6
MAAS implements a system of tags based on the physical properties of
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the nodes. The idea behind this is that you can use the tags to
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identify nodes with particular abilities which may be useful when it
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comes to deploying services.
11
A real world example of this might be to identify nodes which have
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fast GPUs installed, if you were planning on deploying software which
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used CUDA or OpenCL which would make use of this hardware.
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Before we can create a tag we need to know how we will select which nodes it
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gets applied to. MAAS collects hardware information from the nodes using the
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`"lshw" utility <http://ezix.org/project/wiki/HardwareLiSter>`_ to return
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detailed information in XML format. The definitions used in creating a tag are
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then constructed using XPath expressions.
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If you are unfamiliar with XPath expressions, it is well worth checking out the
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`w3schools documentation <http://www.w3schools.com/xpath/xpath_syntax.asp>`_.
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For the lshw XML, we will just check all the available nodes for some
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In our example case, we might want to find GPUs with a clock speed of over
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In this case, the relevant XML node from the output will be labelled "display"
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and does have a property called clock, so it will look like this::
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//node[@id="display"]/clock > 1000000000
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Now we have a definition, we can go ahead and create a tag.
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Once we have sorted out what definition we will be using, creating the
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tag is easy using the ``maas-cli`` command. You will need to :ref:`be
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logged in to the API first <api-key>`::
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$ maas-cli maas tags new name='gpu' \
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comment='GPU with clock speed >1GHz for running CUDA type operations.' \
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definition='//node[@id="display"]/clock > 1000000000'
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The comment is really for your benefit. It pays to keep the actual tag name
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short and to the point as you will be using it frequently in commands, but it
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may subsequently be hard to work out what exactly was the difference between
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tags like "gpu" and "fastgpu" unless you have a good comment. Something which
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explains the definition in plain language is always a good idea!
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To check which nodes this tag applies to we can use the tag command::
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$ maas-cli maas tag nodes gpu
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The process of updating the tags does take some time - not a lot of time, but
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if nothing shows up straight away, try running the command again after a minute
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You can use the tag in the web interface to discover applicable nodes, but the
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real significance of it is when using juju to deploy services. Tags can be used
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with juju constraints to make sure that a particular service only gets deployed
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on hardware with the tag you have created.
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To use the 'gpu' tag we created to run a service called 'cuda' we would use::
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$ juju deploy --constraints maas-tags=gpu cuda
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You could list several tags if required, and mix in other juju constraints if
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$ juju deploy --constraints "mem=1024 maas-tags=gpu,intel" cuda
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Manually assigning tags
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-----------------------
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MAAS supports the creation of arbitrary tags which don't depend on XPath
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definitions ("nodes which make a lot of noise" perhaps). If a tag is created
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without specifying the definition parameter then it will simply be ignored by
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tag refresh mechanism, but the MAAS administrator will be able to manually add
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and remove the tag from specific nodes.
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In this example we are assuming you are using the 'maas' profile and you want
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to create a tag called 'my_tag'::
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$ maas-cli maas tags new name='my_tag' comment='nodes which go ping'
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$ maas-cli maas tag update-nodes my_tag add="<system_id>"
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The first line creates a new tag but omits the definition, so no nodes are
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automatically added to it. The second line applies that tag to a specific node
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referenced by its system id property.
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You can easily remove a tag from a particular node, or indeed add
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and remove them at the same time::
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$ maas-cli maas tag update-nodes my_tag add=<system_id_1> \
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add=<system_id_2> add=<system_id_3> remove=<system_id_4>
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As the rule is that tags without a definition are ignored when rebuilds are
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done, it is also possible to create a normal tag with a definition, and then
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subsequently edit it to remove the definition. From this point the tag behaves
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as if you had manually created it, but it still retains all the existing
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associations it has with nodes. This is particularly useful if you have some
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hardware which is conceptually similar but doesn't easily fit within a single
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$ maas-cli maas tag new name='my_tag' comment='nodes I like ' \
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definition='contains(//node[@id=network]/vendor, "Intel")'
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$ maas-cli maas tag update my_tag definition=''
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$ maas-cli mass tag update-nodes my_tag add=<system_id>
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If you add and remove the same node in one operation, it ends up having
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the tag removed (even if the tag was in place before the operation).
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docs/tags.rst
