Machine and Unit numbering

Within an environment juju keeps track of sequences for machines and units. (Prior to 1.25 just machines). This means that for the life of an environment the number associated with a machine will only ever increase. For example:

juju add-machine # Add machine 0
juju add-machine # Add machine 1
juju terminate-machine # Remove machine 1
juju add-machine # Add machine 2

From 1.25 onwards unit numbering follows the same scheme. For each application name the number will only every increase. For example:

juju deploy mongodb -n 2 # Adds units 0,1
juju remove-application mongodb # Removed units 0,1
juju deploy mongodb -n 2 # Adds units 2,3

Prior to 1.25 units would only ever increase for a single application. If you removed the application and created a new one of the same name the numbering would start again from 0.

For example:

juju deploy mongodb -n 2 # Adds units 0,1
juju remove-application mongodb # Removed units 0,1
juju deploy mongodb -n 2 # Adds units 0,1