One of the service discovery methods Prometheus supports is Consul. Let's look at how to use it.

Finding targets happens in two stages. First a service discovery method such as Consul returns potential targets with metadata. Secondly relabelling allows you to choose which of those targets you want to scrape, and how to convert the metadata into target labels. We've looked at the theory of relabelling previously, so let's make this more concrete.

Lets say you wanted to monitor all services with a prod tag and use the Consul service name as the job label. Your scrape configuration would look like:

  - job_name: dummy
      - server: 'localhost:8500'
      - source_labels: [__meta_consul_tags]
        regex: .*,prod,.*
        action: keep
      - source_labels: [__meta_consul_service]
        target_label: job

The first relabel action says to keep processing only those targets who have a prod tag. Prometheus exposes the Consul tags as a comma separated list in the label called __meta_consul_tags, with leading and trailing commas added for convenience.

The second relabel action says to copy the service name from the __meta_consul_service label to the job label. This is take advantage of the default values for relabel actions, as a straight copy from one label to another is common.


What if you wanted to do something more complicated?

Let's say you wanted the job label to come from a tag with the format job-XXX. We could replace the last relabel action with:

      - source_labels: [__meta_consul_tags]
        regex: .*,job-([^,]+),.*
        replacement: '${1}'
        target_label: job

This extracts the part of the Consul tag that's of interest to us, and copies it to the job label.  ([^,]+) means to capture at least one character which is not a comma.


This is just a small taste of what's possible. With the flexibility of relabelling you can map however you think about your services into Prometheus labels!