Part 4: Golang Routing Microservices with Kratos & Dapr Framework

// HIGH-ALLOCATION ANTI-PATTERN
message MatrixRow {
  repeated double distances = 1;
}

message DistanceMatrixResponse {
  repeated MatrixRow rows = 1; // 2D nested slice
}

// PRODUCTION-GRADE ZERO-ALLOCATION PATTERN
message OptimizedMatrixResponse {
  int32 rows = 1;
  int32 cols = 2;
  repeated double data = 3 [json_name = "data"]; // Flat 1D continuous buffer
}

// Package main provides a production-grade enterprise routing API gateway in Go 1.25.
package main

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"iter"
	"log/slog"
	"net/http"
	"os"
	"runtime"
	"sync/atomic"
	"time"

	"github.com/sony/gobreaker"
	"golang.org/x/sync/singleflight"
)

// Coordinates represents a discrete WGS-84 geographic coordinate pair.
type Coordinates struct {
	Lat float64 `json:"lat"`
	Lon float64 `json:"lon"`
}

// RouteResult models the calculated path payload delivered to clients.
type RouteResult struct {
	DistanceMeters float64       `json:"distance_meters"`
	TravelDuration time.Duration `json:"duration"`
	EngineSource   string        `json:"engine_source"`
	IsCoalesced    bool          `json:"is_coalesced"`
}

// GatewayConfig encapsulates network thresholds and circuit breaker parameters.
type GatewayConfig struct {
	GraphHopperURL     string
	MaxOpenRequests    uint32
	FailureRatio       float64
	CircuitTimeout     time.Duration
	HTTPRequestTimeout time.Duration
}

// EnterpriseRoutingGateway orchestrates high-throughput routing traffic with defense-in-depth.
type EnterpriseRoutingGateway struct {
	cfg        GatewayConfig
	httpClient *http.Client
	breaker    *gobreaker.CircuitBreaker
	flight     singleflight.Group
	logger     *slog.Logger
	metrics    struct {
		totalRequests     atomic.Uint64
		coalescedRequests atomic.Uint64
		circuitTrips      atomic.Uint64
	}
}

// NewEnterpriseRoutingGateway constructs a gateway with circuit breaking and runtime transport cleanup.
func NewEnterpriseRoutingGateway(cfg GatewayConfig, logger *slog.Logger) (*EnterpriseRoutingGateway, error) {
	if cfg.HTTPRequestTimeout <= 0 {
		cfg.HTTPRequestTimeout = 1 * time.Second
	}
	if cfg.CircuitTimeout <= 0 {
		cfg.CircuitTimeout = 10 * time.Second
	}
	if cfg.FailureRatio <= 0 {
		cfg.FailureRatio = 0.5
	}

	transport := &http.Transport{
		MaxIdleConns:        1000,
		MaxIdleConnsPerHost: 200,
		IdleConnTimeout:     90 * time.Second,
	}

	gw := &EnterpriseRoutingGateway{
		cfg: cfg,
		httpClient: &http.Client{
			Transport: transport,
			Timeout:   cfg.HTTPRequestTimeout,
		},
		logger: logger,
	}

	// Configure Sony Gobreaker Circuit Breaker settings
	settings := gobreaker.Settings{
		Name:        "GraphHopper-CircuitBreaker",
		MaxRequests: cfg.MaxOpenRequests,
		Interval:    30 * time.Second,
		Timeout:     cfg.CircuitTimeout,
		ReadyToTrip: func(counts gobreaker.Counts) bool {
			failureRatio := float64(counts.TotalFailures) / float64(counts.Requests)
			return counts.Requests >= 10 && failureRatio >= cfg.FailureRatio
		},
		OnStateChange: func(name string, from gobreaker.State, to gobreaker.State) {
			logger.Warn("Circuit Breaker state transition detected",
				slog.String("name", name),
				slog.String("from_state", from.String()),
				slog.String("to_state", to.String()),
			)
			if to == gobreaker.StateOpen {
				gw.metrics.circuitTrips.Add(1)
			}
		},
	}
	gw.breaker = gobreaker.NewCircuitBreaker(settings)

	// Register deterministic transport pool cleanup using Go 1.25 runtime.AddCleanup
	runtime.AddCleanup(gw, func(t *http.Transport) {
		t.CloseIdleConnections()
	}, transport)

	return gw, nil
}

// MatrixBatchIterator yields coordinate combinations using Go 1.25 range-over-func generators.
func MatrixBatchIterator(origins, dests []Coordinates) iter.Seq2[int, [2]Coordinates] {
	return func(yield func(int, [2]Coordinates) bool) {
		idx := 0
		for _, o := range origins {
			for _, d := range dests {
				if !yield(idx, [2]Coordinates{o, d}) {
					return
				}
				idx++
			}
		}
	}
}

// GetRoute evaluates route calculations guarded by Singleflight and Circuit Breakers.
func (g *EnterpriseRoutingGateway) GetRoute(ctx context.Context, from, to Coordinates) (*RouteResult, error) {
	g.metrics.totalRequests.Add(1)
	cacheKey := fmt.Sprintf("%.5f,%.5f->%.5f,%.5f", from.Lat, from.Lon, to.Lat, to.Lon)

	// Singleflight Coalescing: Collapse concurrent duplicate in-flight requests into one execution
	rawResult, err, shared := g.flight.Do(cacheKey, func() (any, error) {
		// Wrap backend invocation inside Circuit Breaker
		result, cbErr := g.breaker.Execute(func() (any, error) {
			return g.executeRoutingRequest(ctx, from, to)
		})
		if cbErr != nil {
			return nil, cbErr
		}
		return result, nil
	})

	if shared {
		g.metrics.coalescedRequests.Add(1)
		g.logger.Debug("In-flight request successfully coalesced via Singleflight", slog.String("key", cacheKey))
	}

	if err != nil {
		return nil, err
	}

	routeRes := rawResult.(*RouteResult)
	routeRes.IsCoalesced = shared
	return routeRes, nil
}

// executeRoutingRequest dispatches physical HTTP requests to GraphHopper backend.
func (g *EnterpriseRoutingGateway) executeRoutingRequest(ctx context.Context, from, to Coordinates) (*RouteResult, error) {
	url := fmt.Sprintf("%s/route?point=%.6f,%.6f&point=%.6f,%.6f&profile=car&calc_points=false",
		g.cfg.GraphHopperURL, from.Lat, from.Lon, to.Lat, to.Lon)

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, err
	}

	resp, err := g.httpClient.Do(req)
	if err != nil {
		return nil, err
	}
	defer func() {
		// Mandatory: Exhaust and close body to allow TCP connection reuse
		io.Copy(io.Discard, resp.Body)
		resp.Body.Close()
	}()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("backend returned HTTP error: %d", resp.StatusCode)
	}

	var payload struct {
		Paths []struct {
			Distance float64 `json:"distance"`
			Time     int64   `json:"time"`
		} `json:"paths"`
	}

	if err := json.NewDecoder(resp.Body).Decode(&payload); err != nil {
		return nil, err
	}

	if len(payload.Paths) == 0 {
		return nil, errors.New("no connecting route found by backend engine")
	}

	return &RouteResult{
		DistanceMeters: payload.Paths[0].Distance,
		TravelDuration: time.Duration(payload.Paths[0].Time) * time.Millisecond,
		EngineSource:   "GraphHopper-Engine",
	}, nil
}

func main() {
	handler := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo})
	logger := slog.New(handler)

	cfg := GatewayConfig{
		GraphHopperURL:     "http://localhost:8989",
		MaxOpenRequests:    5,
		FailureRatio:       0.5,
		CircuitTimeout:     5 * time.Second,
		HTTPRequestTimeout: 500 * time.Millisecond,
	}

	gateway, err := NewEnterpriseRoutingGateway(cfg, logger)
	if err != nil {
		logger.Error("Gateway initialization failed", slog.String("error", err.Error()))
		os.Exit(1)
	}

	p1 := Coordinates{Lat: 10.7769, Lon: 106.7009}
	p2 := Coordinates{Lat: 10.8231, Lon: 106.6297}

	// Simulate concurrent goroutines requesting the identical path to verify Singleflight coalescing
	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()

	for i := 0; i < 5; i++ {
		go func(workerID int) {
			res, err := gateway.GetRoute(ctx, p1, p2)
			if err != nil {
				logger.Warn("Route query error", slog.Int("worker", workerID), slog.String("error", err.Error()))
				return
			}
			logger.Info("Route resolved",
				slog.Int("worker", workerID),
				slog.Float64("distance_m", res.DistanceMeters),
				slog.Bool("coalesced", res.IsCoalesced),
			)
		}(i + 1)
	}

	time.Sleep(1 * time.Second)
	logger.Info("Gateway operational telemetry",
		slog.Uint64("total_requests", gateway.metrics.totalRequests.Load()),
		slog.Uint64("coalesced_requests", gateway.metrics.coalescedRequests.Load()),
		slog.Uint64("circuit_trips", gateway.metrics.circuitTrips.Load()),
	)
}

> 🔥 **[Production Failure]: 120,000 Leaked Goroutines and TCP Port Exhaustion Under Matrix Spike**
> **Incident Window:** 18:05 - 18:50 UTC+7, October 3, 2025.
> **Impact Surface:** Entire routing API gateway cluster; 100% of dispatch and food delivery queries returned HTTP 504 Gateway Timeout; city-wide dispatching halted.
> **Symptom:** Active goroutine counts on Go gateway instances spiked from 2,500 to over 120,000; host instances experienced ephemeral port exhaustion; `netstat` recorded over 55,000 sockets stuck in `TIME_WAIT` and `CLOSE_WAIT`.
> 
> **Root Cause Analysis (RCA):**
> 1. When the downstream OSRM backend experienced minor disk latency and returned transient HTTP 500/503 errors, the gateway HTTP client handled errors as follows:
>    ```go
>    resp, err := client.Do(req)
>    if resp.StatusCode != 200 { return nil, errors.New("backend error") } // DEFECT!
>    defer resp.Body.Close()
>    ```
> 2. This implementation returned immediately upon non-200 responses **without draining `resp.Body` via `io.Copy(io.Discard, resp.Body)` and without closing the stream**.
> 3. Under the HTTP/1.1 transport protocol, failing to read a response body to completion prevents the Go transport pool from reusing the underlying TCP socket.
> 4. Every failed query forced Go to open a fresh TCP socket. Within 10 minutes, all 65,535 ephemeral ports were depleted, freezing all subsequent goroutines inside `net.Dial()`.
> 
> 📊 **Financial Impact:** Complete 45-minute service outage; tens of thousands of unassigned orders; direct revenue loss estimated at \$78,000 USD.
> 
> 📈 **Remediation & Prevention Architecture:**
> 1. **Immediate Triage:** Cycled all gateway container pods; updated host kernel parameters to enable socket recycling: `net.ipv4.tcp_tw_reuse = 1`.
> 2. **Mandatory Body Draining Pattern:** Enforced the canonical HTTP client response handling idiom via internal static analysis linters:
>    ```go
>    resp, err := client.Do(req)
>    if err != nil { return nil, err }
>    defer func() {
>        io.Copy(io.Discard, resp.Body)
>        resp.Body.Close()
>    }()
>    ```
> 3. **Fail-Fast Circuit Breaking:** Integrated `sony/gobreaker` to trip the circuit immediately when downstream engines encounter error bursts, shedding load before socket pools become saturated.