monero-exporter/pkg/exporter/exporter.go

package exporter

import (
	"context"
	"fmt"
	"net"
	"net/http"

	"github.com/go-logr/logr"
	"github.com/go-logr/zapr"
	"github.com/prometheus/client_golang/prometheus/promhttp"
	"go.uber.org/zap"
)

const (
	defaultBindAddress   = ":9000"
	defaultTelemetryPath = "/metrics"
)

// Exporter is responsible for bringing up a web server that collects metrics
// that have been globally registered via prometheus collectors (e.g., see
// `pkg/collector`).
//
type Exporter struct {
	bindAddress   string
	telemetryPath string

	listener net.Listener
	log      logr.Logger
}

// WithTelemetryPath overrides the default path under which the prometheus
// metrics are reported.
//
// For instance:
//   - /
//   - /metrics
//   - /telemetry
//
func WithTelemetryPath(v string) Option {
	return func(e *Exporter) {
		e.telemetryPath = v
	}
}

// WithBindAddress overrides the default address at which the prometheus
// metrics HTTP server would bind to.
//
// Examples:
//   - :8080
//   - 127.0.0.2:1313
//
func WithBindAddress(v string) Option {
	return func(e *Exporter) {
		e.bindAddress = v
	}
}

// Option allows overriding the exporter's defaults
//
type Option func(e *Exporter)

// New instantiates a new exporter with defaults, unless options are passed.
//
func New(opts ...Option) (*Exporter, error) {
	defaultLogger, err := zap.NewDevelopment()
	if err != nil {
		return nil, fmt.Errorf("zap new development: %w", err)
	}

	e := &Exporter{
		bindAddress:   defaultBindAddress,
		telemetryPath: defaultTelemetryPath,
		log:           zapr.NewLogger(defaultLogger.Named("exporter")),
	}

	for _, opt := range opts {
		opt(e)
	}

	return e, nil
}

// Run initiates the HTTP server to serve the metrics.
//
// ps.: this is a BLOCKING method - make sure you either make use of goroutines
// to not block if needed.
//
func (e *Exporter) Run(ctx context.Context) error {
	var err error

	e.listener, err = net.Listen("tcp", e.bindAddress)
	if err != nil {
		return fmt.Errorf("listen on '%s': %w", e.bindAddress, err)
	}

	doneChan := make(chan error, 1)

	go func() {
		defer close(doneChan)

		e.log.WithValues(
			"addr", e.bindAddress,
			"path", e.telemetryPath,
		).Info("listening")

		http.Handle(e.telemetryPath, promhttp.Handler())
		if err := http.Serve(e.listener, nil); err != nil {
			doneChan <- fmt.Errorf(
				"failed listening on address %s: %w",
				e.bindAddress, err,
			)
		}
	}()

	select {
	case err = <-doneChan:
		if err != nil {
			return fmt.Errorf("donechan err: %w", err)
		}
	case <-ctx.Done():
		return fmt.Errorf("ctx err: %w", ctx.Err())
	}

	return nil
}

// Close gracefully closes the tcp listener associated with it.
//
func (e *Exporter) Close() (err error) {
	if e.listener == nil {
		return nil
	}

	e.log.Info("closing")
	if err := e.listener.Close(); err != nil {
		return fmt.Errorf("close: %w", err)
	}

	return nil
}