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Machine Constraints

Machine constraints allow you to choose the hardware to which your services will be deployed.

Constraints can be set for environments and services, with service constraints overriding environment constraints, the default values set by juju when otherwise unspecified. Changes to constraints do not affect any unit that has already been assigned to a machine.

Constraint can be set by the juju set-constraints command, taking an optional --service arg, and any number of key=value pairs. When the service name is specified, the constraints are set on that service; otherwise they are set on the environment.

Valid choices for the value are generally dependent on the particular constraint, with one exception:

  • An empty value always means "not constrained". This allows you to ignore environment settings at the service level without having to explicitly remember and re-set the juju default values. Note that there is no way to change the juju default values, though environment settings will override them.

The commands juju deploy, juju bootstrap, and juju add-machine have a --constraints flag which expects a single string of space-separated constraints, understood as above. Deployment constraints will be set on the service before the first unit is deployed, and bootstrap constraints will be set on the environment and used to provision the initial master machine.

The juju get-constraints command is used to see the currently applicable constraints. When called without an argument, it outputs the environment constraints as key=value pairs. Alternatively, you can request yaml or json with the --format flag. By passing the name of a service as an argument, it will output the constraints on that particular service.


Deploy MySQL on a machine with at least 32GiB of RAM, and at least 8 ECU of CPU power (architecture will be inherited from the environment, or default to amd64):

juju deploy --constraints "cpu-cores=8 mem=32G" mysql

Deploy to t1.micros on AWS:

juju bootstrap --constraints "cpu-power=0 cpu-cores=0 mem=512M"

Launch all future "mysql" machines with at least 8GiB of RAM and 4 ECU:

juju set-constraints --service mysql mem=8G cpu-cores=4

Output current environment constraints:

juju get-constraints

Output constraints for mysql

juju get-constraints mysql

Provider Constraints

See a complete listing of constraints for details on what each constraint means. Two of the most commonly used are:

  • cpu-cores: The minimum processing power of the machine, roughly indicated by how many cores are available.
  • mem: The minimum memory for the machine, defaulting to 512MB.

Working with constraints

Here are some examples of working with constraints.

When bootstrapping an environment, you can set the constraints directly:

juju bootstrap --constraints arch=i386

The above command did two things:

  • Set the environment constraints to require machines with an i386 architecture, leaving the other defaults untouched; this is precisely equivalent to: bash juju bootstrap --constraints "arch=i386 cpu-cores= mem= " ...but rather more convenient to type.

  • Started the bootstrap/master machine with the above constraints.

Because the environment constraints were set, subsequent deployments will use the same values:

juju deploy mysql

...but other services can be started with their own constraints:

juju deploy wordpress --constraints mem=1024

Note that if you try to deploy a machine or service with a constraint that cannot be fulfilled by the environment, the deployment will fail. Running juju status will show an error in the status for that machine. You can fix the problem by removing the machine (and the service assigned to the machine, if any) and retrying the deployment with different constraints.

MAAS constraints

If you are deploying to a MAAS provider, you may use the additional constraint tags=, followed by a comma-delimited list of tags. Only MAAS nodes thus tagged will be considered appropriate for deploying the service.


juju deploy mysql --constraints tags=foo,bar

...will deploy MySQL only to a node which has been tagged with both "foo" and "bar".