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?
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.