More Object Design with GoF Patterns CSSE 574: Session 7, Part 2

Transcription

1 More Object Design with GoF Patterns CSSE 574: Session 7, Part 2 Steve Chenoweth Phone: Office (812) Cell (937) chenowet@rose-hulman.edu

2 Topics in this discussion More Object Design with GoF Patterns With more in the next set of slides 2

3 Applying Patterns to NextGen POS Iteration 3 Local caching Failover to local services Support for third-party POS devices Handling payments

4 Failover & Performance with Local Caching What is a cache? How does a cache usually work? Why use a local cache for NextGen POS? Performance Improve recoverability

5 Search Strategy for Product Information 1. Look in memory (in map stored by ProductCatalog) 2. Look on local hard drive cache 3. Retrieve from remote persistence service A traditional strategy for semi-real-time data needs like Inventory lookup (here, POS item scanning) Interactive account accessing (say, cell phones) Images from andhttp://

6 Applying the Adapter Pattern True Adapters

7 Using a Factory to Set Up for Local Caching :Store 2: create(pc) create :Register 1: create the local service is returned Eliminate local cache? pc: ProductCatalog 1.1: psa = getproductsadapter() :ServicesFactory : create( externalservice ) 1.1.1: create psa : LocalProducts externalservice : DBProductsAdapter Change external data source? the local service gets a reference to the adapter for the external service

8 Product Lookup with In-memory Miss

9 Product Lookup with Local Cache Miss

10 Using Threads to Freshen the Cache Async msg

11 How s the final iteration going? Used by permission.

12 Handling Failure in NextGen POS What should happen if there is a local cache miss and the external product information service fails?

13 DCD: Exceptions Caught and Thrown ome UML tools allow one to specify (in a dialog box) the exceptions that an operation throws. usagestatistics : Map PersistenceFacade Object get(key, Class) throws DBUnavailableException, FatalException put(key, Object) { exceptions= (DBUnavailableException, FatalException) }... FatalException DBUnavailableException exceptions 13

14 Showing Exception in Sequence Diagrams ps = getdescription(id) : DBProducts Adapter : Persistence Facade : java.sql.statement note the difference between synchronous and asynchronous message arrowheads in the UML «exception» ProductInfoUnavailableException() ps = get(...) «exception» DBUnavailableException() resultset = executequery(...) «exception» SQLException() Name the Problem not UML notation: the Thrower Convert Exceptions pattern All asynchronous messages, including exceptions, are illustrated with a stick arrowhead. Exceptions are shown as messages indicated by the exception class name. An optional «exception» or «signal» stereotype is legal (an exception is a kind of signal in the UML), if increased visibility is desired. stopping the message line at this point indicates the PersistenceFacade object is catching the exception

15 How should NextGen POS handle this exception? Common exception handling patterns Use a central error logging object to record all exceptions for diagnosis by developers Use a standard, application-independent, non-ui object to notify users Can delegate to multiple different UI notifications Protected Variation for changes in reporting

16 Failover to Local Services with a Proxy

17 Proxy GoF Pattern Problem: How do we control access to some subject object if we want to avoid giving direct access? Solution: Add a level of indirection with a proxy object that implements the same methods as the subject and conditionally delegates to it.

18 Structure of the Proxy Pattern Client subject : ISubjectInterface dobar() 1 1 foo() «interface» ISubjectInterface {... whatever subject.foo()... whatever } subject actually references an instance of Proxy, not RealSubject {... pre-processing realsubject.foo()... post-processing } Proxy realsubject : ISubjectInterface foo() "realsubject" will actually reference an instance of RealSubject foo() RealSubject

19 Proxy in NextGen POS Posting sales to the accounting service Send postsale(sale) to a redirection proxy Proxy attempts to post to external service If it fails, then proxy stores result locally

20 Proxy in NextGen POS DCD

21 Proxy in NextGen POS Object Diagram

22 CD for Proxy to failover to local storage if the remote accounting service is down