.NET Database Technologies: Open-Source Frameworks

Looking beyond Microsoft Many third-party ORM tools and frameworks are available for.NET and for other languages/platforms High level of maturity in the Java world in particular Open-source projects and commercial products We will look at two (closely-related) open-source frameworks which implement different patterns  Castle ActiveRecord  NHibernate

Active Record An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data Implemented by Castle ActiveRecord Part of Castle Project, which also includes Windsor, Dynamic Proxy, etc. Built on top of NHibernate

ActiveRecordBase Active record pattern  instance properties representing a record in the database  instance methods acting on that specific record  static methods acting on all records ActiveRecord implements this through ActiveRecordBase class All persistent classes inherit from it ActiveRecordBase implements a basic set of storage and retrieval operations, e.g. Create, FindAll, Save, Update Persistent classes can implement additional operations

ActiveRecord mapping Active record pattern assumes a fairly direct correspondence between objects and database “an object that wraps a row in a database table” Castle ActiveRecord is a bit more flexible than that basic description of pattern implies Makes use of capabilities of Nhibernate Offers a subset of NHibernate mapping functionality, including inheritance mapping Mapping is done using class, fields and property level attributes ActiveRecord is able to infer table and column names if you omit them  Convention over configuration

ActiveRecord queries Persistent classes can retrieve data and populate objects in the following ways:  Calling methods of base class  Calling methods of base class and using NHibernate criteria  Using SimpleQuery class to execute HQL queries (Hibernate Query Language)  Native SQL queries (through NHibernate)  LINQ(?)

Another pattern – Query Object An object that represents a database query. A Query Object is a structure of objects, including criteria, that can form itself into a SQL query You can create this query by referring to classes and fields rather than tables and columns In this way those who write the queries can do so independently of the database schema and changes to the schema can be localized in a single place

ActiveRecord and PI Not a chance! ActiveRecord is intended to be simple to use, without the need to learn NHibernate mapping schema Favours convention over configuration (this is not specifically a property of the Active Record pattern) Suitable for Rapid Application Development rather than Domain Driven Development

Not just.NET - Ruby on Rails Probably the best known Active Record implementation Works with the Ruby language Persistent classes inherit from ActiveRecord::Base class Rails also implements MVC and favours convention over configuration throughout Other implementations exist for PHP, Java, etc.

Data Mapper/Metadata Mapping - NHibernate The Data Mapper is a layer of software that separates the in-memory objects from the database A Metadata Mapping allows developers to define the mappings in a simple tabular form, which can then be processed by generic code to carry out the details of reading, inserting, and updating the data NHibernate is an ORM which implements these patterns Relatively mature technology, ported from Hibernate which is long established for Java

NHibernate Unit of Work Session is roughly equivalent to EF’s ObjectContext Implements Unit of Work and Identity Map patterns Session instance is created by a SessionFactory object which is built by reading configuration file Session opens a Transaction in which to do the work Common practice to wrap Session in a custom Unit of Work at a higher level of abstraction

NHibernate mapping Mappings are defined in XML files or by annotations Very powerful and flexible mapping capabilities Detailed control of loading strategies, etc. Can map select, update, etc to stored queries Configuration (database connection, etc) also defined in XML files

NHibernate queries NHibernate can retrieve data and populate objects in the following ways (among others):  HQL queries  Criteria queries  Native SQL queries  LINQ  Named queries (HQL or SQL) defined in mapping

NHibernate and PI NHibernate persistent classes are POCOs Good adherence to PI principle Favoured by adherents of DDD

Fluent NHibernate Uses Fluent API to define NHibernate configurations and mappings in code Type-safe, no XML Similar to EF Code-First mappings (actually, it’s the other way round) Favours convention over configuration

Fluent APIs Term first coined by Evans and Fowler A fluent interface is normally implemented by using method chaining to relay the instruction context of a subsequent call The API is primarily designed to be readable and to flow Can be detrimental to debugging, as a fluent chain constitutes a single statement in which debuggers may not allow setting up intermediate breakpoints Becoming common for applications such as defining mappings, configurations and query objects (e.g. LINQ extension methods syntax)

ORMs and performance Using any ORM can be expected to have an impact on performance compared to ADO.NET  Overhead of extra layer on top of ADO.NET  Select N+1 problem  Sub-optimal SQL may be generated Hard to find reliable data to base decisions on  Some academic research (van Zyl, Cvetkovic)  Benchmarks ORMeter ( PolePosition (  Opinions and experience Weigh benefits against performance hit

Optimising ORM performance SQL Profiling Careful consideration of fetching/loading strategy within the application Careful design of mapping where possible Using stored procedures where appropriate Compiled queries and query-plan caching (LINQ) Pre-Build Store Views (EF) Tracking vs. No Tracking (EF) 2 nd level caching (NHibernate) etc...