The ActiveMQ broker runs under the activemq user account by default. Most systems will limit non-root users' usage of resources using the user limit (ulimit) mechanism. When the broker runs as the activemq user, the number of open files permissible by the activemq user will limit the number of open connections that the process can have (each connection consumes an open file handle). Each node in the message queue needs an active connection. The number of nodes connected to the broker will be roughly equal to the number of open files allowed. After running a broker on my EL6 system, the default ulimit value is 1024, and the number of nodes that I would typically be able to reach via the broker is 1000. You can check the limit by running ulimit as the activemq user, as follows:

activemq@puppet $ ulimit -n
1024

The limit can be increased by creating a configuration file in /etc/security/limits.d or adding an entry to /etc/security/limits.conf. The syntax for these...