kuma-cp: generate MonitoringAssignment for each Dataplane in a Mesh

[yskopets]

Summary

• kuma-cp: generate MonitoringAssignment for each Dataplane in a Mesh

[jakubdyszkiewicz]

What is the use case of multiple targets?

shouldn't metrics path be in target labels also? In proto definition the docs says

  // Describes a single target that needs to be monitored.
  message Target {

    // Labels associated with that particular target.
    //
    // E.g.,
    // `[
    //    "__address__" :      "192.168.0.1:8080",
    //    "__metrics_path__" : "/metrics"]`,
    //    "instance" :         "backend-01",
    //  ]`.
    map<string, string> labels = 1;
  }

[yskopets]

This is done to meet constraints of Prometheus' file_sd implementation and custom-sd adapter that is recommended for use by all file_sd-based sds.

More detailed version:

• Prometheus model for all sds except for file_sd looks like this:

  // Group is a set of targets with a common label set(production , test, staging etc.).
  type Group struct {
   // Targets is a list of targets identified by a label set. Each target is
   // uniquely identifiable in the group by its address label.
   Targets []model.LabelSet
   // Labels is a set of labels that is common across all targets in the group.
   Labels model.LabelSet
  
   // Source is an identifier that describes a group of targets.
   Source string
  }
  

• that is why Kuma's MonitoringAssignment was designed to be close that model
• however, file_sd uses different model for reading data from a file

  struct {
   Targets []string       `yaml:"targets"`
   Labels  model.LabelSet `yaml:"labels"`
  }
  

  Notice that Targets is just a list of addresses rather than a list of model.LabelSet
• because of that mismatch, some form of conversion is unavoidable inside kuma-prometheus-sd
• the next component that imposes its constraints is custom-sd - adapter that is recommended for use by all file_sd-based sds
• this adapter is doing conversion from Prometheus model into file_sd model and it expects that Targets has only 1 label - __address__ and the rest of the labels come from Labels
• so, we need to convert MonitoringAssignment into a model that custom-sd expects. It could happen on server side, it could happen on client side. Given that we're trying to minimize amount of logic on the client side, the choice was made in favour of server side

[jakubdyszkiewicz]

Thanks for in-depth explanation. Can you leave the comment in the code with shorter explanation that target should only contain address because it's converted to file_sd eventually?

[yskopets]

Done

[golangcibot]

constrainsts is a misspelling of constraints (from misspell)

Suggested change

// Beware of the following constrainsts when it comes to integration with Prometheus: