Tomcat's maximum capacity

Our development team has 14 small applications deployed to Tomcat 5.5 currently (moving to 6.0 shortly). I say small because the number of users for each app is low. I'm wondering what performance metrics and indicators I should be looking at to decide when it is time to introduce an additional application server.

Aside from memory usage and CP开发者_开发知识库U utilization, what should I look out for? How do I know when it's time to scale up or out?

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I would be monitoring:

• average HTTP response times
• output of iostat (or whatever counterpart on Windows if you're running Win)
• network saturation
• possible thread deadlocks (not sure how multithreaded your app is)
• database load, if there is one

Generally it is hard to advise when to scale up/out, each situation is different. But it is always worth to have a plan - in case if your customer base will double in the next few months.

Scaling up is usually cheaper (to a point). I recommend this site for scalability-related stuff.