Use Write Caching to Optimize High Volume Data Driven Applications
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Published: 04 Apr 2005
Unedited - Community Contributed
Abstract
The typical use of caching applies to read caching, or caching data in the application to avoid reading it from the database. However, for applications that must frequently write back to the database, write caching can provide dramatic improvements to performance. This article describes how to implement write caching for an ASP.NET application. Use of SQL Server XML capabilities is also touched on.
by Steven Smith
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Scenario

For the past several years, I’ve been running a small online advertising network (http://ads.aspalliance.com/) which, as I write this, serves about 50 million ads per month. A requirement of this application is that impressions and clicks be tracked so that my customers can evaluate the results of their campaigns. My system uses a SQL Server database that is located on a dedicated server on the same network as my clustered web servers (hosted at ORCSWeb). In this situation, there are dozens of requests per second that must be tracked and persisted to the database, not to mention the requests for data to actually serve the ads. This kind of activity logging is an example of a high volume database operation which can be optimized through the use of caching and batch updating.

In the simplest case, every request for data can be fetched from the database, and every incremental change in statistics can be written back to the database, either as a series of INSERT statements or a combination of INSERT and UPDATE statements. Assuming a fast network connection and a properly tuned database, this can perform quite adequately up to a volume of transactions. However, applying some caching, even for just a brief instant, both for the necessary reads as well as the writes, can dramatically boost the overall performance of the application.


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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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