Use Write Caching to Optimize High Volume Data Driven Applications
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by Steven Smith
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Write Caching, or Batch Updating

Before moving to the next set of tests, let’s talk about write caching, batch updates, or whatever else you’d like to call it.  In the typical read caching scenario, the database holds the most accurate picture of the actual data, and the application maintains a separate, potentially less-current copy of that data that it periodically updates.  With write caching, these roles are reversed--the web application holds the most current version of the application data, or at least the most current piece of it.  The database has an older version of the data, or an older subset of the data.  Implementing write caching in a data driven .NET application can be done in a number of ways.

Two mechanisms are needed to implement write caching: a method of storing the updates within the application and a method of persisting the updates to the database.  Earlier I compared storage of the data in memory versus on the file system and decided that my preference is in-memory storage.  Once that decision has been made, there are still many ways to go about implementing the in-memory storage of the data.  This will depend to some extent on how detailed is the data being logged. 

When I first began, I started with a very simple Application variable, which I incremented with each request.  This worked okay, until I decided I also needed to track clicks separately, for each advertisement and by date.  Once I started needing multiple counters, I decided it was time to break down and implement my own classes to perform the in-memory storage of the update data.


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User Comments

Title: Modern SQL XML Support   
Name: Steve Smith
Date: 2008-08-08 2:33:41 PM
Comment:
Jason Follas posted recently on how to do the SQL XML work more efficiently in SQL 2005+:
http://jasonfollas.com/blog/archive/2008/06/19/coding-in-sql-server-an-evolution.aspx
Title: MSMQ   
Name: Derek
Date: 2008-01-14 9:37:07 AM
Comment:
Thanks for the article. Having some experience with MSMQ I can tell you it will probably perform well, but we've had a lot of issues with using it in a clustered environment. This is a well-known shortcoming of MSMQ.

We've gone to some 3rd party pub-sub solutions that work much more efficiently and work well in a clustered environment.
Title: Using a synchronized HashTable   
Name: Tim
Date: 2005-08-22 1:41:25 PM
Comment:
Also, how would this be affected by using Hashtable.Synchronized(...) to create the hashtable. It appears that automatically protects the hashtable's Add method and the indexing property Set accessor with a lock, which would protect you during writes.
Title: What about the code for updating the item in Cache?   
Name: Tim
Date: 2005-08-22 12:08:14 PM
Comment:
Can you show sample code for updating the item in Cache? For example, if item is added to cache with certain expiration, etc., when you update it, do you have to add it back with the same parameters, or do you just reference it like Cache[myitem] = updatedValue, and it will retain all previously set values...

Thanks
Title: Programmer   
Name: Microfolx
Date: 2005-05-21 10:33:19 AM
Comment:
This is a very nice site. Keep it up.
Title: Re: Consistency   
Name: Brett
Date: 2005-05-20 5:15:54 PM
Comment:
Nice article... I might consider trying normal XML writers rather than StringBuilder. Or even XML serialization. Probably slightly slower, but reduces errors.

On the point regarding charging for page views. I have two suggestions. One is to periodically reload the current click count from the database and not worry about being exact. Hey if they get 1 million or 1,005,000, does it really matter? This isn't the space shuttle. The second would be to switch to a non-caching code path when you get close to whatever your maximum value is. So when you hit, say 990,000 views (based on your hit rate, etc.) then switch to the non-caching version for that customer.
Title: Consistency   
Name: Brian
Date: 2005-05-20 2:03:29 PM
Comment:
I had to accomplish this exact same scenario. The problem I ran into was the caching. Lets say that one of your advertisers pays you to show their ad 1 million times. How can you insure that it doesn’t go over? If you are holding the most current counters in cache, won’t each process be looking at their own cache, not to mention each server in the web farm and all it’s processes be looking at their own cache.

Another general Cache question. When I write something to cache, isn’t it only available for that particular process? So if you setup your application to have 5 worker processes, won’t that have 5 separate cache entries?
Title: web farm   
Name: parker
Date: 2005-04-24 3:19:08 AM
Comment:
How do you account for disperate sessions in a web farm scenario? Is the cache shared amoung the servers?
Title: Too Good   
Name: Zulfiqar Damani
Date: 2005-04-18 12:46:36 AM
Comment:
It presented very well, also it was interesting too.
Title: Thanks   
Name: Happel
Date: 2005-04-14 8:23:23 AM
Comment:
Thanks for sharing this article, very interesting.






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