Tagging

Auto tags, or tags with expressions

These kind of tags have an associated XPath expression that is evaluated against hardware information as obtained from running lshw during node commissioning.

New or updated tag definition

When a new tag is created or an existing tag is modified, its expression must be evaluated for every node known to the region. It’s a moderately computationally intensive process, so the work is spread out to cluster controllers. Here’s how:

1. The region dispatches a provisioningserver.tasks.update_node_tags job to each cluster for each tag it wants evaluated.

   It sends the tag name and its definition, an XPath expression.

   See maasserver.model.tags.Tag.save and maasserver.populate_tags.

2. The task is run (by Celery) on each cluster. This calls provisioningserver.tags.process_node_tags.

3. The system IDs for all nodes in that cluster (aka node group) are fetched by calling back to the region.

   See provisioningserver.tags.get_nodes_for_node_group.

4. Nodes are then processed in batches. For each batch:

   1. Hardware details are obtained from the region for the batch as a whole.

      See provisioningserver.tags.get_hardware_details_for_nodes.

   2. The tag expression is evaluated against each node’s hardware details. The result of the expression, cast as a boolean, determines if the tag applies to this node.

      See provisioningserver.tags.process_batch.

   3. The results are sent back to the region for the batch as a whole.

      See provisioningserver.tags.post_updated_nodes.

New or updated commissioning result

When a new commissioning result comes in containing lshw or lldp XML output, every tag with an expression must be evaluated against the result so that the node is correctly tagged.

To do this, maasserver.api.VersionIndexHandler.signal calls populate_tags_for_single_node just before saving all the changes. This happens in the region. While it’s a computationally expensive operation, the overhead of spinning this work out to a cluster controller negates any benefit that might gained by doing so.

Manual tags, or tags without expressions

The manual part refers to how these tags are associated with nodes. Instead of being automatically associated as the result of evaluating the tag expression, these tags must be manually associated with a node. A manual tag is denoted by the absence of an expression.