Go (Golang) Microservice for Email with Circuit Breakers & Retries

package main

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"log/slog"
	"net/http"
	"time"

	"github.com/sony/gobreaker"
)

type EmailClient struct {
	apiKey     string
	httpClient *http.Client
	cb         *gobreaker.CircuitBreaker
	logger     *slog.Logger
}

type SendRequest struct {
	To      string `json:"to"`
	Subject string `json:"subject"`
	Text    string `json:"text"`
	HTML    string `json:"html,omitempty"`
}

func NewEmailClient(apiKey string, logger *slog.Logger) *EmailClient {
	// Configure Circuit Breaker: Tripped when 5 consecutive failures occur within 30s
	st := gobreaker.Settings{
		Name:        "SadaSendOutboundBreaker",
		MaxRequests: 3,
		Interval:    30 * time.Second,
		Timeout:     10 * time.Second,
		ReadyToTrip: func(counts gobreaker.Counts) bool {
			failureRatio := float64(counts.TotalFailures) / float64(counts.Requests)
			return counts.Requests >= 5 && failureRatio >= 0.6
		},
		OnStateChange: func(name string, from gobreaker.State, to gobreaker.State) {
			logger.Warn("circuit breaker state transition", "name", name, "from", from.String(), "to", to.String())
		},
	}

	transport := &http.Transport{
		MaxIdleConns:        100,
		MaxIdleConnsPerHost: 20,
		IdleConnTimeout:     90 * time.Second,
	}

	return &EmailClient{
		apiKey: apiKey,
		httpClient: &http.Client{
			Transport: transport,
			Timeout:   5 * time.Second,
		},
		cb:     gobreaker.NewCircuitBreaker(st),
		logger: logger,
	}
}

func (c *EmailClient) Send(ctx context.Context, req SendRequest) error {
	_, err := c.cb.Execute(func() (interface{}, error) {
		payload, err := json.Marshal(req)
		if err != nil {
			return nil, fmt.Errorf("marshal payload: %w", err)
		}

		httpReq, err := http.NewRequestWithContext(ctx, "POST", "https://api.sadasend.com/emails", bytes.NewBuffer(payload))
		if err != nil {
			return nil, fmt.Errorf("create request: %w", err)
		}

		httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
		httpReq.Header.Set("Content-Type", "application/json")

		resp, err := c.httpClient.Do(httpReq)
		if err != nil {
			c.logger.Error("outbound network error", "err", err)
			return nil, err
		}
		defer resp.Body.Close()

		if resp.StatusCode >= 500 {
			return nil, fmt.Errorf("upstream server error: status %d", resp.StatusCode)
		}
		if resp.StatusCode >= 400 {
			// Client errors (4xx) should not trip the circuit breaker
			c.logger.Warn("client rejection from sadasend", "status", resp.StatusCode)
			return nil, nil
		}

		return nil, nil
	})

	return err
}