I have an asp.net web application running on IIS 7 set-up in web-garden mode. I want to clear runtime cache items across all worker processes using a single-step. I can setup a database key-value, but that would mean a thread executing on each worker process, on each of my load-balanced-scenario web servers will poll for changes on that key-value and flush cache. That would be a very bad mechanism as I flush cache items once per day at max. Also I cannot implement a push notification using the SqlCacheDependency with Service Broker notifications as I have a MySql db. Any thoughts? Is there any dirty work-around? One possible workaround, expose an aspx page, and hit that page multiple times using the ip and port on which the site is hosted instead of the domain name – ex: http://ip.ip.ip.ip:82/CacheClear.aspx, so that a request for that page might be sent to all the worker processes within that webserver, and on Page_Load, clear the cache items. But this is a really dirty hack and may not work in cases when all requests are sent to the same worker process.
Answers:
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Method 1
You need to setup inter-process communication.
For caching there are two commonly used ways of doing this:
- Setup a shared cache (memcached or the like.)
- Setup a message queue (e.g. ms-mqueue or rabbitMq) and use it to spread state to the local caches.
A shared cache is the ultimate solution as it means the whole cache is distributed but it is also the most complex: it needs to be set up so the cache load is properly distributed between nodes and make sure it doesn’t become a bottle neck.
The second option requires more code on your part but it is easier if you don’t want to share the cache content (as in your case.)
The easiest is to setup a listener thread or task to handle the cache clear or individual entries invalidation messages. This thread will be dormant if there are no messages so the impact on performance is minimal.
You can also forgo the listener thread by handling messages as part of the usual iis request pipeline. I.e. set up a filter/module that checks for messages in the queue and processes them before handling the request; but performance wise the first option is (slightly) better.
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