When a definition is supplied during a tag's creation the tag is automatically applied to all the nodes that satisfy the definition:
maas $PROFILE tags create name=$TAG_NAME \ comment='$TAG_COMMENT' definition='$TAG_DEFINITION'
maas $PROFILE tags create name='gpu' \ comment='GPU with clock speed >1GHz for running CUDA type operations.' \ definition='//node[@id="display"]/'clock units="Hz"' > 1000000000'
We recommend that each tag have a short name and a comment that fully describes it. Having both will help with usage and for recalling a tag's meaning long after it was created.
To list all tags present on the region controller:
maas $PROFILE tags read
To list what nodes (or machines) a tag applies to:
maas $PROFILE tag nodes $TAG_NAME maas $PROFILE tag machines $TAG_NAME
Although a tag can be used in the web UI as a node search filter the primary benefit of tags is realized when Juju is utilized for application deployment.
For example, to use the 'gpu' tag to deploy a (hypothetical) service called 'cuda':
juju deploy --constraints tags=gpu cuda
You can also use multiple tags in addition to the normal Juju constraints:
juju deploy --constraints "mem=1024 tags=gpu,intel" cuda
It is possible to assign tags to nodes manually by simply omitting the definition and applying the tag to a node by referencing its system id:
maas $PROFILE tags create name=$TAG_NAME comment='$TAG_COMMENT' maas $PROFILE tag update-nodes $TAG_NAME add=$SYSTEM_ID
To remove a tag:
maas $PROFILE tag update-nodes $TAG_NAME remove=$SYSTEM_ID
In the same operation, a tag can be added to some nodes and removed from others:
maas $PROFILE tag update-nodes $TAG_NAME \ add=$SYSTEM_ID_1 add=$SYSTEM_ID_2 remove=$SYSTEM_ID_3
It is also possible to create a tag with a definition (thereby map to certain nodes), remove the definition (but retain the mapping), and then add the tag manually to specific nodes. This is useful for hardware which is conceptually similar but do not all satisfy a single tag definition. Here are the commands you would use to do this:
maas $PROFILE tags create name=$TAG_NAME \ comment='$TAG_COMMENT' definition='$TAG_DEFINITION' maas $PROFILE tag update $TAG_NAME definition='' maas $PROFILE tag update-nodes $TAG_NAME add=$SYSTEM_ID