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New Message on BDOTNET
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From: dnyanesh
Message 5 in Discussion
Hi All
Ques: What is Response.writeSubstitution used for?
Ans: Substitution ASP.NET v1.x introduced a powerful feature known as Partial
Page Caching. This feature allowed developers to construct ASP.NET pages which
were partly dynamic and partly cached. Regions marked as dynamic are executed
on each request while areas marked as cached are executed only once and cached
until a specific dependency is enforced. The cached regions are separated into
user controls with appropriate cache directives, and the dynamic content either
remains in the parent Page or is contained in user controls without cache
directives.
This partial page caching approach works very well for scenarios where most of
the page content is dynamic and/or it is easy to encapsulate cached content
into isolated user controls. It does not work as well for the inverse scenario,
when the majority of page content should be cached and only a small portion is
to be dynamic. For example, consider a page of news stories extracted from a
database, which contains a single rotating advertisement. In this scenario, the
news stories (and surrounding page header, footer, and site navigation
interface) might be easily cached, since they change relatively infrequently.
However, the rotating advertisement should change on each page request, to
display a random advertisement to each site visitor.
ASP.NET Whidbey introduces a new feature called Post-Cache Substitution, which
is aimed at optimizing the development experience for this mostly-cached page
scenario. Rather than requiring page developers to mark page regions (user
controls) as cached, post-cache substitution allows them to output cache an
entire page and then simply identify regions of the page that should be exempt
from caching. It also allows control developers to prevent their rendering from
from being cached. In the above example, an AdRotator control that takes
advantage of post-cache substitution would be able to serve a different
advertisement on each request even if its parent page were cached.
Using the Substitution APIPage developers can easily take advantage of
post-cache substitution in their output-cached pages. A developer simply writes
a method with a prescribed SubstitutionCallback signature that takes a single
HttpContext parameter and returns a String, which is the content to substitute
at a given location. The developer then calls Response.WriteSubstitution,
passing the callback method as a parameter. The callback method can either be a
static method on the containing Page or UserControl, or a static or instance
method on another arbitrary object, and must be thread-safe.
<%@ OutputCache Duration="60" VaryByParam="none" %> <script language="C#"
runat="server"> public static String GetCurrentDate (HttpContext context) {
return DateTime.Now.ToString(); } </script> ... cached content ...
<form runat="server"> <% Response.WriteSubstitution (new
HttpResponseSubstitutionCallback(GetCurrentDate)) %> </form> On the first
request to the page, WriteSubstitution performs these steps: Calls the
HttpResponseSubstitutionCallback delegate to produce the output. Adds a
substitution buffer to the response, which retains the delegate to call on
future requests, as well as the first-time output from step 1. Degrades
client-side cacheability from public to server-only, so that the will not be
cached on the client, ensuring future requests to the page re-invoke the
delegate. On subsequent requests, the cache module intercepts incoming requests
and retrieves the associated stored buffers. When writing the substitution
buffer, the delegate is invoked to produce new output, which is written to the
response. Regards, Dnyaneshwar Parkhe
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