Presentation on theme: "Object Relational Mapping John M. Miller Perpetual Data Systems."— Presentation transcript:
Object Relational Mapping John M. Miller Perpetual Data Systems
John M. Miller jMiller@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
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?
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
Why are domain models important? Because Why is OR Mapping Important? 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.Orders[Number=1001].Items[Item=33].Product.Price Constraints become easier to implement If AProduct.OnHand < AItem.Quantity Then raise BizRuleXceptn
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
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 can’t be done, not practical, a waste of time, etc. It costs time and money The dread impedance mismatch Wikipedia, C2
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."
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
The Impedance Mismatch Cultural Created by different camps Software Architects The equally dread Database Administrator Night & Day, Oil & Water, Pickles & Ketchup
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?
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 didn’t want to publish ObjectSpaces and then release an incompatible WinFS
So why did Microsoft back away? WinFS Windows object store Due to ship with Longhorn Pushed back until after Longhorn Because It wasn’t going to be ready in time Object Persistence is harder than they thought E12
So why did Microsoft back away? To the net effect of leaving the market without a Microsoft OR Mapper until? 2008? 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?
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
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
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
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? 1.Call me 2.Create the object model you want to persist 3.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
The Object Egg You create the domain object model first using standard OOA&D methods Then derive the data structure from the object model Problems Number of tables Normalization Fragile Structures
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
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
What next? 2/1/2005 - San Diego.NET Developer Group Meeting Implementing.NET Applications using Conceptual Models
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