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Machine and Unit numbering

Within a model Juju keeps track of sequences for machines and units. (Prior to 1.25 just machines). This means that for the life of a model, 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.