• <ins id="pjuwb"></ins>
    <blockquote id="pjuwb"><pre id="pjuwb"></pre></blockquote>
    <noscript id="pjuwb"></noscript>
          <sup id="pjuwb"><pre id="pjuwb"></pre></sup>
            <dd id="pjuwb"></dd>
            <abbr id="pjuwb"></abbr>

            Javen-Studio 咖啡小屋

            http://javenstudio.org - C++ Java 分布式 搜索引擎
            Naven's Research Laboratory - Thinking of Life, Imagination of Future

              C++博客 :: 首頁 :: 新隨筆 :: 聯系 :: 聚合  :: 管理 ::
              24 隨筆 :: 57 文章 :: 170 評論 :: 4 Trackbacks
            Enterprise Library 4.1 - October 2008
            Design of the Caching Application Block

            The Caching Application Block is designed to do the following:

            • It provides a set of APIs that are manageable in size.
            • It allows developers to incorporate the standard caching operations into their applications without having to learn the internal workings of the application block.
            • It uses the Enterprise Library configuration tools for easy configuration.
            • It performs efficiently.
            • It is thread safe. Code is considered to be thread safe when it can be called from multiple programming threads without unwanted interaction among those threads.
            • It ensures that the backing store remains intact if an exception occurs while it is being accessed.
            • It ensures that the states of the in-memory cache and the backing store remain synchronized.

            This topic describes the design of the caching system, describing the highlights and specific design details. Other topics in this section include Design of the Expiration Process and Design of the Scavenging Process.

            Figure 1 illustrates the interrelationships between the key classes in the Caching Application Block.

            Dd203150.b4fde6b0-2e54-4dd0-a340-18e14ee2bf18(en-us,MSDN.10).png

            Figure 1
            Design of the Caching Application Block

            When you initialize an instance of the CacheManager using the CacheFactory, it internally creates a CacheManagerFactory object, which in turn creates a Cache object. After the Cache object is created, all data in the backing store is loaded into an in-memory representation that is contained in the Cache object. Applications can then make requests to the CacheManager object to retrieve cached data, add data to the cache, and remove data from the cache.

            When an application uses the GetData method to send a request to the CacheManager object to retrieve an item, the CacheManager object forwards the request to the Cache object. If the item is in the cache, it is returned from the in-memory representation in the cache to the application. If it is not in the cache, the request returns null. If the item is expired, the item also returns null.

            When an application uses the Add method to send a request to the CacheManager object to add an item to the cache, the CacheManager object again forwards the request to the Cache object. If there is already an item with the same key, the Cache object first removes it before adding the new item to the in-memory store and the backing store. If the backing store is the default backing store, NullBackingStore, the data is written only to memory. If the number of cached items exceeds a predetermined limit when the item is added, the BackgroundScheduler object begins scavenging. When adding an item, the application can use an overload of the Add method to specify an array of expiration policies, the scavenging priority, and an object that implements the ICacheItemRefreshAction interface. This object can be used to refresh an expired item from the cache.

            When adding an item that is not already in the in-memory hash table, the Cache object first creates a dummy cache item and adds it to the in-memory hash table. It then locks the cache item in the in-memory hash table, adds the item to backing store, and finally replaces the existing cache item in the in-memory hash table with the new cache item. (In the case where the item was already in the in-memory hash table, it replaces the dummy item.) If there is an exception while writing to the backing store, it removes the dummy item added to the in-memory hash table and does not continue. The Caching Application Block enforces a strong exception safety guarantee. This means that if an Add operation fails, the state of the cache rolls back to what it was before it tries to add the item. In other words, either an operation is completed successfully or the state of the cache remains unchanged. (This is also true for the Remove and Flush methods.)

            The BackgroundScheduler object periodically monitors the lifetime of items in the cache. When an item expires, the BackgroundScheduler object first removes it and then, optionally, notifies the application that the item was removed. At this point, it is the responsibility of the application to refresh the cache.

            The CacheManager class is the interface between the application and the rest of the Caching Application Block. All caching operations occur through this class. For developers who will be using the application block unmodified, the CacheManager object provides all the methods needed to add, retrieve, and remove items from the cache. Every method call made through the CacheManager object is thread safe.

            To create an instance of a CacheManager object, the application uses the CacheFactory class, which in turn uses the CacheManagerFactory class. The CacheManagerFactory class creates all the internal classes needed to implement a CacheManager object.

            Each name applies to only one cache. To create instances of multiple caches, use multiple names. Note that different caches, meaning caches with different names, cannot share the same backing store. There can be only one backing store for each CacheManager object.

            The Cache object receives requests from the CacheManager object and implements all operations between the backing store and in-memory representation of the cached data. It contains a hash table that holds the in-memory representation of the data. (This is the form that users see.) An item of data is packaged as a CacheItem object. This object includes the data itself, together with other information such as the item's key, its priority, the RefreshAction object, and the expiration policy (or array of policies). It is stored in the hash table. The Cache object also uses a synchronized hash table to control access to the items in the cache, both from the application and from the BackgroundScheduler. The Cache object provides thread safety for the entire Caching Application Block.

            The BackgroundScheduler object is responsible for expiring aging cache items and scavenging lower-priority cache items. A PollTimer object triggers the expiration cycle, and a numeric limit triggers the scavenging process. These are set in the configuration file.

            The BackgroundScheduler object is an implementation of the active object pattern. This means that any other object (in this case, the PollTimer) talks to the BackgroundScheduler as if it existed on the thread of the calling object. After it is called, the BackgroundScheduler packages the request as a message and puts it in a queue collection object instead of immediately executing the requested behavior. (Remember that this all occurs in the caller's thread.) This queue is an example of the Producer-Consumer pattern. When the BackgroundScheduler is ready to process the message, an internal thread pulls the message from the queue. In effect, the BackgroundScheduler serializes all scavenging and expiration requests.

            From its own thread, the BackgroundScheduler object sequentially removes messages from the queue and then executes the request. For the expiration process, it calls the Run method in the ExpirationTimeoutExpiredMsg class. For the scavenging process, it calls the Run method in the StartScavengingMsg class. The advantage of performing operations serially on a single thread is that it guarantees that the code will run in a single-threaded environment. This makes both the code and its effects simpler to understand.

            The cache storage classes that are included with the Caching Application Block are the DataBackingStore class, the IsolatedStorageBackingStore class, and the NullBackingStore class. If you are interested in developing your own backing store, your class must either implement the IBackingStore interface or inherit from the abstract BaseBackingStore class, which implements the IBackingStore interface. This class contains implementations of common policies and utilities that can be used by all backing stores.

            The DataBackingStore class is used when the backing store is the Data Access Application Block. Using the configuration tools, it is configured to use a named database instance. The IsolatedStorageBackingStore class stores cache items in domain-specific isolated storage. Using the Configuration Console, it is configured to use a named isolated storage. The Caching Application Block communicates with all backing stores through the IBackingStore interface.

            The DataBackingStore and IsolatedStorageBackingStore classes can encrypt cache item data before it is persisted to storage. The encryption of cache item data is enabled through configuration. Using the configuration tools, cache storage can be configured to use a named symmetric encryption algorithm provider. The named provider is also used when reading data from the cache storage to decrypt the data before populating the cache with the item data.


            posted on 2008-12-26 17:16 Javen-Studio 閱讀(839) 評論(0)  編輯 收藏 引用
            欧美伊香蕉久久综合类网站| 久久精品18| 精品永久久福利一区二区| 色欲久久久天天天综合网| 亚洲综合伊人久久综合| 久久99国产精品久久99| 亚洲精品国精品久久99热| 精品熟女少妇AV免费久久| 国产国产成人精品久久| 亚洲另类欧美综合久久图片区| 亚洲av成人无码久久精品| 伊人色综合久久天天| 亚洲午夜久久久影院伊人| AA级片免费看视频久久| 中文字幕无码精品亚洲资源网久久 | 久久这里只有精品18| 91精品国产91热久久久久福利| 久久精品国产男包| 亚洲精品97久久中文字幕无码| 婷婷久久综合九色综合98| 欧美精品乱码99久久蜜桃| 精品国产热久久久福利| 久久综合九色综合精品| 精品国产乱码久久久久久郑州公司| 2021久久精品免费观看| 国内精品欧美久久精品| 欧美精品一本久久男人的天堂| 狠狠色丁香久久婷婷综合五月| 久久中文字幕人妻熟av女| 久久www免费人成精品香蕉| 亚洲精品高清国产一久久| 久久九九青青国产精品| 2022年国产精品久久久久| 久久99国内精品自在现线| 日产精品久久久久久久| 亚洲精品乱码久久久久久久久久久久 | 久久婷婷激情综合色综合俺也去| 思思久久99热免费精品6| 久久强奷乱码老熟女| 久久久久无码专区亚洲av| 久久久久免费视频|