Learn how to query data in Prometheus and how to use wildcards
Hello and welcome to this “snippet-sized” post about Prometheus queries! Prometheus is a time-series database which means it is build to collect a lot of datasets that show values over time, for example the result of a HTTP request or the RAM usage of a server. At Synoa we use Prometheus to monitor the health of our APIs and systems. I won’t go into how Prometheus is setup, that’s stuff for a different article, but instead this article focuses on how to query data with wildcards in Prometheus, using Prometheus own query language.
First we need to define a bit of test data. Assume we have the following datasets in Prometheus:
http_status{job="customer-dev",instance="https://dev.some-api.link/service-a",env="dev"}
http_status{job="customer-dev",instance="https://dev.some-api.link/service-b",env="dev"}
http_status{job="customer-dev",instance="https://dev.some-api.link/service-c",env="dev"}
http_status{job="customer-prd",instance="https://some-api.link/service-a",env="prd"}
http_status{job="customer-prd",instance="https://some-api.link/service-b",env="prd"}
http_status{job="customer-prd",instance="https://some-api.link/service-c",env="prd"}
Here we have 6 datasets describing service-a, service-b, and service-c running in the PRD (production) and DEV (development) environment. To get all production services we could query like this:
Prometheus Query
http_status{env="prd"}Result
http_status{job="customer-prd",instance="https://some-api.link/service-a",env="prd"}
http_status{job="customer-prd",instance="https://some-api.link/service-b",env="prd"}
http_status{job="customer-prd",instance="https://some-api.link/service-c",env="prd"}
Wildcards in queries
Coming from MySQL you may think a wildcard could look like http_status{job="customer-*"}, but that’s not the case with Prometheus. Prometheus uses a Regex-like pattern and the wildcard character is .+ (read: dot plus) combined with tilde character (~) instead of just the equal sign (=). So the query becomes http_status{job~="customer-.+"}. In the example below using the .+ wildcard character we search for metrics where the instance label ends with service-c.
{% note “info” %} Prometheus uses the tilde character ~ to indicate a query contains a wildcard. Inside the label-query the “dot plus” (.+) character combination is used where all characters are accepted. {% endnote %}
Prometheus Query
http_status{instance=~".+service-c"}Result
http_status{job="customer-dev",instance="https://dev.some-api.link/service-c",env="dev"}
http_status{job="customer-prd",instance="https://some-api.link/service-c",env="prd"}
Depending on how your metrics are labels querying can be hard or easy. At Synoa I decided to include special labels like env, system, as well as “good” job names. The job label always has the format customer-env-system, e.g. customer-prd-magento or customer-env-ecs. If I want to get all customer metrics I query like http_status{job="customer-.+"}, if I want to see all dev system metrics I can query for http_status{job="customer-dev-.+"}, and so on!
If you got a better label system or a must-have label let me know on Mastodon @KevinGimbel@fosstodon.org.
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