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Wednesday, August 30, 2006

Caching in ASP.NET

Caching means just storing the frequently used items in memory.

ASP.NET supports three types of caching for Web-based applications:


  • Page Level Caching (called Output Caching)
  • Page Fragment Caching (often called Partial-Page Output Caching)
  • Programmatic or Data Caching

Output Caching

Page level, or output caching, caches the HTML output of dynamic requests to ASP.NET Web pages. The way ASP.NET implements this (roughly) is through an Output Cache engine. Each time an incoming ASP.NET page request comes in, this engine checks to see if the page being requested has a cached output entry. If it does, this cached HTML is sent as a response; otherwise, the page is dynamically rendered, it's output is stored in the Output Cache engine.


Output Caching is particularly useful when you have very static pages.

Output caching is easy to implement. By simply using the @OuputCache page directive, ASP.NET Web pages can take advantage of this powerful technique. The syntax looks like this:


<%@OutputCache Duration="60" VaryByParam="none" %> 


The Duration parameter specifies how long, in seconds, the HTML output of the Web page should be held in the cache. When the duration expires, the cache becomes invalid and, with the next visit, the cached content is flushed, the ASP.NET Web page's HTML dynamically generated, and the cache repopulated with this HTML. The VaryByParam parameter is used to indicate whether any GET (QueryString) or POST (via a form submit with method="POST") parameters should be used in varying what gets cached. In other words, multiple versions of a page can be cached if the output used to generate the page is different for different values passed in via either a GET or POST.


The VaryByParam is a useful setting that can be used to cache different "views" of a dynamic page whose content is generated by GET or POST values. For example, you may have an ASP.NET Web page that reads in a Part number from the QueryString and displays information about a particular widget whose part number matches the QueryString Part number. Imagine for a moment that Output Caching ignored the QueryString parameters altogether (which you can do by setting VaryByParam="none"). If the first user visited the page with QueryString /ProductInfo.aspx?PartNo=4, she would see information out widget #4. The HTML for this page would be cached. The next user now visits and wished to see information on widget #8, a la /ProductInfo.aspx?PartNo=8. If VaryByParam is set to VaryByParam="none", the Output Caching engine will assume that the requests to the two pages are synonymous, and return the cached HTML for widget #4 to the person wishing to see widget #8! To solve for this problem, you can specify that the Output Caching engine should vary its caches based on the PartNo parameter by either specifying it explicitly, like VaryByParam="PartNo", or by saying to vary on all GET/POST parameters, like: VaryByParam="*".


Partial-Page Output Caching

More often than not, it is impractical to cache entire pages. For example, you may have some content on your page that is fairly static, such as a listing of current inventory, but you may have other information, such as the user's shopping cart, or the current stock price of the company, that you wish to not be cached at all. Since Output Caching caches the HTML of the entire ASP.NET Web page, clearly Output Caching cannot be used for these scenarios: enter Partial-Page Output Caching.


Partial-Page Output Caching, or page fragment caching, allows specific regions of pages to be cached. ASP.NET provides a way to take advantage of this powerful technique, requiring that the part(s) of the page you wish to have cached appear in a User Control. One way to specify that the contents of a User Control should be cached is to supply an OutputCache directive at the top of the User Control. That's it! The content inside the User Control will now be cached for the specified period, while the ASP.NET Web page that contains the User Control will continue to serve dynamic content. (Note that for this you should not place an OutputCache directive in the ASP.NET Web page that contains the User Control - just inside of the User Control.)


These two techniques cached either the full HTML output of an ASP.NET Web page, or a portion of the HTML output of an ASP.NET Web page (by caching the HTML output of a User Control).


Data Caching

Sometimes, more control over what gets cached is desired. ASP.NET provides this power and flexibility by providing a cache engine. Programmatic or data caching takes advantage of the .NET Runtime cache engine to store any data or object between responses. That is, you can store objects into a cache, similar to the storing of objects in Application scope in classic ASP


Realize that this data cache is kept in memory and "lives" as long as the host application does. In other words, when the ASP.NET application using data caching is restarted, the cache is destroyed and recreated. Data Caching is almost as easy to use as Output Caching or Fragment caching: you simply interact with it as you would any simple dictionary object. To store a value in the cache, use syntax like this:


Cache["foo"] = bar; // C#

Cache("foo") = bar ' VB.NET 


To retrieve a value, simply reverse the syntax like this:

bar = Cache["foo"]; // C#

bar = Cache("foo") ' VB.NET 


Note that after you retrieve a cache value in the above manner you should first verify that the cache value is not null prior to doing something with the data. Since Data Caching uses an in-memory cache, there are times when cache elements may need to be evicted. That is, if there is not enough memory and you attempt to insert something new into the cache, something else has gotta go! The Data Cache engine does all of this scavenging for your behind the scenes, of course. However, don't forget that you should always check to ensure that the cache value is there before using it. This is fairly simply to do - just check to ensure that the value isn't null/Nothing. If it is, then you need to dynamically retrieve the object and restore it into the cache.


For example, if we were storing a string myString in the cache whose value was set from some method named SetStringToSomething(), and we wanted to read the value of myString from the cache, we'd want to:


Read the myString from the cache: str = Cache("myString")

Ensure that str wasn't null/Nothing. If it was, we'd want to get the value of str from SetStringToSomething(), and then put it in the cache, like so:

'Try to read the cache entry MyString into str

str = Cache("myString")


'Check if str is Nothing

If str is Nothing then

  'If it is, populate str from SetStringToSomething()

  str = SetStringToSomething()


  'Now insert str into the cache entry myString

  Cache("myString") = str

End If


Besides using the dictionary-like key/value assignment, as shown in the example above, you can also use the Insert or Add method to add items to the cache. Both of these methods are overloaded to accommodate a variety of situations. The Add and the Insert methods operate exactly the same except the Add method returns a reference to the object being inserted to the cache. Because of the similarity of the two methods, I will concentrate on the Insert method. Note that the Insert method allows you to simply add items to the cache using a key and value notation as well. For example to simply add an instance of the object bar to the cache named foo, use syntax like this:


Cache.Insert("foo", bar); // C#

Cache.Insert("foo", bar) ' VB.NET 

(Note that this is synonymous to using the Cache("foo") = bar syntax we looked at earlier.)


Note that with inserting items into the Data Cache using the Cache(key) = value method or the Cache.Insert(key, value) we have no control over when (if ever) the items are evicted from the cache. However, there are times when we'd like to have control over when items leave the cache. For example, perhaps we want to have an inserted item in the cache to only live for n seconds, as with Output Caching. Or perhaps we'd like to have it exit the cache n seconds after it's last accessed. With Data Caching, you can optionally specify when the cache should have a member evicted.


Additionally, you can have an item evicted from the cache when a file changes. Such an eviction dependency is called a file dependency, and has many real-world applications, especially when working with XML files. For example, if you want to pull data out of an XML file, but you don't want to constantly go to disk to read the data, you can tell the ASP.NET caching engine to expire the cached XML file whenever the XML file on disk is changed. To do this, use the following syntax:

Cache.Insert("foo", bar, new CacheDependancy(Server.MapPath("BarData.xml"))) 


By using this syntax, the cache engine takes care of removing the object bar from the cache when BarData.xml file is changed. Very cool! There are also means to have the inserted cache value expire based on an interval, or at an absolute time, as discussed before. For more information on these methods, consult the documentation for the Cache.Insert method.

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saraswathi said...

nice article