Summary

A developer asks:

How can I programmatically count the number of logged in users on a single node and across all nodes that comprise my system?

Suggested Approach

Information about logged in users, including how long and often they're logged in, is written to database table pr4_log_usage.  The data you seek can be gleaned from querying the table, whose columns are fully exposed. 

To prevent excessively long query times against the very large pr4_log_usage table, make sure to include date/time selection criteria in your query.