space is a high-level coordination tool for
  glueing agents into an application
• Different programming paradigm
• Instead of relying on message-passing,
• applications cooperate through the flow of
  objects in and out of spaces
• Space-based models (JavaSpaces, TSpaces) based on
  tuple space concepts

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CameraBuyer
public class CameraBuyer public static void
main(String args) try System.setSecurit
yManager(new RMISecurityManager()) TupleSpace
space (TupleSpace) Naming.lookup("rmi//"
args0 "/" args1) String make
args2 Tuple cameraTemplate new
Tuple("camera", new Field("make",
make), new Field("price", Integer.class))
System.out.println("Buyer.in "
cameraTemplate) Tuple camera
space.in(cameraTemplate) System.out.println("M
atch " camera) System.out.println("Camera.p
rice " camera.getField("price"))
catch (Exception e) System.err.println(e)

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Agenda Parallelism

• Work units organized around activities
• Processes masters and workers

Master
Workers