Lab12 Complete

app_go/README.md

@@ -1,7 +1,7 @@
 # DevOps Info Service (Go)
 
 ## Overview
-Simple Go web service that exposes system/runtime details, health and readiness checks, Prometheus metrics, and structured JSON logs.
+Simple Go web service that exposes system/runtime details, a file-backed visits counter, health and readiness checks, Prometheus metrics, and structured JSON logs.
 
 ## Prerequisites
 - Go 1.25+
@@ -20,10 +20,23 @@ HOST=127.0.0.1 PORT=8080 ./devops-info-service.out
 
 ## Endpoints
 - `GET /` - service + system + runtime + request info
+- `GET /visits` - current visits counter stored in `/data/visits`
 - `GET /health` - health check
 - `GET /ready` - readiness check
 - `GET /metrics` - Prometheus metrics exposition
 
+## Visits Counter
+- The root handler increments the counter on every `GET /`.
+- The counter is persisted as plain text in `/data/visits`.
+- If the file is missing, the service starts from `0`.
+- If the file is malformed, the service logs a warning and treats the value as `0`.
+
+## Local Docker Check
+For Lab 12, run the monitoring stack with a writable `/data` volume for the Go container and verify that:
+- repeated `GET /` calls increment the counter
+- `GET /visits` returns the current count
+- the counter survives a container restart because the backing file is persisted on the host
+
 ## Configuration
 
 | Variable | Default | Description |

app_go/go.mod

@@ -13,6 +13,6 @@ require (
 	github.com/prometheus/common v0.67.5 // indirect
 	github.com/prometheus/procfs v0.20.1 // indirect
 	go.yaml.in/yaml/v2 v2.4.4 // indirect
-	golang.org/x/sys v0.42.0 // indirect
+	golang.org/x/sys v0.43.0 // indirect
 	google.golang.org/protobuf v1.36.11 // indirect
 )

app_go/go.sum

@@ -28,8 +28,8 @@ go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
 go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 go.yaml.in/yaml/v2 v2.4.4 h1:tuyd0P+2Ont/d6e2rl3be67goVK4R6deVxCUX5vyPaQ=
 go.yaml.in/yaml/v2 v2.4.4/go.mod h1:gMZqIpDtDqOfM0uNfy0SkpRhvUryYH0Z6wdMYcacYXQ=
-golang.org/x/sys v0.42.0 h1:omrd2nAlyT5ESRdCLYdm3+fMfNFE/+Rf4bDIQImRJeo=
-golang.org/x/sys v0.42.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/sys v0.43.0 h1:Rlag2XtaFTxp19wS8MXlJwTvoh8ArU6ezoyFsMyCTNI=
+golang.org/x/sys v0.43.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
 google.golang.org/protobuf v1.36.11 h1:fV6ZwhNocDyBLK0dj+fg8ektcVegBBuEolpbTQyBNVE=
 google.golang.org/protobuf v1.36.11/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=

app_go/main.go

@@ -8,6 +8,7 @@ import (
 	"net"
 	"net/http"
 	"os"
+	"path/filepath"
 	"runtime"
 	"strconv"
 	"strings"
@@ -20,7 +21,7 @@ import (
 
 const (
 	serviceName        = "devops-info-service"
-	serviceVersion     = "1.10.0"
+	serviceVersion     = "1.12.0"
 	serviceDescription = "DevOps course info service"
 	serviceFramework   = "Go net/http"
 	serviceLoggerName  = "devops_info_service"
@@ -75,11 +76,17 @@ type StatusResponse struct {
 	UptimeSeconds int64  `json:"uptime_seconds"`
 }
 
+type VisitsResponse struct {
+	Visits int `json:"visits"`
+}
+
 var (
 	// startTime is used for uptime calculations.
-	startTime = time.Now().UTC()
-	logMu     sync.Mutex
-	logOutput io.Writer = os.Stdout
+	startTime      = time.Now().UTC()
+	logMu          sync.Mutex
+	visitsMu       sync.Mutex
+	logOutput      io.Writer = os.Stdout
+	visitsFilePath           = "/data/visits"
 	// metricsRegistry only exposes service metrics, matching the Python app.
 	metricsRegistry   = prometheus.NewRegistry()
 	httpRequestsTotal = prometheus.NewCounterVec(
@@ -123,6 +130,7 @@ var (
 	// endpoints is a static list used to mirror the Python app output.
 	endpoints = []EndpointInfo{
 		{Path: "/", Method: http.MethodGet, Description: "Service information."},
+		{Path: "/visits", Method: http.MethodGet, Description: "Visits counter."},
 		{Path: "/health", Method: http.MethodGet, Description: "Health check."},
 		{Path: "/ready", Method: http.MethodGet, Description: "Readiness check."},
 		{Path: "/metrics", Method: http.MethodGet, Description: "Prometheus metrics."},
@@ -260,14 +268,80 @@ func clientIP(r *http.Request) string {
 	return r.RemoteAddr
 }
 
+func readVisitsCount() int {
+	data, err := os.ReadFile(visitsFilePath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return 0
+		}
+
+		emitLog("WARNING", serviceLoggerName, "failed to read visits counter", map[string]any{
+			"error": err.Error(),
+			"path":  visitsFilePath,
+		})
+		return 0
+	}
+
+	trimmed := strings.TrimSpace(string(data))
+	if trimmed == "" {
+		emitLog("WARNING", serviceLoggerName, "invalid visits counter, resetting to zero", map[string]any{
+			"path":  visitsFilePath,
+			"value": "",
+		})
+		return 0
+	}
+
+	count, err := strconv.Atoi(trimmed)
+	if err != nil || count < 0 {
+		emitLog("WARNING", serviceLoggerName, "invalid visits counter, resetting to zero", map[string]any{
+			"path":  visitsFilePath,
+			"value": trimmed,
+		})
+		return 0
+	}
+
+	return count
+}
+
+func writeVisitsCount(count int) error {
+	if err := os.MkdirAll(filepath.Dir(visitsFilePath), 0o755); err != nil {
+		return err
+	}
+
+	return os.WriteFile(visitsFilePath, []byte(fmt.Sprintf("%d\n", count)), 0o644)
+}
+
+func getVisitsCount() int {
+	visitsMu.Lock()
+	defer visitsMu.Unlock()
+
+	return readVisitsCount()
+}
+
+func incrementVisitsCount() int {
+	visitsMu.Lock()
+	defer visitsMu.Unlock()
+
+	count := readVisitsCount() + 1
+	if err := writeVisitsCount(count); err != nil {
+		emitLog("WARNING", serviceLoggerName, "failed to persist visits counter", map[string]any{
+			"error": err.Error(),
+			"path":  visitsFilePath,
+			"value": count,
+		})
+	}
+
+	return count
+}
+
 // listEndpoints returns the advertised endpoints for the root response.
 func listEndpoints() []EndpointInfo {
 	return endpoints
 }
 
 func normalizeEndpointLabel(path string) string {
 	switch path {
-	case "/", "/health", "/metrics", "/ready":
+	case "/", "/health", "/metrics", "/ready", "/visits":
 		return path
 	default:
 		return "unmatched"
@@ -336,6 +410,7 @@ func queryString(r *http.Request) string {
 // mainHandler serves GET /.
 func mainHandler(w http.ResponseWriter, r *http.Request) {
 	recordEndpointCall("/")
+	incrementVisitsCount()
 	payload := RootResponse{
 		Service:   getServiceInfo(),
 		System:    getSystemInfo(),
@@ -347,6 +422,14 @@ func mainHandler(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, payload)
 }
 
+// visitsHandler serves GET /visits.
+func visitsHandler(w http.ResponseWriter, r *http.Request) {
+	recordEndpointCall("/visits")
+	writeJSON(w, http.StatusOK, VisitsResponse{
+		Visits: getVisitsCount(),
+	})
+}
+
 // healthHandler serves GET /health.
 func healthHandler(w http.ResponseWriter, r *http.Request) {
 	recordEndpointCall("/health")
@@ -395,6 +478,8 @@ func router(w http.ResponseWriter, r *http.Request) {
 	switch {
 	case r.URL.Path == "/" && r.Method == http.MethodGet:
 		mainHandler(w, r)
+	case r.URL.Path == "/visits" && r.Method == http.MethodGet:
+		visitsHandler(w, r)
 	case r.URL.Path == "/health" && r.Method == http.MethodGet:
 		healthHandler(w, r)
 	case r.URL.Path == "/metrics" && r.Method == http.MethodGet:

app_go/main_test.go

@@ -6,6 +6,8 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"testing"
@@ -81,6 +83,18 @@ func performRequest(handler http.Handler, method, path string) *httptest.Respons
 	return recorder
 }
 
+func withTempVisitsFile(t *testing.T) string {
+	t.Helper()
+
+	oldPath := visitsFilePath
+	visitsFilePath = filepath.Join(t.TempDir(), "visits")
+	t.Cleanup(func() {
+		visitsFilePath = oldPath
+	})
+
+	return visitsFilePath
+}
+
 func metricValue(metricsText, sampleName string, labels map[string]string) (float64, bool) {
 	for _, line := range strings.Split(metricsText, "\n") {
 		line = strings.TrimSpace(line)
@@ -194,6 +208,57 @@ func TestIndexReturnsExpectedJSONStructureAndTypes(t *testing.T) {
 
 	for _, route := range []string{
 		http.MethodGet + " /",
+		http.MethodGet + " /visits",
+		http.MethodGet + " /health",
+		http.MethodGet + " /ready",
+		http.MethodGet + " /metrics",
+	} {
+		if !routeIndex[route] {
+			t.Fatalf("expected endpoint %q to be listed", route)
+		}
+	}
+}
+
+func TestVisitsEndpointDefaultsToZeroWhenFileMissing(t *testing.T) {
+	restore := captureLogOutput(io.Discard)
+	defer restore()
+
+	withTempVisitsFile(t)
+
+	recorder := performRequest(http.HandlerFunc(router), http.MethodGet, "/visits")
+	if recorder.Code != http.StatusOK {
+		t.Fatalf("expected status %d, got %d", http.StatusOK, recorder.Code)
+	}
+
+	payload := decodeJSONResponse[VisitsResponse](t, recorder)
+	if payload.Visits != 0 {
+		t.Fatalf("expected visits to default to 0, got %d", payload.Visits)
+	}
+
+	if _, err := os.Stat(visitsFilePath); !os.IsNotExist(err) {
+		t.Fatalf("expected visits file to remain absent, got err=%v", err)
+	}
+}
+
+func TestRootIncrementsVisitsCounterAndPersistsFile(t *testing.T) {
+	restore := captureLogOutput(io.Discard)
+	defer restore()
+
+	withTempVisitsFile(t)
+
+	first := performRequest(http.HandlerFunc(router), http.MethodGet, "/")
+	if first.Code != http.StatusOK {
+		t.Fatalf("expected status %d, got %d", http.StatusOK, first.Code)
+	}
+
+	payload := decodeJSONResponse[RootResponse](t, first)
+	routeIndex := map[string]bool{}
+	for _, endpoint := range payload.Endpoints {
+		routeIndex[endpoint.Method+" "+endpoint.Path] = true
+	}
+	for _, route := range []string{
+		http.MethodGet + " /",
+		http.MethodGet + " /visits",
 		http.MethodGet + " /health",
 		http.MethodGet + " /ready",
 		http.MethodGet + " /metrics",
@@ -202,6 +267,71 @@ func TestIndexReturnsExpectedJSONStructureAndTypes(t *testing.T) {
 			t.Fatalf("expected endpoint %q to be listed", route)
 		}
 	}
+
+	data, err := os.ReadFile(visitsFilePath)
+	if err != nil {
+		t.Fatalf("expected visits file to be created: %v", err)
+	}
+	if got := strings.TrimSpace(string(data)); got != "1" {
+		t.Fatalf("expected visits file to contain 1 after first root request, got %q", got)
+	}
+
+	second := performRequest(http.HandlerFunc(router), http.MethodGet, "/")
+	if second.Code != http.StatusOK {
+		t.Fatalf("expected status %d, got %d", http.StatusOK, second.Code)
+	}
+
+	data, err = os.ReadFile(visitsFilePath)
+	if err != nil {
+		t.Fatalf("expected visits file to remain readable: %v", err)
+	}
+	if got := strings.TrimSpace(string(data)); got != "2" {
+		t.Fatalf("expected visits file to contain 2 after second root request, got %q", got)
+	}
+
+	visits := performRequest(http.HandlerFunc(router), http.MethodGet, "/visits")
+	if visits.Code != http.StatusOK {
+		t.Fatalf("expected status %d, got %d", http.StatusOK, visits.Code)
+	}
+
+	count := decodeJSONResponse[VisitsResponse](t, visits)
+	if count.Visits != 2 {
+		t.Fatalf("expected visits endpoint to report 2, got %d", count.Visits)
+	}
+}
+
+func TestVisitsEndpointFallsBackToZeroForMalformedFile(t *testing.T) {
+	restore := captureLogOutput(io.Discard)
+	defer restore()
+
+	withTempVisitsFile(t)
+
+	if err := os.WriteFile(visitsFilePath, []byte("broken"), 0o644); err != nil {
+		t.Fatalf("failed to seed malformed visits file: %v", err)
+	}
+
+	recorder := performRequest(http.HandlerFunc(router), http.MethodGet, "/visits")
+	if recorder.Code != http.StatusOK {
+		t.Fatalf("expected status %d, got %d", http.StatusOK, recorder.Code)
+	}
+
+	payload := decodeJSONResponse[VisitsResponse](t, recorder)
+	if payload.Visits != 0 {
+		t.Fatalf("expected malformed counter to fall back to 0, got %d", payload.Visits)
+	}
+
+	after := performRequest(http.HandlerFunc(router), http.MethodGet, "/")
+	if after.Code != http.StatusOK {
+		t.Fatalf("expected status %d, got %d", http.StatusOK, after.Code)
+	}
+
+	data, err := os.ReadFile(visitsFilePath)
+	if err != nil {
+		t.Fatalf("expected visits file to be repaired by root request: %v", err)
+	}
+	if got := strings.TrimSpace(string(data)); got != "1" {
+		t.Fatalf("expected repaired visits file to contain 1, got %q", got)
+	}
 }
 
 func TestHealthReturnsExpectedJSONStructureAndTypes(t *testing.T) {