Vector Matching in PromQL

When working with Prometheus, you’re likely familiar with performing operations between vectors and scalars, even if you haven’t explicitly thought about it. Queries like:

up == 0
node_arp_entries > 100

are examples of operations between instant vectors and scalars. However, PromQL also allows operations between two instant vectors, provided they follow specific matching rules.

Operations on Instant Vectors

Consider the following metrics:

node_filesystem_avail_bytes{instance="node1", job="node", mountpoint="/home"} 234
node_filesystem_avail_bytes{instance="node1", job="node", mountpoint="/var"} 485
node_filesystem_size_bytes{instance="node1", job="node", mountpoint="/home"} 28434
node_filesystem_size_bytes{instance="node1", job="node", mountpoint="/var"} 3485

You can perform an operation like:

node_filesystem_avail_bytes / node_filesystem_size_bytes * 100

This results in:

{instance="node1", job="node", mountpoint="/var"} 5485

Rules for Vector Matching

To perform operations between two instant vectors, the vectors must have matching labels. The default rule is that all labels should be identical. However, PromQL provides mechanisms to modify matching behavior.

One-to-One Matching