Object Relational Mapping

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Object Relational Mapping

• John M. Miller
• Perpetual Data Systems

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John M. MillerjMiller_at_pdata.com

• Independent Software Architect, Designer
  Developer
• PDC 2002 Speaker
• Collaborator on adding advanced business rule
  support to a commercial .Net application
  framework
• Member of a team submitting a response to the OMG
  request for a method for formalizing business
  rules
• XSLT transformations from ORM conceptual models
  into .NET entity business object implementation

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Overview

• What is Object Relational Mapping
• Why did Microsoft back away?
• Is OR Mapping dead?
• What if it isn't?
• What does it mean for .NET application
  architectures?
• Which comes first the data chicken or object egg?

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What is Object Relational Mapping

• Something that persists objects into relational
  databases
• Can be
• a software layer using dynamic SQL generation
• a method of generating static SQL
• Implicit with Domain object models
• DataSets need not apply

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Why is OR Mapping Important?

• Why are domain models important?
• Because

I said so

• They abstract the data access code hiding the
  database details
• They encapsulate the database in an interface
  that more closely matches the conceptual domain.
• Object Models are more expressive for certain
  kinds of object interactions
• Constraints become easier to implement
• Object notation is a natural way of navigating a
  join path
• ACustomer.OrdersNumber1001.ItemsItem33.Produ
  ct.Price
• Constraints become easier to implement
• If AProduct.OnHand lt AItem.Quantity Then raise
  BizRuleXceptn

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Mappers

• Domain models put the responsibility of
  reconciling design differences between the object
  and data models on the persistence layer
• Persistence Layers
• Static
• Hand coded
• Generated
• Dynamic

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Problems with OR Mapping

• A weakness in the mapper limits the capability of
  the object model and in turn the capability of
  the application
• No standard object query language
• It cant be done, not practical, a waste of time,
  etc.
• It costs time and money
• The dread impedance mismatch

Wikipedia, C2
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The Impedance Mismatch

• "When you notice that you're experiencing
  considerable pain, and you recognize that a
  substantial amount of the pain occurs in and
  around the interface between your code and the
  relational database, then you've found the
  Object/Relational Impedance Mismatch."

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The Impedance Mismatch

• Technical
• Object Modeling and Data Modeling model
  fundamentally different things
• Objects Process
• Data persistence is secondary
• Data Structure
• What about static class diagrams?
• Fundamentally flawed for complex structural
  modeling
• Different Optimizations
• Identification schemes are a problem

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The Impedance Mismatch

• Cultural
• Created by different camps
• Software Architects
• The equally dread Database Administrator
• Night Day, Oil Water, Pickles Ketchup

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The Impedance Mismatch

• Reality
• Neither domain models nor relational databases
  are going away.
• Mappers are the price we pay for playing
• Does it really exist?

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So why did Microsoft back away?

• ObjectSpaces
• Billed as the Microsoft OR Mapper
• Due to ship with Visual Studio 2005
• Rolled into WinFS
• Because
• Overlapped with some of the WinFS persistence
  story
• Different APIs and Microsoft wanted to unify the
  APIs
• Microsoft didnt want to publish ObjectSpaces and
  then release an incompatible WinFS

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So why did Microsoft back away?

• WinFS
• Windows object store
• Due to ship with Longhorn
• Pushed back until after Longhorn
• Because
• It wasnt going to be ready in time
• Object Persistence is harder than they thought
• E12

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So why did Microsoft back away?

• To the net effect of leaving the market without a
  Microsoft OR Mapper until?
• 2008?
• 2009?
• 2010?
• After first creating a buzz with the initial
  previews of ObjectSpaces and generating
  significant market momentum
• Leaving the development world to wonder
• Is OR Mapping dead?

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Is OR Mapping Dead?

• In the Microsoft community
• Almost
• There are several implementations that were
  available before ObjectSpaces that are still
  around
• And a few created since
• One that emulates the ObjectSpaces preview API
• In the Java community
• Not even close
• Widely adopted
• Not if, but which one

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What if it isn't?

• You could miss out on a technology that could
  make an application easier to design, implement
  and maintain
• What is the .NET OR faithful to do?
• Wait for Microsoft
• Adopt an existing .NET OR Mapper until Microsoft
  does something
• Try WORM
• Adopt an existing .NET OR Mapper permanently
• nHibernate is hot right now
• Switch to Java
• Like you, they get OR mappers

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What does it mean for .NET application
architectures?

• XML and DataSets will continue to rule the DAL
  world until a dominate OR mapper for .NET appears
• OR Mapping will continue to be used sparingly and
  grow as the mappers mature
• Adoption rate in the .NET community should mimic
  the adoption rate for OR mappers in the Java
  community, but at faster pace
• Should be widely adopted in the .NET community in
  3-4 years
• Hold your breath when WinFS ships

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Which comes first the data chicken or object egg?

• Lets say you are starting a new project with
• Complex business logic
• Complex data relationships
• Interested in creating a domain model and and
  relational database.
• What do you do first?
• Call me
• Create the object model you want to persist
• Create the data model that you know you need
• If you do either 2 or 3 you have to deal with the
  dread impedance mismatch

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The Object Egg

• You create the domain object model first using
  standard OOAD methods
• Then derive the data structure from the object
  model
• Problems
• Number of tables
• Normalization
• Fragile Structures

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The Data Chicken

• You create the data model first using standard
  data modeling methods
• The derive the domain model from the data model
• Problems

• You tell me?

• Fidelity Loss

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What then?

• Conceptual Models
• Design the structures conceptually first using a
  formal conceptual modeling method
• Derive both the entity object model and data
  model from the conceptual model
• Problems
• Your domain object model needs to collaborate
  with the entity objects
• No standards
• Complex transformation

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What next?

• 2/1/2005 - San Diego .NET Developer Group Meeting
• Implementing .NET Applications using Conceptual
  Models

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Thank You!

• jMiller_at_pdata.com
• pdata.com/jMM
• shrinkster.com/3bt
• en.wikipedia.org/wiki/Object-relational_mapping
• c2.com/cgi/wiki?ObjectRelationalMapping
• theserverside.com/books/review/HibernateReview.tss
  
• devx.com/vb2themax/Article/19894
• sddotnetdg.org/Events/SDDOTNETMeetings/default.as
  px