ASP.NET applications can make use of the System.Web.Caching.Cache
object to cache data and prevent repeated expensive calls
to a database or other store. However, ideally an application should make use of caching at the point where data is retrieved from the database, which typically is inside a Business Objects or Model layer. One of the key features of using a UI pattern such as Model-View-Presenter (MVP) or Model-View-Controller (MVC) is that the Model and Presenter (or Controller) layers are developed without any knowledge of the UI layer. Introducing a dependency on System.Web into the Model layer would break this independence of the Model from the View.
This article gives a solution
to this problem, using dependency injection
to inject the caching implementation into the Model layer at runtime. This allows caching
to be used within the Model layer, without any knowledge of the actual caching mechanism that will be used.
Create a sample application
to use the caching solution
Create a test SQL Server database
This solution uses a SQL Server database with the same Sales data used in my previous post on calculating running totals. The advantage of using this data is that it gives nice slow queries that will exaggerate the effect of using caching!
To create the data, first create a new SQL database called CacheSample. Next run the following script
to create the Sale table and populate it:
USE CacheSample
GO
CREATE TABLE Sale(DayCount smallint, Sales money)
CREATE CLUSTERED INDEX ndx_DayCount ON Sale(DayCount)
go
INSERT Sale VALUES (1,120)
INSERT Sale VALUES (2,60)
INSERT Sale VALUES (3,125)
INSERT Sale VALUES (4,40)
DECLARE @DayCount smallint, @Sales money
SET @DayCount = 5
SET @Sales = 10
WHILE @DayCount < 5000
BEGIN
INSERT Sale VALUES (@DayCount,@Sales)
SET @DayCount = @DayCount + 1
SET @Sales = @Sales + 15
END
Next create a stored procedure
to calculate the running total, and return a specified number of rows from the Sale table, using the following script:
USE [CacheSample]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author: Robin
-- Create date:
-- Description:
-- =============================================
CREATE PROCEDURE [dbo].[spGetRunningTotals]
-- Add the parameters for the stored procedure here
@HighestDayCount smallint = null
AS
BEGIN
-- SET NOCOUNT ON added
to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
IF @HighestDayCount IS NULL
SELECT @HighestDayCount = MAX(DayCount) FROM dbo.Sale
DECLARE @SaleTbl TABLE (DayCount smallint, Sales money, RunningTotal money)
DECLARE @DayCount smallint,
@Sales money,
@RunningTotal money
SET @RunningTotal = 0
SET @DayCount = 0
DECLARE rt_cursor CURSOR
FOR
SELECT DayCount, Sales
FROM Sale
ORDER BY DayCount
OPEN rt_cursor
FETCH NEXT FROM rt_cursor INTO @DayCount,@Sales
WHILE @@FETCH_STATUS = 0 AND @DayCount <= @HighestDayCount
BEGIN
SET @RunningTotal = @RunningTotal + @Sales
INSERT @SaleTbl VALUES (@DayCount,@Sales,@RunningTotal)
FETCH NEXT FROM rt_cursor INTO @DayCount,@Sales
END
CLOSE rt_cursor
DEALLOCATE rt_cursor
SELECT DayCount, Sales, RunningTotal
FROM @SaleTbl
END
GO
Create the Sample ASP.NET application
In Visual Studio create a new solution and add a class library project called CacheSample.BusinessObjects and an ASP.NET web application called CacheSample.UI.
The CacheSample.BusinessObjects project will contain a single class
to represent a Sale data item, with all the code
to retrieve the sales from the database included in it for simplicity (normally I would at least have a separate Repository or other
object that is responsible for retrieving data, and probably a data access layer as well, but for this sample I want
to keep it simple).
The C# code for the Sale class is shown below:
using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
namespace CacheSample.BusinessObjects
{
public class Sale
{
public Int16 DayCount { get; set; }
public decimal Sales { get; set; }
public decimal RunningTotal { get; set; }
public static IEnumerable<Sale> GetSales(int? highestDayCount)
{
List<Sale> sales = new List<Sale>();
SqlParameter highestDayCountParameter = new
SqlParameter("@HighestDayCount", SqlDbType.SmallInt);
if (highestDayCount.HasValue)
highestDayCountParameter.Value = highestDayCount;
else
highestDayCountParameter.Value = DBNull.Value;
string connectionStr =
System.Configuration.ConfigurationManager
.ConnectionStrings["CacheSample"].ConnectionString;
using(SqlConnection sqlConn = new SqlConnection(connectionStr))
using (SqlCommand sqlCmd = sqlConn.CreateCommand())
{
sqlCmd.CommandText = "spGetRunningTotals";
sqlCmd.CommandType = CommandType.StoredProcedure;
sqlCmd.Parameters.Add(highestDayCountParameter);
sqlConn.Open();
using (SqlDataReader dr = sqlCmd.ExecuteReader())
{
while (dr.Read())
{
Sale newSale = new Sale();
newSale.DayCount = dr.GetInt16(0);
newSale.Sales = dr.GetDecimal(1);
newSale.RunningTotal = dr.GetDecimal(2);
sales.Add(newSale);
}
}
}
return sales;
}
}
}
The static GetSale() method makes a call
to the spGetRunningTotals stored procedure and then reads each row from the returned SqlDataReader into an instance of the Sale class, it then returns a List of the Sale objects, as IEnnumerable<Sale>. A reference
to System.Configuration needs
to be added
to the CacheSample.BusinessObjects project so that the connection
string can be read from the web.config file.
In the CacheSample.UI ASP.NET project, create a single web page called ShowSales.aspx, and make this the default start up page. This page will contain a single button
to call the GetSales() method and a label
to display the results. The html mark up and the C# code behind are shown below:
ShowSales.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="ShowSales.aspx.cs" Inherits="CacheSample.UI.ShowSales" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Cache Sample - Show All Sales</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Button ID="btnTest1" runat="server" onclick="btnTest1_Click"
Text="Get All Sales" />
<asp:Label ID="lblResults" runat="server"></asp:Label>
</div>
</form>
</body>
</html>
ShowSales.aspx.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using CacheSample.BusinessObjects;
namespace CacheSample.UI
{
public partial class ShowSales : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void btnTest1_Click(object sender, EventArgs e)
{
System.Diagnostics.Stopwatch stopWatch =
new System.Diagnostics.Stopwatch();
stopWatch.Start();
var sales = Sale.GetSales(null);
var lastSales = sales.Last();
stopWatch.Stop();
lblResults.Text =
string.Format(
"Count of Sales: {0}, Last DayCount: {1}, Total Sales: {2}. Query took {3} ms",
sales.Count(),
lastSales.DayCount,
lastSales.RunningTotal,
stopWatch.ElapsedMilliseconds);
}
}
}
Finally we need
to add a connection
string to the CacheSample SQL Server database, called CacheSample,
to the web.config file:
<?xmlversion="1.0"?>
<configuration>
<connectionStrings>
<addname="CacheSample"
connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;Initial Catalog=CacheSample"
providerName="System.Data.SqlClient" />
</connectionStrings>
<system.web>
<compilationdebug="true"targetFramework="4.0" />
</system.web>
</configuration>
Run the application and click the button a few times
to see how long each call
to the database takes. On my system, each query takes about 450ms.
Next I shall look at a solution
to use the ASP.NET caching
to cache the data returned by the query, so that subsequent requests
to the GetSales() method are much faster.
Adding Data Caching Support
I am going
to create my caching support in a separate project called CacheSample.Caching, so the next step is
to add a class library
to the solution. We shall be using the application configuration
to define the implementation of our caching system, so we need a reference
to System.Configuration adding
to the project.
ICacheProvider<T> Interface
The first step in adding caching
to our application is
to define an interface, called ICacheProvider, in the CacheSample.Caching project, with methods
to retrieve any data from the cache or
to retrieve the data from the data source if it is not present in the cache. Dependency Injection will then be used
to inject an implementation of this interface at runtime, allowing the users of the interface (i.e. the CacheSample.BusinessObjects project)
to be completely unaware of how the caching is actually implemented. As data of any type maybe retrieved from the data source, it makes sense
to use generics in the interface, with a generic type parameter defining the data type associated with a particular instance of the cache interface implementation. The C# code for the ICacheProvider interface is shown below:
using System;
using System.Collections.Generic;
namespace CacheSample.Caching
{
public interface ICacheProvider
{
}
public interface ICacheProvider<T> : ICacheProvider
{
T Fetch(string key,
Func<T> retrieveData,
DateTime? absoluteExpiry,
TimeSpan? relativeExpiry);
IEnumerable<T> Fetch(string key,
Func<IEnumerable<T>> retrieveData,
DateTime? absoluteExpiry,
TimeSpan? relativeExpiry);
}
}
The empty non-generic interface will be used as a type in a Dictionary generic collection later
to store instances of the ICacheProvider<T> implementation for reuse, I prefer
to use a base interface when doing this, as I think the alternative of using
object makes for less clear code.
The ICacheProvider<T> interface defines two overloaded Fetch methods, the difference between these is that one will return a single instance of the type T and the other will return an IEnumerable<T>, providing support for easy caching of collections of data items.
Both methods will take a key parameter, which will uniquely identify the cached data, a delegate of type Func<T> or Func<IEnumerable<T>> which will provide the code
to retrieve the data from the store if it is not present in the cache, and absolute or relative expiry policies
to define when a cached item should expire. Note that at present there is no support for cache dependencies, but I shall be showing a method of adding this in part two of this article.
CacheProviderFactory Class
We need a mechanism of creating instances of our ICacheProvider<T> interface, using Dependency Injection
to get the implementation of the interface.
To do this we shall create a CacheProviderFactory static class in the CacheSample.Caching project. This factory will provide a generic static method called GetCacheProvider<T>(), which shall return instances of ICacheProvider<T>. We can then call this factory method with the relevant data type (for example the Sale class in the CacheSample.BusinessObject project)
to get a instance of ICacheProvider for that type (e.g. call CacheProviderFactory.GetCacheProvider<Sale>()
to get the ICacheProvider<Sale> implementation).
The C# code for the CacheProviderFactory is shown below:
using System;
using System.Collections.Generic;
using CacheSample.Caching.Configuration;
namespace CacheSample.Caching
{
public static class CacheProviderFactory
{
private static Dictionary<Type, ICacheProvider> cacheProviders
= new Dictionary<Type, ICacheProvider>();
private static
object syncRoot = new object();
///<summary>
/// Factory method
to create or retrieve an implementation of the
/// ICacheProvider interface for type <typeparamref name="T"/>.
///</summary>
///<typeparam name="T">
/// The type that this cache provider instance will work with
///</typeparam>
///<returns>An instance of the implementation of ICacheProvider for type
///<typeparamref name="T"/>, as specified by the application
/// configuration</returns>
public static ICacheProvider<T> GetCacheProvider<T>()
{
ICacheProvider<T> cacheProvider = null;
// Get the Type reference for the type parameter T
Type typeOfT = typeof(T);
// Lock the access
to the cacheProviders dictionary
// so multiple threads can work with it
lock (syncRoot)
{
// First check if an instance of the ICacheProvider implementation
// already exists in the cacheProviders dictionary for the type T
if (cacheProviders.ContainsKey(typeOfT))
cacheProvider = (ICacheProvider<T>)cacheProviders[typeOfT];
else
{
// There is not already an instance of the ICacheProvider in
// cacheProviders for the type T
// so we need
to create one
// Get the Type reference for the application's implementation of
// ICacheProvider from the configuration
Type cacheProviderType =
Type.GetType(CacheProviderConfigurationSection.Current.
CacheProviderType);
if (cacheProviderType != null)
{
// Now get a Type reference for the Cache Provider with the
// type T generic parameter
Type typeOfCacheProviderTypeForT =
cacheProviderType.MakeGenericType(new Type[] { typeOfT });
if (typeOfCacheProviderTypeForT != null)
{
// Create the instance of the Cache Provider and add it
to
// the cacheProviders dictionary for future use
cacheProvider =
(ICacheProvider<T>)Activator.
CreateInstance(typeOfCacheProviderTypeForT);
cacheProviders.Add(typeOfT, cacheProvider);
}
}
}
}
return cacheProvider;
}
}
}
As this code uses Activator.CreateInstance()
to create instances of the ICacheProvider<T> implementation, which is a slow process, the factory class maintains a Dictionary of the previously created instances so that a cache provider needs
to be created only once for each type.
The type of the implementation of ICacheProvider<T> is read from a custom configuration section in the application configuration file, via the CacheProviderConfigurationSection class, which is described below.
CacheProviderConfigurationSection Class
The implementation of ICacheProvider<T> will be specified in a custom configuration section in the application’s configuration.
To handle this create a folder in the CacheSample.Caching project called Configuration, and add a class called CacheProviderConfigurationSection
to this folder. This class will extend the System.Configuration.ConfigurationSection class, and will contain a single
string property called CacheProviderType. The C# code for this class is shown below:
using System;
using System.Configuration;
namespace CacheSample.Caching.Configuration
{
internal class CacheProviderConfigurationSection : ConfigurationSection
{
public static CacheProviderConfigurationSection Current
{
get
{
return (CacheProviderConfigurationSection)
ConfigurationManager.GetSection("cacheProvider");
}
}
[ConfigurationProperty("type", IsRequired=true)]
public
string CacheProviderType
{
get
{
return (string)this["type"];
}
}
}
}
Adding Data Caching
to the Sales Class
We now have enough code in place
to add caching
to the GetSales() method in the CacheSample.BusinessObjects.Sale class, even though we do not yet have an implementation of the ICacheProvider<T> interface.
We need
to add a reference
to the CacheSample.Caching project
to CacheSample.BusinessObjects so that we can use the ICacheProvider<T> interface within the GetSales() method.
Once the reference is added, we can first create a unique
string key based on the method name and the parameter value, so that the same cache key is used for repeated calls
to the method with the same parameter values. Then we get an instance of the cache provider for the Sales type, using the CacheProviderFactory, and pass the existing code
to retrieve the data from the database as the retrievalMethod delegate in a call
to the Cache Provider Fetch() method. The C# code for the modified GetSales() method is shown below:
public static IEnumerable<Sale> GetSales(int? highestDayCount)
{
string cacheKey =
string.Format("CacheSample.BusinessObjects.GetSalesWithCache({0})",
highestDayCount);
return CacheSample.Caching.CacheProviderFactory.
GetCacheProvider<Sale>().Fetch(cacheKey,
delegate()
{
List<Sale> sales = new List<Sale>();
SqlParameter highestDayCountParameter = new
SqlParameter("@HighestDayCount", SqlDbType.SmallInt);
if (highestDayCount.HasValue)
highestDayCountParameter.Value = highestDayCount;
else
highestDayCountParameter.Value = DBNull.Value;
string connectionStr =
System.Configuration.ConfigurationManager.
ConnectionStrings["CacheSample"].ConnectionString;
using (SqlConnection sqlConn = new SqlConnection(connectionStr))
using (SqlCommand sqlCmd = sqlConn.CreateCommand())
{
sqlCmd.CommandText = "spGetRunningTotals";
sqlCmd.CommandType = CommandType.StoredProcedure;
sqlCmd.Parameters.Add(highestDayCountParameter);
sqlConn.Open();
using (SqlDataReader dr = sqlCmd.ExecuteReader())
{
while (dr.Read())
{
Sale newSale = new Sale();
newSale.DayCount = dr.GetInt16(0);
newSale.Sales = dr.GetDecimal(1);
newSale.RunningTotal = dr.GetDecimal(2);
sales.Add(newSale);
}
}
}
return sales;
},
null,
new TimeSpan(0, 10, 0));
}
This example passes the code
to retrieve the Sales data from the database
to the Cache Provider as an anonymous method, however it could also be written as a lambda. The main advantage of using an anonymous function (method or lambda) is that the code inside the anonymous function can access the parameters passed
to the GetSales() method. Finally the absolute expiry is set
to null, and the relative expiry set
to 10 minutes,
to indicate that the cache entry should be removed 10 minutes after the last request for the data.
As the ICacheProvider<T> has a Fetch() method that returns IEnumerable<T>, we can simply return the results of the Fetch() method
to the caller of the GetSales() method.
This should be all that is needed for the GetSales() method
to now retrieve data from a cache after the first time the data has be retrieved from the database.
Implementing a ASP.NET Cache Provider
The final step is
to actually implement the ICacheProvider<T> interface, and add the implementation details
to the web.config file for the dependency injection.
The cache provider implementation needs
to have access
to System.Web. Therefore it could be placed in the CacheSample.UI project, or in its own project that has a reference
to System.Web. Implementing the Cache Provider in a separate project is my favoured approach.
Create a new project inside the solution called CacheSample.CacheProvider, and add references
to System.Web and CacheSample.Caching
to this project. Add a class
to the project called AspNetCacheProvider. Make the class a generic class by adding the generic parameter <T> and indicate that the class implements ICacheProvider<T>.
The C# code for the AspNetCacheProvider class is shown below:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Caching;
using CacheSample.Caching;
namespace CacheSample.CacheProvider
{
public class AspNetCacheProvider<T> : ICacheProvider<T>
{
#region ICacheProvider<T> Members
public T Fetch(string key, Func<T> retrieveData,
DateTime? absoluteExpiry, TimeSpan? relativeExpiry)
{
return FetchAndCache<T>(key, retrieveData,
absoluteExpiry, relativeExpiry);
}
public IEnumerable<T> Fetch(string key, Func<IEnumerable<T>> retrieveData,
DateTime? absoluteExpiry, TimeSpan? relativeExpiry)
{
return FetchAndCache<IEnumerable<T>>(key, retrieveData,
absoluteExpiry, relativeExpiry);
}
#endregion
#region Helper Methods
private U FetchAndCache<U>(string key, Func<U> retrieveData,
DateTime? absoluteExpiry, TimeSpan? relativeExpiry)
{
U value;
if (!TryGetValue<U>(key, out value))
{
value = retrieveData();
if (!absoluteExpiry.HasValue)
absoluteExpiry = Cache.NoAbsoluteExpiration;
if (!relativeExpiry.HasValue)
relativeExpiry = Cache.NoSlidingExpiration;
HttpContext.Current.Cache.Insert(key, value,
null, absoluteExpiry.Value, relativeExpiry.Value);
}
return value;
}
private bool TryGetValue<U>(string key, out U value)
{
object cachedValue = HttpContext.Current.Cache.Get(key);
if (cachedValue == null)
{
value = default(U);
return false;
}
else
{
try
{
value = (U)cachedValue;
return true;
}
catch
{
value = default(U);
return false;
}
}
}
#endregion
}
}
The two interface Fetch() methods call a private method called FetchAndCache(). This method first checks for a element in the HttpContext.Current.Cache with the specified cache key, and if so tries
to cast this
to the specified type (either T or IEnumerable<T>). If the cached element is found, the FetchAndCache() method simply returns it. If it is not found in the cache, the method calls the retrievalMethod delegate
to get the data from the data source, and then adds this
to the HttpContext.Current.Cache.
The final step is
to add the AspNetCacheProvider class
to the relevant custom configuration section in the CacheSample.UI.Web.Config file.
To do this there needs
to be a <configSections> element added as the first element in <configuration>. This will match a custom section called <cacheProvider> with the CacheProviderConfigurationSection. Then we add a <cacheProvider> element, with a type property set
to the fully qualified assembly name of the AspNetCacheProvider class, as shown below:
<?xmlversion="1.0"?>
<configuration>
<configSections>
<sectionname="cacheProvider"
type="CacheSample.Base.Configuration.CacheProviderConfigurationSection, CacheSample.Base" />
</configSections>
<connectionStrings>
<addname="CacheSample"
connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;Initial Catalog=CacheSample"
providerName="System.Data.SqlClient" />
</connectionStrings>
<cacheProvidertype="CacheSample.CacheProvider.AspNetCacheProvider`1, CacheSample.CacheProvider, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null">
</cacheProvider>
<system.web>
<compilationdebug="true"targetFramework="4.0" />
</system.web>
</configuration>
One point
to note is that the fully qualified assembly name of the AspNetCacheProvider class includes the notation `1 after the class name, which indicates that it is a generic class with a single generic type parameter.
The CacheSample.UI project needs
to have references added
to CacheSample.Caching and CacheSample.CacheProvider so that the actual application is aware of the relevant cache provider implementation.
Conclusion
After implementing this solution, you should have a working cache provider mechanism, that will allow the middle and data access layers
to implement caching support when retrieving data, without any knowledge of the actually caching implementation. If the UI is not ASP.NET based, if for example it is Winforms or WPF, the implementation of ICacheProvider<T> would be written around whatever technology is available. It could even be a standalone caching system that takes full responsibility for adding and removing items from a global store.
The next part of this article will show how this caching mechanism may be extended
to provide support for cache dependencies, such as the System.Web.Caching.SqlCacheDependency.
Another possible extension would be
to cache the cache provider implementations instead of storing them in a static Dictionary in the CacheProviderFactory. This would prevent a build up of seldom used cache providers in the application memory, as they could be removed from the cache if not used often enough, although in reality there are probably unlikely
to be vast numbers of cache provider implementation instances, as most applications do not have a massive number of business
object or model types.
