Discussion - New `PeriodicCollector` to handle a slow `Collector`?

[dhoard]

Current Scenario:

Currently, all Collectors are collected inline... i.e. when...

public Enumeration<Collector.MetricFamilySamples> metricFamilySamples();

... and similar methods are called. This works well for Collectors that are performant.

However, some Collectors have code paths that are computationally expensive to call during a scrape interval.

Solution 1 - Decrease the scrape interval/increase the scrape timeout.

This will work, but delays the collection of all metrics, simply because 1 of N Collectors is slow.

Solution 2 - Refactor the slow Collector

Refactoring the Collector may not be possible due to the nature of the code, timing, locks, etc. Additionally, the code may be closed source.

Solution 3 - Add a new PeriodCollector

We could create a PeriodicCollector implementation (totally optionally to use) that can wrap an existing Collector and perform the collection on a periodic interval (fixed rate or fix delay) and decouple the collection of metrics.

Thoughts?

[dhoard]

Initial implementation of a PeriodicCollector and PeridocCollectorTest...

https://github.com/dhoard/client_java/blob/periodic_collector/simpleclient/src/main/java/io/prometheus/client/PeriodicCollector.java

https://github.com/dhoard/client_java/blob/periodic_collector/simpleclient/src/test/java/io/prometheus/client/PeriodicCollectorTest.java