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When a user does not make any action on the platform i.e he is inactive for a time lapse, he is considered as offline.
The time lapse is configurable in exo.properties file
using this parameter exo.user.status.offline.delay.
The parameter is expressed in millisecond and the value default is 240000 milliseconds.
# The delay when we consider a user as offline. Default value is 240000 milliseconds
exo.user.status.offline.delay=240000