.NET/Java PDF, Tiff, Barcode SDK Library

SqlCommand cm = new SqlCommand("select * from authors", cn); new SqlDataAdapter(cm).Fill(ds); dv = new DataView (ds.Tables[0], "", "state", DataViewRowState.CurrentRows); Cache.Insert("AuthorData", dv); } return dv; } protected void ddlState_SelectedIndexChanged(object sender, EventArgs e) { DataView dv = GetAuthors(); dv.RowFilter = string.Format("state = '{0}'", ddlState.SelectedValue); gvAuthors.DataSource = dv; gvAuthors.DataBind(); } Here again, you could go back to the database with each request that comes into the server, but you ll be much better off keeping a cache of this data in memory and using the DataView to create an index on the data you ll be querying upon. If the users can choose the column they want to apply the filter to, you can apply a strategy similar to the one you used in the sorting example, creating a cached DataView per column being queried upon. The problem with an in-memory cache is, of course, concurrency and dirty data. When someone comes along and makes a change to the data, the cache entry is no longer useful to you. In 11, we take a look at the new facility that s been added to 2.0 to create a dependency between a cache entry and the relational database where the entry came from, the SqlCacheDependency. We also examine some overall strategies for managing latency and concurrency when modifying data.

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Our simple example consists of creating a schema for the scenario illustrated in Figure 8-1. The entity Component consists of zero or more parts.

let twoBitAdder (x1,x2) (y1,y2) (sum1,sum2) carryInner carry = halfAdder x1 y1 sum1 carryInner &&& fullAdder x2 y2 carryInner sum2 carry Note the close relation between the diagram for the 2-bit adder and its representation as code. You can read the implementation as a specification of the diagram, and vice versa. The types of these functions are, however, a little less informative: val val val val val sumBit : Prop -> Prop -> Prop carryBit : Prop -> Prop -> Prop halfAdder : Prop -> Prop -> Prop -> Prop -> Prop fullAdder : Prop -> Prop -> Prop -> Prop -> Prop -> Prop twoBitAdder : Prop * Prop -> Prop * Prop -> Prop * Prop -> Prop -> Prop -> Prop

Using the traditional relational schema based approach, this scenario can be modeled as a parent table, components_rel, and a child table, parts_rel, with a foreign key relationship as shown in the following code. First, we create the parent table components_rel (the prefix rel reminds us that this is part of a pure relational approach): benchmark@ORA10G> create table components_rel 2 ( 3 component_id number primary key, 4 component_name varchar2(50) 5 ); Table created. Next, we create the child table parts_rel. For simplicity, assume that one part can belong to at most one component. benchmark@ORA10G> create table parts_rel 2 ( 3 component_id number references components_rel on delete cascade, 4 part_id number primary key, 5 part_name varchar2(50), 6 part_desc varchar2(500) 7 ); Table created.

Version 2.0 of the Framework introduces a factory model for creating instances of data access objects. Using this factory enables you to write code that s generic across different database vendors. This feature is available through ADO, but to write vendor-neutral code in .NET requires using an interface-based late bound programming model or one of the backwards compatibility Managed Providers, such as OLEDB or ODBC. The interface-based approach involves programming against the interfaces common to all Managed Providers, and then using Reflection or a custom-written factory to actually load instances of types at runtime. The backwards compatibility layer introduces a serious performance hit to your managed applications. This is particularly painful if the databases you re supporting do provide Managed Providers coded specifically for those vendors platforms. You don t ever want to, for example, use the OLEDB Managed Provider to talk to SQL Server 2000. So in 2.0, there s a Provider Factory model. This factory enables you to let configuration entries drive what database you re to use, but get instances of types from specific Managed Providers at runtime. So if you re using SQL Server, you get a SqlConnection object to use from your code. If you re using Oracle, you get an OracleConnection. The services of the Provider Factory are exposed through a couple of types that Microsoft added to the System.Data.Common namespace. DbProviderFactories exposes a couple of simple shared methods (see Table 10-6). Table 10-6. The Shared Methods of the DbProviderFactories type