Monitoring and Metrics

UDS Core leverages Pepr to handle setup of Prometheus scraping metrics endpoints, with the particular configuration necessary to work in a STRICT mTLS (Istio) environment. We handle this with both mutations of existing service monitors and generation of service monitors via the Package CR.

Mutations

All service monitors are mutated to set the scrape scheme to HTTPS and set the TLS Config to what is required for Istio mTLS scraping (see this doc for details). Beyond this, no other fields are mutated. Supporting existing service monitors is useful since some charts include service monitors by default with more advanced configurations, and it is in our best interest to enable those and use them where possible.

Assumptions are made about STRICT mTLS here for simplicity, based on the istio-injection namespace label. Without making these assumptions we would need to query PeerAuthentication resources or another resource to determine the exact workload mTLS posture.

Note: This mutation is the default behavior for all service monitors but can be skipped using the annotation key uds/skip-sm-mutate (with any value). Skipping this mutation should only be done if your service exposes metrics on a PERMISSIVE mTLS port.

Package CR monitor field

UDS Core also supports generating service monitors from the monitor list in the Package spec. Charts do not always support service monitors, so generating them can be useful. This also provides a simplified way for other users to create service monitors, similar to the way we handle VirtualServices today. A full example of this can be seen below:

...
spec:
  monitor:
    - selector: # Selector for the service to monitor
        app: foobar
      portName: metrics # Name of the port to monitor
      targetPort: 1234 # Corresponding target port on the pod/container (for network policy)
      # Optional properties depending on your application
      description: "Metrics" # Add to customize the service monitor name
      podSelector: # Add if pod labels are different than `selector` (for network policy)
        app: barfoo
      path: "/mymetrics" # Add if metrics are exposed on a different path than "/metrics"

This config is used to generate service monitors and corresponding network policies to setup scraping for your applications. The ServiceMonitors will go through the mutation process to add tlsConfig and scheme to work in an istio environment.

This spec intentionally does not support all options available with a ServiceMonitor. While we may add additional fields in the future, we do not want to simply rebuild the ServiceMonitor spec since mutations are already available to handle Istio specifics. The current subset of spec options is based on the bare minimum necessary to craft resources.

NOTE: While this is a rather verbose spec, each of the above fields are strictly required to craft the necessary service monitor and network policy resources.

Notes on Alternative Approaches

In coming up with this feature a few alternative approaches were considered but not chosen due to issues with each one. The current spec provides the best balance of a simplified interface compared to the ServiceMonitor spec, and a faster/easier reconciliation loop.

Generation based on service lookup

An alternative spec option would use the service name instead of selectors/port name. The service name could then be used to lookup the corresponding service and get the necessary selectors/port name (based on numerical port). There are however 2 issues with this route:

1. There is a timing issue if the Package CR is applied to the cluster before the app chart itself (which is the norm with our UDS Packages). The service would not exist at the time the Package is reconciled. We could lean into eventual consistency here, if we implemented a retry mechanism for the Package, which would mitigate this issue.
2. We would need an “alert” mechanism (watch) to notify us when the service(s) are updated, to roll the corresponding updates to network policies and service monitors. While this is doable it feels like unnecessary complexity compared to other options.

Generation of service + monitor

Another alternative approach would be to use a pod selector and port only. We would then generate both a service and servicemonitor, giving us full control of the port names and selectors. This seems like a viable path, but does add an extra resource for us to generate and manage. There could be unknown side effects of generating services that could clash with other services (particularly with istio endpoints). This would otherwise be a relative straightforward approach and is worth evaluating again if we want to simplify the spec later on.