Caching

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Caching

• Chapter 12

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Caching

• For high-performance apps
• Caching storing frequently-used items in memory
• Accessed more quickly
• Cached Web Form bypasses
• servers processing HTML
• Servers running web form to create a response
• Response is processed directly from memory

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Things that can be stored in Cache

• Web Forms
• Parts of web forms
• Application Data

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Caching Web Forms

• Good Caching Candidates
• Frequently used
• Contains fairly static data
• First series of Demos ? Use OutputCache directive
• Demos show different ways to control caching
• Demo 1 WebFormCaching1.aspx
• Response loaded into memory for 60 secs (on
  client)
• Subsequent requests during period get stored
  response
• After 60 secs new response is cached

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Another way to control Caching

• Previous examples used OutputCache directive
• From code, use Response objects Cache property
• Returns a HttpCachePolicy object
• HttpCachePolicy members are similar to attributes
  of OutputCache directive

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Multiple Caches from Single Web Form

• Demo2 WebFormCaching2.aspx
• VaryByParam attribute
• New form cached when query string or HTTP POST
  varies
• Demo up to 5 different cached responses
• One for each time zone

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Vary by Response Header

• Demo 3 VaryByHeader.aspx
• Different forms cached depending on Language
  Requested in Response.Header
• Change regional settings in Windows Control Panel
  
• One form cached per language requested

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Cache Response / Custom String

• Demo 4 CustomString.aspx
• Custom string set in Global.aspx
• Demo caches a response based on the browser used
  to request the page
• (Netscape Mozilla??)

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HttpCachePolicy members vs OutputCache directive
attributes
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Controlling Caching thru Code

• Demo 5 UsingResponseCache
• Uses Response.Cache object
• Sets Cache duration at 5 seconds
• Similar to Duration attribute of OutputCache
  directive
• lt_at_ OutputCache Duration"5" VaryByParam"None"
  Location"Any" gt

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Where are items stored?

• Default anywhere they are accepted
• Client
• Proxy server
• Host Server

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Caching parts of a page

• Given Large table that seldom varies
• Put table in user control and cache user control
  response
• Fragment caching

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Caching Part of a Web Form with a User Control

• Place controls and content in a web user control
• Set caching attributes for that control
• Create an instance of the user control on the web
  form
• Demo 6 PartialCaching1.aspx
• Uses PartialCaching1.ascx

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Caching Application Data

• Cache frequently used data
• Usually Read-Only
• To store changed cache data before it is removed
  from memory
• Use CacheItemRemovedCallback delegate to save
  changes

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Cache Object

• Intrinsic object
• Stores frequently used items (quick retrieval)
• Global available from anywhere in web app
• Similar to Application Object

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Ways to store data in Cache object

• Assignment
• Assign a value to a key in the Cache object
• Cache(NewItem) Some String Data
• Insert method
• Uses (optional) parameters to set policies and
  duration
• Add method
• Parameters are required
• Returns a reference to cached data

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Using Cache Object
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Parameters for Add and Insert
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Using Cache

• Demo 7 UsingCache.aspx
• Walkthru
• Stores items in Cache Load event
• Retrieve item from Cache PreRender event

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How Long does an item stay in Cache?

• Add/Insert Parameters
• Really these give only indirect control
• When memory gets low, ASP.NET
• Removes expired members from cache
• Unloads other members based on their priority and
  when last accessed

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Cache Priority Settings
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Using Cache Data before it is Removed from Memory

• Add/Insert Methods
• onRemoveCallback
• Demo 8 DataCache1.aspx

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Updating Cache when Data Changes

• Data stored in cache may represent data stored
  elsewhere (say in a database).
• Establish relationship between cached data and
  external
• File
• Folder
• Group of files
• Use Add/Insert dependency parameter
• Accepts a CacheDependency object

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CacheDependency object

• Identifies file, folder, or set of files to watch
  for change
• ASP.NET checks timestamps of objects in
  CacheDependency object
• Compares with DateTime for cached item
• If later, unloads the item from the cache

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Using CacheDependency object

• Demo 9 DataCache2.aspx
• NewItem in Cache is linked to NewItem.TXT file
• When new data is written to the file, data is
  removed from Cache

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Demo 10 C

• Demo10 CachingLab.aspx
• Displays a list of customers from Northwind db
• Users select from dropdown list
• Click button to view orders in data grid
• Web Form cached for 1 minute
• Since response can vary by which customer is
  selected, use VaryByParam to store multiple
  responses one for each selected customer

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Page_Load Event

• CustList is a ListItemCollection object
• If this is the first time through (IsPostback)
• If the list is not in the cache
• Store it in the cache (CacheCustList())
• Else
• take the Customer List out of the cache
• Move the CustList Details to the DropDownList

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CacheCustList Method

• Connect to NorthWind
• Create a DataReader for the Customer records
• For each row provided by the datareader
• Add the Customer Data to the CustList
• Put the CustList in the Cache 5 min
• Close the connection

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butGetOrders_Click

• Create a new dataset to hold the orders for the
  selected customer
• If the Order Data for this customer is not in the
  Cache, If IsNothing(Cache(drpCustList.SelectedItem
  .Value))
• Put it there CacheOrders
• Else
• Set the dataset the Cached orders
• Bind the order data to the grid

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