How use pkl with a interface which holds a config for a repository configuration

[austin-barrington]

Hello,

I am very new to PKL and as such I might be missing something here.

I am writing a program which takes in a configuration that uses an interface to abstract the config of datasources. Datasources are configuration entities for clients to write to different databases. e.g. influxdb, prom, clickhouse.

Config Example:

outputs:
  - name: "influx2"
    type: "influxdb_v1"
    config:
      host: http://localhost:8086
      database: slo
  - name: "prometheus"
    type: "prometheus"
    config:
      url: http://localhost:9090/metrics

My structs are:

type Configuration struct {
	Inputs  []Datasource `pkl:"inputs"`
	Outputs []Datasource `pkl:"outputs"`
}

type Datasource struct {
	Name string `pkl:"name"`
	Type string `pkl:"type"`

	// Config is a datasource interface that can be used to store any configuration
	// this is defined by the type of the datasource
	Config interface{} `pkl:"config"`
}

// Database Configuration
type InfluxV1Configuration struct {
	Host     string `yaml:"host"`
	Username string `yaml:"username"`
	Password string `yaml:"password"`
	Database string `yaml:"database"`
}

This is where Datasource.Config can equal InfluxV1Configuration or another kind of struct.

My code when loading:

config := entities.Configuration{}

ctx := context.Background()

evaluator, err := pkl.NewEvaluator(ctx, pkl.PreconfiguredOptions)
if err != nil {
	return config, err
}
defer func() {
	cerr := evaluator.Close()
	if err == nil {
		err = cerr
	}
}()
if err := evaluator.EvaluateModule(ctx, pkl.FileSource(file), &config); err != nil {
	return config, err
}

fmt.Printf("config: %+v\n", config)
os.Exit(0)

Error: panic: reflect.Set: value of type pkl.Object is not assignable to type entities.Datasource

This is because config is an interface (tested by removing it). So thoughts on how I go about sorting this?

[austin-barrington]

Gave up and used code gen which after tinkering seemed to work.

Final output:

spec.pkl

@go.Package { name = "src/entities/pkl" }
module srenity.entities.Configuration;

import "package://pkg.pkl-lang.org/pkl-go/[email protected]#/go.pkl" 

inputs: Listing<Datasource>
outputs: Listing<Datasource>
@go.Name { value = "SLOs" }
slos: Listing<SLO>

class Datasource {
    name: String
    type: String

    // golang interface
    config: Dynamic
}

class SLO {
    name: String
    description: String
    output: String
    @go.Name { value = "SLIs" }
    slis: Listing<SLI>
}

class SLI {
    name: String
    description: String
    input: String
    interval: Duration
    query: String
    goal: Float
}

generate: pkl-gen-go ./pkl/spec.pkl

You can then use a config like so:

amends "./spec.pkl"
inputs = new {
    new {
        name = "influx1"
        type = "influxdb_v1"
        config = new {
            url = "http://localhost:8086"
        }
    }
}
outputs = new {
    new {
        name = "influx2"
        type = "influxdb_v1"
        config = new {
            url = "http://localhost:8086"
        }
    }
}
slos = new {
    new {
        name = "My SLO"
        description = "My SLO description"
        output = "influx2"
        slis {
            new {
                name = "What is the % of successful requests?"
                description = ""
                interval = 10.s
                input = "influx1"
                query = "SELECT (mean(http_success) / mean(http_fails)) * 100 FROM http_requests WHERE time > now() - 5m GROUP BY time(1m)"
                goal = 99.9
            }
            new {
                name = "What is the % of successful requests?"
                description = ""
                interval = 10.s
                input = "influx1"
                query = "SELECT (mean(http_success) / mean(http_fails)) * 100 FROM http_requests WHERE time > now() - 5m GROUP BY time(1m)"
                goal = 99.9
            }
        }
    }
}

The bit that got me was that module srenity.entities.Configuration; created a top level go struct that I did not need to add a class into the spec.pkl for.

[holzensp]

Glad you found this yourself. Btw, for quicker / better response; this sort of issue, you'll want to file on https://github.com/apple/pkl-go

[austin-barrington]

Thanks, I shall do so! Thank you for open sourcing this as it's very useful.