api_call_latency

API call latency SLIs/SLOs details

User stories

  • As a user of vanilla Kubernetes, I want some guarantee how quickly I get the response from an API call.
  • As an administrator of Kubernetes cluster, if I know characteristics of my external dependencies of apiserver (e.g custom admission plugins, webhooks and initializers) I want to be able to provide guarantees for API calls latency to users of my cluster.

Other notes

  • We obviously can’t give any guarantee in general, because cluster administrators are allowed to register custom admission plugins, webhooks and/or initializers, which we don’t have any control about and they obviously impact API call latencies.
  • As a result, we define the SLIs to be very generic (no matter how your cluster is set up), but we provide SLO only for default installations (where we have control over what apiserver is doing). This doesn’t provide a false impression, that we provide guarantee no matter how the cluster is setup and what is installed on top of it.
  • At the same time, API calls are part of pretty much every non-trivial workflow in Kubernetes, so this metric is a building block for less trivial SLIs and SLOs.
  • The SLO for latency for read-only API calls of a given type may have significant buffer in threshold. In fact, the latency of the request should be proportional to the amount of work to do (which is number of objects of a given type in a given scope) plus some constant overhead. For better tracking of performance, we may want to define purely internal SLI of “latency per object”. But that isn’t in near term plans.

Caveats

  • The SLO has to be satisfied independently from used encoding in user-originated requests. This makes mix of client important while testing. However, we assume that all core components communicate with apiserver using protocol buffers.
  • In case of GET requests, user has an option opt-in for accepting potentially stale data (being served from cache) and the SLO again has to be satisfied independently of that. This makes the careful choice of requests in tests important.

TODOs

  • We may consider treating non-namespaced resources as a separate bucket in the future. However, it may not make sense if the number of those may be comparable with namespaced ones.

Test scenario

TODO: Describe test scenario.