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CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
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Figure 10-5. An HTML table generated using the generic data Provider Factory to talk to SQL Server Table 10-7. Generic Database Types, the Factory Method on DbProviderFactory That Creates Them, and Their Role in Life System.Data.Common Type
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A generic command object, exposing Connection, CommandText, CommandType, a Parameters collection, and Execute methods for sending queries to the database. A generic object that will build the Insert, Update, and Delete Commands for a DataAdapter. A generic Connection object that uses a ConnectionString and Open method to establish communication with any database server. A helper type that abstracts away the details of the syntax of a connection string for a specific provider. A generic DataAdapter for manufacturing DataTables and moving DataSet changes back to the database. A generic Parameter for sending arguments to parameterized SQL statements or to procedures defined within the database.
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CreateConnection
DbConnectionStringBuilder
CreateConnectionStringBuilder
DbDataAdapter
CreateDataAdapter
DbParameter
CreateParameter
CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
Of course, the whole point of a Provider Factory is to select a Managed Provider at runtime instead of at design time. This can be especially helpful for software vendors who want to write applications that work against SQL Server or Oracle, for example; and their customers control this with the configuration when they install the application. You can change the last example to pull the name of the provider out of the connection information using the providerName attribute of the add element in the connectionStrings section of the configuration file. <appSettings> <add key="EnvironmentName" value="localPubs"/> </appSettings> <connectionStrings> <add name=" localPubs" connectionString="server=.;database=pubs;uid=sa;pwd=" providerName="System.Data.SqlClient"/> <add name=" OraclePubs" connectionString="Oracle Connetion String..." providerName="System.Data.OracleClient"/> </connectionStrings> By adding the provider name in your configuration file, you can change the code to have these configuration entries drive the provider, enabling users to dynamically select a provider when they deploy the application. You ll use the providerName setting to drive the invariant name of the provider you ll use (see ConfigFactory.aspx in Web10). protected void Page_Load(object sender, EventArgs e) { string EnvName = ConfigurationManager.AppSettings["EnvironemntName"].ToString(); ConnectionStringSettings css = ConfigurationManager.ConnectionStrings[EnvName]; DbProviderFactory factory = DbProviderFactories.GetFactory(css.ProviderName); DbConnection cn = factory.CreateConnection(); cn.ConnectionString = css.ConnectionString; DbCommand cm = factory.CreateCommand(); cm.Connection = cn; cm.CommandText = "select * from authors where [state] = @state"; DbParameter pm = factory.CreateParameter(); pm.ParameterName = "@state"; pm.Value = "CA"; cm.Parameters.Add(pm);
CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
GridView gv = new GridView(); cn.Open(); gv.DataSource = cm.ExecuteReader(CommandBehavior.CloseConnection); gv.DataBind(); form1.Controls.Add(gv); } Adding the Provider Factory restores a feature of ADO that was lost when you moved to ADO.NET version 1.x: the ability to easily switch your backend database at deployment or even at runtime. Using these features won t be for everyone, but it will be very nice for those who do have the requirement of shipping code that can dynamically adjust to different vendor s databases.
Concurrency
Concurrency, or more specifically, the problem of what to do with a dirty read is always an issue in distributed application development. The problem is simple. User A reads a row of data, and begins to examine it within his browser window, contemplating changing it. Meanwhile, user B comes along and makes a change to the same row. User A finally applies a change, and posts it to the server for submission to the database. The row in the table has changed since user A first retrieved the data, so user A is said to have a dirty read of the data, and if user A posts those changes, it will result in a dirty write (see Figure 10-6).
Figure 10-6. A dirty read from the database The question is: How should the updating process deal with this situation How can the updating process even learn of it And once it does, what should be done These questions must always be asked when inventing an application s architecture. How they are solved depends on the business requirements of the application, and the amount of control the development team has over the database whose data is being consumed. The problem is much different when it s a custom database than it is when the database is vendor supplied and changes cannot be made to it. These are the factors that determine which of the following solutions will be employed.
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