避免10 个常见ASP.NET缺陷 使网站平稳运行
SQL Server Session State: Another Perf Killer
SQL Server 会话状态：另一个性能杀手
ASP.NET makes it easy to store session state in databases: just flip a switch in web.config and session state magically moves to a back-end database. This is a critical feature for applications that run on Web farms because it allows every server in the farm to share a common repository for session state. The added database activity slows the performance of individual requests, but the loss in performance is offset by an increase in scalability.
This is all fine and good until you take a moment to ponder the following points:
- Even in an application that uses session state, most pages do not use session state.
- By default, the ASP.NET session state manager performs two accesses—one read access and one write access—to the session data store in every request, regardless of whether the page requested uses session state.
ASP.NET 使得在数据库中存储会话状态变得简单：只需切换 web.config 中的开关，会话状态就会轻松地移动到后端数据库。对于在 Web 领域中运行的应用程序来说，这是一项重要功能，因为它允许该领域中的每个服务器共享会话状态的一个公共库。添加的数据库活动降低了单个请求的性能，但是可伸缩性的提高弥补了性能的损失。
这看起来都还不错，但是您略微考虑一下下列几点，情况就会有所不同：
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即使在使用会话状态的应用程序中，大多数页也不使用会话状态。 |
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默认情况下，ASP.NET 会话状态管理器对每个请求中的会话数据存储执行两个访问（一个读取访问和一个写入访问），而不管请求的页是否使用会话状态。 |
In other words, when you use the SQL Server™ session state option, you pay the price (two database accesses) in every request—even in requests for pages that do nothing with session state. This directly (and adversely) affects the throughput of the entire site.
换句话说，当您使用 SQL Server™ 会话状态选项时，您在每个请求中都要付出代价（两个数据库访问）— 甚至在与会话状态无关的页面的请求中。这会直接对整个网站的吞吐量造成负面影响。
图 5 消除不必要的会话状态数据库访问
So what do you do about it? Simple: you disable session state in pages that don’t use it. It’s always a good idea to do so, but it’s especially important when session state is stored in a database. Figure 5 shows how to disable it. If a page doesn’t use session state at all, include an EnableSessionState="false" in its Page directive, like so:
<%@ Page EnableSessionState="false" ... %>
This directive prevents the session state manager from reading and writing the session state database in each request. If a page reads data from session state but doesn’t write it (that is, doesn’t modify the contents of the user’s session), then set EnableSessionState to ReadOnly, as shown here:
<%@ Page EnableSessionState="ReadOnly" ... %>
Finally, if a page requires read/write access to session state, then either omit the EnableSessionState attribute or set it to true:
<%@ Page EnableSessionState="true" ... %>
那么您应该怎么办呢？很简单：禁用不使用会话状态的页中的会话状态。这样做总是一个好办法，但是当会话状态存储在数据库中时，该方法尤其重要。图 5 显示如何禁用会话状态。如果页面根本不使用会话状态，请在其 Page 指令中包含 EnableSessionState="false"，如下所示：
<%@ Page EnableSessionState="false" ... %>
该指令阻止会话状态管理器在每个请求中读取和写入会话状态数据库。如果页面从会话状态中读取数据，但却不写入数据（即，不修改用户会话的内容），则将 EnableSessionState 设置为 ReadOnly，如下所示：
<%@ Page EnableSessionState="ReadOnly" ... %>
最后，如果页面需要对会话状态进行读/写访问，则省略 EnableSessionState 属性或将其设置为 true：
<%@ Page EnableSessionState="true" ... %>
By taming session state in this way, you’ll ensure that ASP.NET only accesses the session state database when it really needs to. Eliminating unnecessary database accesses is the first step toward building high-performance applications.
通过以这种方式控制会话状态，可以确保 ASP.NET 只在真正需要时才访问会话状态数据库。消除不必要的数据库访问是构建高性能应用程序的第一步。
Incidentally, the EnableSessionState attribute is no secret. It’s been documented since ASP.NET 1.0, yet I rarely see developers take advantage of it. Perhaps this is because it’s not terribly important with the default in-memory session state model. But it’s critical with the SQL Server model.
顺便说一下，EnableSessionState 属性是公开的。该属性自 ASP.NET 1.0 以来就已经进行了说明,但是我至今仍很少见到开发人员利用该属性。也许是因为它对于内存中的默认会话状态模型并不十分重要。但是它对于 SQL Server 模型却很重要。
from:asp学习网/title:避免10 个常见ASP.NET缺陷 使网站平稳运行/ time:2007-5-20 17:41:09
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