Spring provides a generic factory for creating ADO.NET API artifacts such as IDbConnection and IDbCommand. The factory API is very similar to the one introduced in .NET 2.0 but adds extra metadata needed by Spring to support features provided by its DAO/ADO.NET framework such as error code translation to a DAO exception hierarchy. The factory itself is configured by using a standard Spring XML based configuration file though it is unlikely you will need to modify those settings yourself, you only need be concerned with using the factory. Out of the box several popular databases are supported and an extension mechanism is available for defining new database providers or modifying existing ones. A custom database namespace for configuration aids in making terse XML based declarations of Spring's database objects you wish to use.

The downside of Spring's factory as compared to the one in .NET 2.0 is that the types returned are lower level interfaces and not the abstract base classes in System.Data.Common. However, there are still 'holes' in the current .NET 2.0 provider classes that are 'plugged' with Spring's provider implementation. One of the most prominent is the that the top level DbException exposes the HRESULT of the remote procedure call, which is not what you are commonly looking for when things go wrong. As such Spring's provider factory exposes the vendor sql error code and also maps that error code onto a consistent data access exception hierarchy. This makes writing portable exception handlers much easier. In addition, the DbParameter class doesn't provide the most common convenient methods you would expect as when using say the SqlServer provider. If you need to access the BCL provider abstraction, you still can through Spring's provider class. Furthermore, a small wrapper around the standard BCL provider abstraction allows for integration with Spring's transaction management facilities, allowing you to create a DbCommand with its connection and transaction properties already set based on the transaction calling context.

The IDbProvider API is shown below and should look familiar to anyone using .NET 2.0 data providers. Note that Spring's DbProvider abstraction can be used on .NET 1.1 in addition to .NET 2.0

public interface IDbProvider
{
  IDbCommand CreateCommand();

  object CreateCommandBuilder();        

  IDbConnection CreateConnection();

  IDbDataAdapter CreateDataAdapter();

  IDbDataParameter CreateParameter();

  string CreateParameterName(string name);

  string CreateParameterNameForCollection(string name);

  IDbMetadata DbMetadata
  {
      get;
  }               

  string ConnectionString
  {
      set;
      get;
  }

  string ExtractError(Exception e);

  bool IsDataAccessException(Exception e);
    
}

ExtractError is used to return an error string for translation into a DAO exception. On .NET 1.1 the method IsDataAccessException is used to determine if the thrown exception is related to data access since in .NET 1.1 there isn't a common base class for database exceptions. CreateParameterName is used to create the string for parameters used in a CommandText object while CreateParameterNameForCollection is used to create the string for a IDataParameter.ParameterName, typically contained inside a IDataParameterCollection.

The class DbProviderFactory creates IDbProvider instances given a provider name. The connection string property will be used to set the IDbConnection returned by the factory if present. The provider names, and corresponding database, currently configured are listed below.

Note

If your exact version of the database provider is not listed, you can pick the general provider name, i.e. MySql.Data.MySqlClient, and then perform an assembly redirect in App.config. This will often be sufficient to upgrade to newer versions. As shown below <runtime> <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1"> <dependentAssembly> <assemblyIdentity name="Npgsql" publicKeyToken="5d8b90d52f46fda7" culture="neutral"/> <bindingRedirect oldVersion="0.0.0.0-65535.65535.65535.65535 newVersion="2.0.0.0"/> </dependentAssembly> </assemblyBinding> </runtime>

An example using DbProviderFactory is shown below

IDbProvider dbProvider = DbProviderFactory.GetDbProvider("System.Data.SqlClient");

The default definitions of the providers are contained in the assembly resource assembly://Spring.Data/Spring.Data.Common/dbproviders.xml. If the provider you want to use is not provided "out of the box" you can provide additional definitions. To do this follow the format of object definitions defined in the previously mentioned assembly resource.

From Spring 1.3.1 an on you can specify the additional Spring IResource location where additional providers are defined within Spring's XML configuration file. See the next section for an example. Alternatively, you can set the public static property DBPROVIDER_ADDITIONAL_RESOURCE_NAME in DbProviderFactory to a Spring resource location. The default value is file://dbProviders.xml. (That isn't a typo, there is a difference in case with the name of the embedded resource).

It may happen that the version number of an assembly you have downloaded is different than the one listed above. If it is a point release, i.e. the API hasn't changed in anyway that is material to your application, you should add an assembly redirect of the form shown below.

<dependentAssembly>
  <assemblyIdentity name="MySql.Data" 
                           publicKeyToken="c5687fc88969c44d" 
                           culture="neutral"/>
  <bindingRedirect oldVersion="0.0.0.0-65535.65535.65535.65535"
                         newVersion="1.0.10.1"/>
</dependentAssembly>

This redirects any reference to an older version of the assembly MySql.Data to the version 1.0.10.1.

Creating a DbProvider in Spring's XML configuration file is shown below in the typical case of using it to specify the DbProvider property on an AdoTemplate.

<objects xmlns='http://www.springframework.net'
         xmlns:db="http://www.springframework.net/database">

  <db:provider id="DbProvider" 
      provider="System.Data.SqlClient" 
      connectionString="Data Source=(local);Database=Spring;User ID=springqa;Password=springqa;Trusted_Connection=False"/>
  
  <object id="adoTemplate" type="Spring.Data.Core.AdoTemplate, Spring.Data">  
    <property name="DbProvider" ref="DbProvider"/>                
  </object>

</objects>

If you need to register an additional IDbProvider defintions from your own configuration file, set the attribute 'additonalDbProviders' to the IResource location of those definitions. Examples of the format for additional provider definitions can be found within the Spring.Data assembly, location assembly://Spring.Data/Spring.Data.Common/dbproviders.xml. Open it up in Visual Studio or Reflector to see the contents of the dbproviders.xml file.

<objects xmlns='http://www.springframework.net'
         xmlns:db="http://www.springframework.net/database">

  <db:additionalProviders resource="assembly://MyAssembly/MyAssembly.MyNamespace/AdditionalProviders.xml"/>
 
  <db:provider id="DbProvider" 
      provider="System.Data.SqlClient" 
      connectionString="Data Source=(local);Database=Spring;User ID=springqa;Password=springqa;Trusted_Connection=False"/>


</objects>

A custom namespace should be registered in the main application configuration file to use this syntax. This configuration, only for the parsers, is shown below. Additional section handlers are needed to specify the rest of the Spring configuration locations as described in previous chapters.

<configuration>

  <configSections>
    <sectionGroup name="spring">
      <section name="parsers" type="Spring.Context.Support.NamespaceParsersSectionHandler, Spring.Core" />
    </sectionGroup>
  </configSections>

  <spring>
    <parsers>
      <parser type="Spring.Data.Config.DatabaseNamespaceParser, Spring.Data" />
    </parsers>
  </spring>

</configuration>

There are a few options available to help manage your connection strings.

The first option is to leverage the Spring property replacement functionality, as described in Section 5.9.2.1, “Example: The PropertyPlaceholderConfigurer”. This lets you insert variable names as placeholders for values in a Spring configuration file. In the following example specific parts of a connection string have been parameterized but you can also use a variable to set the entire connection string.

An example of such a setting is shown below

<configuration>
  <configSections>
    <sectionGroup name="spring">
      <section name='context' type='Spring.Context.Support.ContextHandler, Spring.Core'/>
    </sectionGroup>

    <section name=
"databaseSettings" type="System.Configuration.NameValueSectionHandler, System, Version=1.0.5000.0,
	Culture=neutral, PublicKeyToken=b77a5c561934e089"
 />
       
  </configSections>

  <spring>
    <context>      
      <resource uri="Aspects.xml" />
      <resource uri="Services.xml" />
      <resource uri="Dao.xml" />
    </context>
  </spring>

  <!-- These properties are referenced in Dao.xml -->
  <databaseSettings>
    <add key="db.datasource" value="(local)" />
    <add key="db.user" value="springqa" />
    <add key="db.password" value="springqa" />
    <add key="db.database" value="Northwind" />
  </databaseSettings>

</configuration>

Where Dao.xml has a connection string as shown below

<objects xmlns='http://www.springframework.net'
         xmlns:db="http://www.springframework.net/database">

  <db:provider id="DbProvider" 
      provider="System.Data.SqlClient" 
      connectionString="${db.datasource};Database=${db.database};User ID=${db.user};Password=${db.password};Trusted_Connection=False"/>
  
  <object id="adoTemplate" type="Spring.Data.Core.AdoTemplate, Spring.Data">  
    <property name="DbProvider" ref="DbProvider"/>                
  </object>

  <!-- configuration of what values to substitute for ${ } variables listed above -->
  <object name="appConfigPropertyHolder"
          type="Spring.Objects.Factory.Config.PropertyPlaceholderConfigurer, Spring.Core">
    <property name="configSections" value="DatabaseConfiguration"/>
  </object> 

</objects>

Please refer to the Section Section 5.9.2.1, “Example: The PropertyPlaceholderConfigurer” for more information.

Spring provides some convenient implementations of the IDbProvider interface that add addtional behavior on top of the standard implementation.

	Note
	These provider implementations do not take into account usage with NHibernate. NHibernate scopes a SessionFactory, where second level caching is managed, to each connection. This forum thread, contains an implementation of the class LocalDelegatingSessionFactoryObject that will create multiple SessionFactories for each database connection.

<object id="DbProvider" type="Spring.Data.Common.UserCredentialsDbProvider, Spring.Data">
  <property name="TargetDbProvider" ref="targetDbProvider"/>
  <property name="Username" value="User ID=defaultName"/>
  <property name="Password" value="Password=defaultPass"/>
</object>
 
<db:provider id="targetDbProvider" provider="SqlServer-2.0"
    connectionString="Data Source=MARKT60\SQL2005;Database=Spring;Trusted_Connection=False"/>

userCredentialsDbProvider.SetCredentialsForCurrentThread("User ID=springqa", "Password=springqa");

19.5.2. MultiDelegatingDbProvider

There are use-cases in which there will need to be a runtime selection of the database to connect to among many possible candidates. This is often the case where the same schema is installed in separate databases for different clients. The MultiDelegatingDbProvider implements the IDbProvider interface and provides an abstraction to the multiple databases and can be used in DAO layer such that the DAO layer is unaware of the switching between databases. MultiDelegatingDbProvider does its job by looking into thread local storage. This storage location stores the name of the dbProvider that is to be used for processing the request.

MultiDelegatingDbProvider is configured using the dictionary property TargetDbProviders. The key of this dictionary contains the name of a dbProvider and its value is a dbProvider object. You can also provide this dictionary as a constructor argument. The property DefaultDbProvider can be set with the name of the DbProvider to use if no provider name is found in thread local storage

During request processing, once you have determined which target dbProvider should be use, in this example database1ProviderName, you should execute the following code is you are using Spring 1.2 M1 or later

// Spring 1.3.0 or later
MultiDelegatingDbProvider.CurrentDbProviderName = "database1ProviderName"

// Spring 1.2 M1 or later
LogicalThreadContext.SetData(MultiDelegatingDbProvider.CURRENT_DBPROVIDER_SLOTNAME, "database1ProviderName")

and the following ocde if you are using earlier versions

// Prior to Spring 1.2 M1
LogicalThreadContext.SetData("dbProviderName", "database1ProviderName")

and then call the data access layer.

	Note
	If you do not change the name of the IDbProvider stored in thread local storage during request processing, say in the web tier where a user is identified, then you will always refer to the default provider if the property DefaultDbProvider has been set. If the DefaultDbProvider property has not been set than an InvalidDataAccessApiUsageException will be thrown.

Here is a sample configuration to build up an object definition for MultiDelegatingDbProvider.

<db:provider id="CreditAndDebitsDbProvider"
    provider="System.Data.SqlClient"
    connectionString="Data Source=MARKT60\SQL2005;Initial Catalog=CreditsAndDebits;User ID=springqa; Password=springqa"/>

<db:provider id="CreditDbProvider"
    provider="System.Data.SqlClient"
    connectionString="Data Source=MARKT60\SQL2005;Initial Catalog=Credits;User ID=springqa; Password=springqa"/>

<object id="dbProviderDictionary" type="Spring.Collections.SynchronizedHashtable, Spring.Core">
  <property name="['DbProvider1']" ref="CreditAndDebitsDbProvider"/>
  <property name="['DbProvider2']" ref="CreditDbProvider"/>
</object>

<object id="DbProvider" type="Spring.Data.MultiDelegatingDbProvider, Spring.Data">
  <property name="TargetDbProviders" ref="dbProviderDictionary"/>
  <property name="DefaultDbProvider" value="CreditDbProvider"/>
</object>

As seen above, MultidelegatingDbProvider works via a thread local storage mechansims. If you prefer to place the logic to switch databases in a single location, within a single class, then create a subclass MultiDelegatingDbProvider and override the method GetTargetProvider. You can then select which provider to return based on your own implementation that does not involve thread local storage.

Note

This class is not recommended for usage with NHibernate. NHibernate usage typically involves caches that are scoped at the level of the SessionFactory. If you switch the database that hibernate is pointing to and do not also managed switching the cache, then the cache will end up with results from two different databases - which of course you don't want to have. The helper class contained in this post may help you if you when using NHibernate with multiple databases.