0022. json_exporter instead of an exporter of our own

• Status: accepted
• Date: 2026-10-22

Context

dynamic.json and info.json are JSON documents, and Prometheus reads only its own text formats. Something has to turn four fields into metrics: the player count, the slots, the version string and the OneSync flag, and, for the build alert, the Recommended build from the platform’s public version list. That component runs on the monitoring host, fetches URLs on the game port, and has to be installed, updated and secured by every user of the pack.

Considered options

1. prometheus-community’s json_exporter (0.8.0) with modules in json_exporter/config.yml.
2. A small exporter of our own (Python or Go) that fetches the endpoints and serves metrics.
3. A script on a timer that writes the values into a file for node_exporter’s textfile collector.

Decision

Chosen option 1, because the four fields are plain values that json_exporter’s JSONPath reads directly, so the pack stays configuration (ADR 0005). json_exporter is a project of the prometheus-community organisation, released with a checksums file for each release, and runs as an ordinary service; setups that already run it only add three modules. The same exporter reads the version list (fxserver_versions). It fetches per scrape, so a value is as fresh as the scrape. Option 2 would give full control (for example a numeric build metric instead of a build label), but it is code to write, package, sign, release and keep secure for a job that configuration does. Option 3 serves values as old as the last timer run and needs a script with network access on the game or monitoring host.

Consequences

• Good: no program of this project runs on the monitoring host; users update json_exporter like any other exporter (toolchain has the download and its sha256).
• Good: the CI job exporters runs the pinned json_exporter release with the shipped modules against the fake server, so a wrong path or name fails CI (ADR 0012).
• Bad: json_exporter fetches any URL that whoever reaches it asks for, so it must listen on loopback or a private address (configuration).
• Bad: JSONPath cannot count or parse, so the build number is a label of fxserver_info that the scrape job cuts out of the version string, and the build alerts read the number from the fxserver role’s textfile metrics instead (ADR 0017). json_exporter ignores keys it does not know, so scripts/check_pack.py reports unknown keys in the modules.