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ref https://linear.app/tryghost/issue/ENG-1505/add-prometheus-metrics-server-to-allow-monitoring-ghost-metrics # Summary This commit includes two main components: a prometheus client class to collect metrics from Ghost, and a standalone metrics server that exposes a /metrics endpoint at a separate port (9416 by default) from the main Ghost app. The prometheus client is a very thin wrapper around [prom-client](https://github.com/siimon/prom-client). We could use prom-client directly, but this approach should make it easier to switch to a different prometheus client package (or make our own) if we ever need to down the line. The list of default metrics this enables is specified in an e2e test [here](https://github.com/TryGhost/Ghost/pull/21192/files#diff-ebc52236be2cd14b40be89220ae961f48d3f837693f7d1da76db292348915941R66-R92). This also gives us the ability to create and collect custom metrics, although none are included in this commit yet. # Configuration The prometheus client and the metrics server are both disabled by default, but can be enabled by setting the metrics_server:enabled flag to true. You can also define a custom host and port using `metrics_server:host` and `metrics_server:port`. ## Why not expose the /metrics endpoint in one of the existing express apps? The standalone express app exists for two main reasons: 1. We don't want these metrics to be public, and the easiest way to accomplish that is to expose the /metrics endpoint at a different port that won't be exposed to the internet. 2. Creating a standalone express instance decouples the metrics endpoint from the Ghost server, so if Ghost is not responding for whatever reason, we should still be able to scrape metrics to understand what's going on internally. ## Impact on Boot & Shut down time The prometheus client is initialized early in the boot process so we can collect metrics during the boot sequence. Testing locally has shown that this increases boot time by ~20ms. The metrics server which exposes the /metrics endpoint is not initialized until after the background services, and it is not awaited, to avoid impacting boot time. None of this code, including the requires, will run if the metrics_server:enabled flag is set to false (or not set). Shutting down the metrics server is added as a cleanup task for the main Ghost server instance, and is setup to shut down with 0 grace period to avoid impacting shut down time. |
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Metrics Server
A standalone server for exporting prometheus metrics from Ghost
Usage
Develop
This is a monorepo package.
Follow the instructions for the top-level repo.
git clonethis repo &cdinto it as usual- Run
yarnto install top-level dependencies.
Test
yarn lintrun just eslintyarn testrun lint and tests