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Monday, July 31, 2006

ASP Tips: Tip 4: Avoid Caching Non-Agile Components in the Application or Session Objects

While caching data in the Application or Session object can be a good idea, caching COM objects can have serious pitfalls. It is often tempting to stuff frequently-used COM objects into the Application or Session objects. Unfortunately, many COM objects, including all of those written in Visual Basic 6.0 or earlier, can cause serious bottlenecks when stored in the Application or Session objects.

Specifically, any component that is not agile will cause performance bottlenecks when cached in the Session or Application objects. An agile component is a component marked ThreadingModel=Both that aggregates the Free-threaded marshaler (FTM), or is a component that is marked ThreadingModel=Neutral. (The Neutral model is new to Windows® 2000 and COM+.) The following components are not agile:

  • Free-threaded components (unless they aggregate the FTM).
  • Apartment-threaded components.
  • Single-threaded component.

Configured components (Microsoft Transaction Server (MTS)/COM+ library and server packages/applications) are not agile unless they are Neutral-threaded. Apartment-threaded components and other non-agile components work best at page scope (that is, they are created and destroyed on a single ASP page).

In IIS 4.0, a component marked ThreadingModel=Both was considered agile. In IIS 5.0, that is no longer sufficient. The component must not only be marked Both, it must also aggregate the FTM. Agility in Server Components describes how to make C++ components written with the Active Template Library aggregate the FTM. Be aware that if your component caches interface pointers, those pointers must themselves be agile, or must be stored in the COM Global Interface Table (GIT). If you can't recompile a Both-threaded component to aggregate the FTM, you can mark the component as ThreadingModel=Neutral. Alternatively, if you don't want IIS performing the agility check (thus, you want to allow non-agile components to be stored at Application or Session scope), you can set AspTrackThreadingModel to True in the metabase. Changing AspTrackThreadingModel is not recommended.

IIS 5.0 will throw an error if you attempt to store a non-agile component created with Server.CreateObject in the Application object. You can work around this by using <object runat="server" scope="application"> in Global.asa, but this is not recommended, as it leads to marshaling and serialization, as explained below.

What goes wrong if you cache non-agile components? A non-agile component cached in the Session object will "lock down" the Session to an ASP worker thread. ASP maintains a pool of worker threads that services requests. Normally, a new request is handled by the first-available worker thread. If a Session is locked down to a thread, then the request has to wait for its associated thread to become available. Here's an analogy that might help: you go to a supermarket, select some groceries, and pay for them at checkout stand #3. Thereafter, whenever you pay for groceries at that supermarket, you always have to pay for them at stand #3, even though other checkout stands might have shorter or even empty lines.

Storing non-agile components at Application scope has an even worse effect on performance. ASP has to create a special thread to run non-agile, Application-scoped components. This has two consequences: all calls have to be marshaled to this thread and all calls are serialized. Marshaling means that the parameters have to be stored in a shared area of memory; an expensive context switch is made to the special thread; the component's method is executed; the results are marshaled into a shared area; and another expensive context switch reverts control to the original thread. Serialization means that all methods are run one at a time. It is not possible for two different ASP worker threads to be executing methods on the shared component simultaneously. This kills concurrency, especially on multiprocessor machines. Worse still, all non-agile Application-scoped components share one thread (the "Host STA"), so the effects of serialization are even more marked.

Confused? Here are some general rules. If you are writing objects in Visual Basic (6.0) or earlier, do not cache them in the Application or Session objects. If you don't know an object's threading model, don't cache it. Instead of caching non-agile objects, you should create and release them on each page. The objects will run directly on the ASP worker thread, so there will be no marshaling or serialization. Performance will be adequate if the COM objects are running on the IIS box, and if they don't take a long time to initialize and destroy. Note that Single-threaded objects should not be used this way. Be careful—VB can create Single-threaded objects! If you have to use Single-threaded objects this way (such as a Microsoft Excel spreadsheet) don't expect high throughput.

ADO recordsets can be safely cached when ADO is marked as Free-threaded. To mark ADO as Free-threaded, use the Makfre15.bat file, which is typically located in the directory \\Program Files\Common\System\ADO.

Warning: ADO should not be marked as Free-threaded if you are using Microsoft Access as your database. The ADO recordset must also be disconnected In general, if you cannot control the ADO configuration on your site (for example, you are an independent software vendor [ISV] who sells a Web application to customers who will manage their own configurations), you are probably better off not caching recordsets.

Dictionary components are also agile objects. The LookupTable loads its data from a data file and is useful for combo-box data as well as configuration information. The PageCache object from Duwamish Books provides dictionary semantics, as does the Caprock Dictionary. These objects, or derivatives thereof, can form the basis of an effective caching strategy. Note that the Scripting.Dictionary object is NOT agile, and should not be stored at Application or Session scope.

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