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I have an application which works well on a single server. My customer now wants to move it to a load-balanced environment. Which things are likely to bite me when doing this?
Currently I know of
Both of these are described here, for example, so I’m looking for things additional to this.
These similar questions, but the first addresses load balancing in general, and I’m looking for a migration guide, and the second addresses a specific problem.
One thing that you might experience is an increased load on your database server. If you implement any serverside caching of data, you will be used to your site hitting the database once for a given dataset, caching the data and then not going to the database again until the cache times out. This is a good strategy for commonly accessed data.
In a load-balanced environment, subsequent requests might go to different servers and the database will be hit twice for the same data. Or more if you have more than 2 servers. This is not bad in itself, but you would be well advised to keep an eye on it. If the database is queueing, the benefits of running a webfarm might be negated. As always, benchmark, profile and run tests.
One way around this is to have sticky-sessions. This is a router-based approach. once a user session is established, all requests from that user are routed to the same server. This has its draw backs, most notably, a potential decrease in the efficiency of the load-balancing, not to mention the problems when you lose a server. Furthermore, it will only help when you are caching mostly user-specific data, such as paged search results, which are only cached for a short time.
Another solution is to have a distributed, in memory cache, such as memcached, Velocity or NCache. There are others as well, but these are the ones that I have worked with.
Another thing to look out for, which is unrelated to the above is: How you deal with file uploads from your users. Many sites allow files to be uploaded by users. If such files have not previously been saved to a central store, such as a database or a common file share, they will need to be.