How is caching performed in ASP.NET?

Consider an Example. An application needs to query a database which has 10 million records after every four seconds to retrieve same type of result. Let us assume that database will get updated in an hour. Now, in this situation data retrieved from the database will remain the same for about 50 minutes. So querying this database after every four seconds will cause network congestion and the load on the database /web server. To solve the above problem we use caching. Caching is used to store data either on web server or client machine for a specified duration. So, in the above example, we cache the data for a fixed period of time (assume 50 minutes), then when the first time query is executed, results will be stored in cache. Now in the next time frame( here it is 50 minutes)the same type of requests for the data will  get the data from the query itself without executing query again on 10 million records. Hence, caching enhances user experience and gain performance in web applications.

ASP.NET provides three kinds of caching techniques:

1) Page level or output caching: In this caching, the complete HTML output of a presented ASP.NET page is stored in the cache and any other requests for the same page is sent from the cache itself. For each request entry of that page is find the cache. If an entry is found HTML output is sent to the client. If it doesn’t find an entry, then it saves a copy of HTML output in the cache for later use. It is extremely beneficial in static pages (pages whose content doesn’t change) as everything that is presented on the page is cached. It is implemented by adding @output cache directive in the web page as shown below:

<%@ OutputCache duration=”100” VaryByParam=”none” %>

Duration specifies time for which page exists in cache and varyByParam attribute is used to cache different copies of the page based on the parameters received in the form of HTTP POST or HTTP GET .For example, when a user requests for personal record of an employee with id =009 and VaryByParam=”none” , page will be presented and output will be cached . Now when another user seeks records for the employee id=007 then the caching engine will not be able to see the difference and the cached output for id=009 will be sent. This problem can be avoided by setting VaryByParam=”emp_id”

2) Partial page output caching: As the name suggests, it is used when we do not require entire page to be cached rather only some of the contents of the page need to be cached. Here we create a single or more user controls for those parts of the page and decide the caching policies of those user controls. To implement this caching we use <partial caching (500)> .When someone requests for the page where this user control is used, the cache engine will store the user control for 500 sec in its output cache and will be retrieved for the subsequent requests 

3) Data caching: it is used on data which is very common. It is very useful when you need to cache your own custom data into cache. For example, when we need to cache a data set which can be used later in the application, we require data caching. It is implemented by cache class or system.web.caching.cache namespace. We use cache class to store the data while handling simple data. It has the format as cache (“myvalue”)= value. Following tasks need to be performed during data caching:

a) Adding items in cache: To add items in cache we use- Cache. Insert (“My value”, value).We can also add expiration time of 5 mins using-Cache.Insert(“myvalue”,value,nothing,datetime.now.addminutes(5))

b) Retrieving data from the cache: It is done by the following statement- value=Cache(“myvalue”).

c) Deleting data from the cache: Although it is done automatically based on expiration time, we can manually do so by- Cache.Remove(“mydata”).