Page Output Caching

Page output caching can be accomplished declaratively using a page @

OutputCache

directive or programmatically using the Response.Cache API.The following code samples

are equivalent:

<script runat="server">

Sub Page_Load (sender As object, e As EventArgs)

Response.Cache.SetExpires(DateTime.Now.AddSeconds(100))

Response.Cache.SetCacheability(HttpCacheability.Public)

Response.Cache.SetValidUntilExpires(true)

' Set the timestamp

timestamp.Text = DateTime.Now.ToString()

End Sub

</script>

<asp:Label id="timestamp" runat="server" />

Or

<%@ OutputCache Duration="100" VaryByParam="none" %>

<script runat="server">

Sub Page_Load(sender As Object, e As EventArgs)

timestamp.Text = DateTime.Now.ToString()

End Sub

</script>

<asp:Label id="timestamp" runat="server" />

Comments

As you can clearly see in the code sample, the page @OutputCache directive is much easier

to implement. However, the lower-level Response.Cache API provides more flexibility

when necessary. In either case, the content for the response is generated once and

stored in memory before being sent back to the client.The contents are never written to

disk, but instead are kept in memory within the application using the Cache API.

In the @OutputCache directive example, the Duration and VaryByParam attributes are

required (if they are not specified, an exception is thrown detailing the requirements of

these attributes).The Duration attribute is a time dependency, in seconds, used to control

the lifetime of the cached response in memory.The VaryByParam attribute specifies

which parameters (in the Request.Params collection) should cause separate versions of

the page to be stored in the cache.

The SetValidUntilExpires() method on Response.Cache is used to control a

nuance of HTTP caching. By default, this method is set to true when using the

@OutputCache directive. Conversely, the default is false when using the

Response.Cache API. SetValidUntilExpires() controls how ASP.NET honors cache

invalidation headers sent by browsers.When users click the Refresh button in the browser,

the browser sends an HTTP header to the server specifying that the requested content

cannot come from a cache.When SetValidUntilExpires() is true, the HTTP

cache invalidation request sent by the browser is ignored and the request can be served

from the cache; when false, the HTTP cache invalidation request is honored and the

request is removed from the cache. It's recommended to always set this to true to guarantee

that the server can respond from the cache, otherwise a simple refresh in the

browser can remove the item from the cache.