I would like to propose adding context to the client_golang promhttp.Handler functions, through a new set of functions in the promhttp package. I believe that there was some discussion around this, but not for a long time.
The goal of this change is for exporters other metric providers to have an opportunity to understand cancellation. The cancellation could be through the http.Request.Context() being cancelled when the http request is closed from the client, but Prometheus also provides a header "X-Prometheus-Scrape-Timeout-Seconds" on scrape requests. Today, if an exporter has an expensive collector, there is no way to know that the results will be thrown away and that collection can be stopped in the event that the scrape from Prometheus has timed out.[1] I have only begun to dig into the work that would be required to propagate the context the whole way down through a Registry and I can already tell that it would not be trivial. The general idea would be to add context versions of promhttp.Handler() and the supporting functions that would pull the context from the http.Request and optionally create a child context with the timeout/deadline from the X-Prometheus-Scrape-Timeout-Seconds header. Downstream, the prometheus.Registry would also need to understand context. I think this may only need an additional GatherCtx() func, but the downstream prometheus.Collector interface would probably need a context-aware counterpart. I believe that this is all possible without any breaking changes, but I have not researched enough to know for sure. I want to put this out to the community and maintainers to get some feedback before spending too much time trying to make these changes. Joe Adams @sysadmind 1. https://github.com/prometheus-community/postgres_exporter/pull/558 -- You received this message because you are subscribed to the Google Groups "Prometheus Developers" group. To unsubscribe from this group and stop receiving emails from it, send an email to prometheus-developers+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/prometheus-developers/25a3ef44-6f02-492f-8f6c-28383cd0d6d8n%40googlegroups.com.
