Thread-safe caching in ASP.NET HttpModules

Using multithreading: Thread-safe caching in ASP.NET HttpModules on newest questions tagged multithreading – Stack Overflow

Here’s the scenario

I have an ASP.NET 4.0 application which has a LOT of legacy URL rewrites. I want to build an HTTP module that will look at a database of these rewrites and do a 301 redirect if needed. There’s tens of thousands of records in this table.

So obviously I need to do some sort of caching so I’m not hitting the database and searching through all those records for each page request.

One of our requirements is to have a fast start up time when our application recycles for some reason (worker process recycles or what have you). So I don’t want to load the whole table at app start up. The app start up is already excruciatingly slow.

My brilliant/ridiculous idea is to,

1. At app start up in my HttpModule create an empty dictionary to hold rewrites.
2. Start a background worker thread. The app continues starting up.
3. The background worker thread updates the dictionary with say, oh, the most 1000 most used rewrites.

Questions

My questions are:

1. Is this ridiculous?
2. Is there such a thing as
   a thread-safe dictionary? The
   background thread might be updating
   the dictionary while new requests
   are coming in.
3. Would all the locking
   that goes on in a thread-safe
   collection slow down incoming requests?

Thanks!!

source: http://stackoverflow.com/questions/6380804/thread-safe-caching-in-asp-net-httpmodules
Using multithreading: using-multithreading