Making use of Tags

MAAS implements a system of tags based on the physical properties of the nodes. The idea behind this is that you can use the tags to identify nodes with particular abilities which may be useful when it comes to deploying services.

A real world example of this might be to identify nodes which have fast GPUs installed, if you were planning on deploying software which used CUDA or OpenCL which would make use of this hardware.

Tag definitions

Before we can create a tag we need to know how we will select which nodes it gets applied to. MAAS collects hardware information from the nodes using the "lshw" utility to return detailed information in XML format. The definitions used in creating a tag are then constructed using XPath expressions.

If you are unfamiliar with XPath expressions, it is well worth checking out the w3schools documentation. For the lshw XML, we will just check all the available nodes for some properties. In our example case, we might want to find GPUs with a clock speed of over 1GHz. In this case, the relevant XML node from the output will be labelled "display" and does have a property called clock, so it will look like this:

//node[@id="display"]/clock > 1000000000

Now we have a definition, we can go ahead and create a tag.

Creating a tag

Once we have sorted out what definition we will be using, creating the tag is easy using the maas command. You will need to be logged in to the [API][api-authentication] first:

maas maas tags new name='gpu' \
 comment='GPU with clock speed >1GHz for running CUDA type operations.' \
 definition='//node[@id="display"]/clock > 1000000000'

The comment is really for your benefit. It pays to keep the actual tag name short and to the point as you will be using it frequently in commands, but it may subsequently be hard to work out what exactly was the difference between tags like "gpu" and "fastgpu" unless you have a good comment. Something which explains the definition in plain language is always a good idea!

To check which nodes this tag applies to we can use the tag command:

maas maas tag nodes gpu

The process of updating the tags does take some time - not a lot of time, but if nothing shows up straight away, try running the command again after a minute or so.

Using the tag

You can use the tag in the web interface to discover applicable nodes, but the real significance of it is when using juju to deploy services. Tags can be used with juju constraints to make sure that a particular service only gets deployed on hardware with the tag you have created.

Example: To use the 'gpu' tag we created to run a service called 'cuda' we would use:

juju deploy --constraints tags=gpu cuda

You could list several tags if required, and mix in other juju constraints if needed:

juju deploy --constraints "mem=1024 tags=gpu,intel" cuda

Manually assigning tags

MAAS supports the creation of arbitrary tags which don't depend on XPath definitions ("nodes which make a lot of noise" perhaps). If a tag is created without specifying the definition parameter then it will simply be ignored by tag refresh mechanism, but the MAAS administrator will be able to manually add and remove the tag from specific nodes.

In this example we are assuming you are using the 'maas' profile and you want to create a tag called 'my_tag':

maas maas tags new name='my_tag' comment='nodes which go ping'
maas maas tag update-nodes my_tag add="<system_id>"

The first line creates a new tag but omits the definition, so no nodes are automatically added to it. The second line applies that tag to a specific node referenced by its system id property.

You can easily remove a tag from a particular node, or indeed add and remove them at the same time:

maas maas tag update-nodes my_tag add=<system_id_1> \
    add=<system_id_2> add=<system_id_3> remove=<system_id_4>

As the rule is that tags without a definition are ignored when rebuilds are done, it is also possible to create a normal tag with a definition, and then subsequently edit it to remove the definition. From this point the tag behaves as if you had manually created it, but it still retains all the existing associations it has with nodes. This is particularly useful if you have some hardware which is conceptually similar but doesn't easily fit within a single tag definition:

maas maas tag new name='my_tag' comment='nodes I like ' \
       definition='contains(//node[@id=network]/vendor, "Intel")'

maas maas tag update my_tag definition=''
maas mass tag update-nodes my_tag add=<system_id>

Note: If you add and remove the same node in one operation, it ends up having the tag removed (even if the tag was in place before the operation).