[FIXED] Tomcat max connections on a per context basis

Issue

I have multiple web applications running under a single Tomcat container. Since they all run under a single Tomcat connector (as defined in the server.xml file), attributes such as maxConnections and maxThreads govern the container as a whole. As a result it is possible for a single application to consume all available Tomcat threads, starving the other applications of threads and making them unresponsive. I would like to be able to define the maximum http threads on a per context basis so that this is no longer possible.

Here's what I've tried so far:

1. Create a custom filter in the application that keeps track of the current thread count and limits additional connections. (Got the filter here: How to set limit to the number of concurrent request in servlet?). I'm not sure I like this solution, as it isn't as full-featured (support for attributes such as acceptCount, maxConnections, maxThreads, and minSpareThreads) as Tomcat provides by default to the container; and adding in the features feels like I am attempting to build what already exists in Tomcat.
2. Create a separate Tomcat connector in the server.xml file for each context. This has a few issues. For one, each connector requires a separate port; this means I'll have to account for this in my apache config. Secondly, I plan to add more webapps regularly; this means a config change followed by a tomcat restart, which is disruptive to clients.

Has anyone else encountered something like this? I feel like there should be a "Tomcat supported" workflow to accomplish what I'm after.

Solution

I'm going to post an answer that was provided to me from the Tomcat user group: http://tomcat.apache.org/tomcat-9.0-doc/config/valve.html#Semaphore_Valve (The Semaphore Valve is not Tomcat 9 specific, but was actually introduced in Tomcat 6). I experimented with this concept, and I found the following practical applications:

1. (Untested) The Semaphore Valve should be able to be nested within the Host element in the server.xml file.
2. (Tested) A [context-name].xml file can be placed inside [tomcat-home]/conf/Catalina/localhost with the valve nested within the Context element.

This is not necessarily the solution that I am going with, as more testing will need to be performed. However, I thought I'd add this as it is a potential answer to the problem.

Update:
As a recap, the SemaphoreValve was an option that was recommended to me through the Tomcat user mailing list as a solution to the issue that I described above. It turns out it was easier to implement than I anticipated. Adding the following to context.xml in the Tomcat/conf directory did the trick:

<Valve className="org.apache.catalina.valves.SemaphoreValve" 
       concurrency="10" 
       fairness="true" />

Thanks to Mark Thomas from the Apache group for supplying the solution.

Answered By - Bryan Larson
Answer Checked By - Gilberto Lyons (JavaFixing Admin)