Adding first version of HealthCheck

Added a expvar style handler for the debug http server to allow health checks (/debug/health).

Signed-off-by: Diogo Monica <diogo@docker.com>

health/health.go

@@ -0,0 +1,212 @@
+package health
+
+import (
+	"encoding/json"
+	"net/http"
+	"sync"
+	"time"
+)
+
+var (
+	mutex            sync.RWMutex
+	registeredChecks = make(map[string]Checker)
+)
+
+// Checker is the interface for a Health Checker
+type Checker interface {
+	// Check returns nil if the service is okay.
+	Check() error
+}
+
+// CheckFunc is a convenience type to create functions that implement
+// the Checker interface
+type CheckFunc func() error
+
+// Check Implements the Checker interface to allow for any func() error method
+// to be passed as a Checker
+func (cf CheckFunc) Check() error {
+	return cf()
+}
+
+// Updater implements a health check that is explicitly set.
+type Updater interface {
+	Checker
+
+	// Update updates the current status of the health check.
+	Update(status error)
+}
+
+// updater implements Checker and Updater, providing an asynchronous Update
+// method.
+// This allows us to have a Checker that returns the Check() call immediately
+// not blocking on a potentially expensive check.
+type updater struct {
+	mu     sync.Mutex
+	status error
+}
+
+// Check implements the Checker interface
+func (u *updater) Check() error {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	return u.status
+}
+
+// Update implements the Updater interface, allowing asynchronous access to
+// the status of a Checker.
+func (u *updater) Update(status error) {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	u.status = status
+}
+
+// NewStatusUpdater returns a new updater
+func NewStatusUpdater() Updater {
+	return &updater{}
+}
+
+// thresholdUpdater implements Checker and Updater, providing an asynchronous Update
+// method.
+// This allows us to have a Checker that returns the Check() call immediately
+// not blocking on a potentially expensive check.
+type thresholdUpdater struct {
+	mu        sync.Mutex
+	status    error
+	threshold int
+	count     int
+}
+
+// Check implements the Checker interface
+func (tu *thresholdUpdater) Check() error {
+	tu.mu.Lock()
+	defer tu.mu.Unlock()
+
+	if tu.count >= tu.threshold {
+		return tu.status
+	}
+
+	return nil
+}
+
+// thresholdUpdater implements the Updater interface, allowing asynchronous
+// access to the status of a Checker.
+func (tu *thresholdUpdater) Update(status error) {
+	tu.mu.Lock()
+	defer tu.mu.Unlock()
+
+	if status == nil {
+		tu.count = 0
+	} else if tu.count < tu.threshold {
+		tu.count++
+	}
+
+	tu.status = status
+}
+
+// NewThresholdStatusUpdater returns a new thresholdUpdater
+func NewThresholdStatusUpdater(t int) Updater {
+	return &thresholdUpdater{threshold: t}
+}
+
+// PeriodicChecker wraps an updater to provide a periodic checker
+func PeriodicChecker(check Checker, period time.Duration) Checker {
+	u := NewStatusUpdater()
+	go func() {
+		t := time.NewTicker(period)
+		for {
+			<-t.C
+			u.Update(check.Check())
+		}
+	}()
+
+	return u
+}
+
+// PeriodicThresholdChecker wraps an updater to provide a periodic checker that
+// uses a threshold before it changes status
+func PeriodicThresholdChecker(check Checker, period time.Duration, threshold int) Checker {
+	tu := NewThresholdStatusUpdater(threshold)
+	go func() {
+		t := time.NewTicker(period)
+		for {
+			<-t.C
+			tu.Update(check.Check())
+		}
+	}()
+
+	return tu
+}
+
+// CheckStatus returns a map with all the current health check errors
+func CheckStatus() map[string]string {
+	mutex.RLock()
+	defer mutex.RUnlock()
+	statusKeys := make(map[string]string)
+	for k, v := range registeredChecks {
+		err := v.Check()
+		if err != nil {
+			statusKeys[k] = err.Error()
+		}
+	}
+
+	return statusKeys
+}
+
+// Register associates the checker with the provided name. We allow
+// overwrites to a specific check status.
+func Register(name string, check Checker) {
+	mutex.Lock()
+	defer mutex.Unlock()
+	_, ok := registeredChecks[name]
+	if ok {
+		panic("Check already exists: " + name)
+	}
+	registeredChecks[name] = check
+}
+
+// RegisterFunc allows the convenience of registering a checker directly
+// from an arbitrary func() error
+func RegisterFunc(name string, check func() error) {
+	Register(name, CheckFunc(check))
+}
+
+// RegisterPeriodicFunc allows the convenience of registering a PeriodicChecker
+// from an arbitrary func() error
+func RegisterPeriodicFunc(name string, check func() error, period time.Duration) {
+	Register(name, PeriodicChecker(CheckFunc(check), period))
+}
+
+// RegisterPeriodicThresholdFunc allows the convenience of registering a
+// PeriodicChecker from an arbitrary func() error
+func RegisterPeriodicThresholdFunc(name string, check func() error, period time.Duration, threshold int) {
+	Register(name, PeriodicThresholdChecker(CheckFunc(check), period, threshold))
+}
+
+// StatusHandler returns a JSON blob with all the currently registered Health Checks
+// and their corresponding status.
+// Returns 503 if any Error status exists, 200 otherwise
+func StatusHandler(w http.ResponseWriter, r *http.Request) {
+	if r.Method == "GET" {
+		w.Header().Set("Content-Type", "application/json; charset=utf-8")
+		checksStatus := CheckStatus()
+		// If there is an error, return 503
+		if len(checksStatus) != 0 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+		}
+		err := json.NewEncoder(w).Encode(checksStatus)
+
+		// Parsing of the JSON failed. Returning generic error message
+		if err != nil {
+			w.Write([]byte("{server_error: 'Could not parse error message'}"))
+		}
+	} else {
+		w.WriteHeader(http.StatusNotFound)
+	}
+}
+
+// Registers global /debug/health api endpoint
+func init() {
+	http.HandleFunc("/debug/health", StatusHandler)
+}