WICKED CODE: Asynchronous Pages In ASP.NET 2.0

摘要

ASP.NET 2.0 is replete with new features ranging from declarative data binding and Master Pages to membership and role management services. But my vote for the coolest new feature goes to asynchronous pages, and here's why. When ASP.NET receives a request for a page, it grabs a thread from a thread pool and assigns that request to the thread. A normal, or synchronous, page holds onto the thread for the duration of the request, preventing the thread from being used to process other requests. If a synchronous request becomes I/O bound—for example, if it calls out to a remote Web service or queries a remote database and waits for the call to come back—then the thread assigned to the request is stuck doing nothing until the call returns. That impedes scalability because the thread pool has a finite number of threads available. If all request-processing threads are blocked waiting for I/O operations to complete, additional requests get queued up waiting for threads to be free. At best, throughput decreases because requests wait longer to be processed. At worst, the queue fills up and ASP.NET fails subsequent requests with 503 "Server Unavailable" errors.