From de13f4d3c3c9be2de5ec73485c01d8e064ab64a9 Mon Sep 17 00:00:00 2001
From: Shawn <44221603+shaspitz@users.noreply.github.com>
Date: Wed, 17 Sep 2025 12:39:20 -0700
Subject: [PATCH] follower health

---
 cl/cmd/singlenode/main.go          |  6 +++
 cl/singlenode/follower/follower.go | 74 +++++++++++++++++++++++++-----
 2 files changed, 69 insertions(+), 11 deletions(-)

diff --git a/cl/cmd/singlenode/main.go b/cl/cmd/singlenode/main.go
index dc1386aa3..b259e73ab 100644
--- a/cl/cmd/singlenode/main.go
+++ b/cl/cmd/singlenode/main.go
@@ -413,11 +413,17 @@ func startFollowerNode(c *cli.Context) error {
 	}
 	bb := blockbuilder.NewMemberBlockBuilder(engineCL, logger.With("component", "BlockBuilder"))
 
+	healthAddr := c.String(healthAddrPortFlag.Name)
+	if healthAddr == "" {
+		return fmt.Errorf("health-addr is required")
+	}
+
 	followerNode, err := follower.NewFollower(
 		logger,
 		repo,
 		syncBatchSize,
 		bb,
+		healthAddr,
 	)
 	if err != nil {
 		logger.Error("Failed to initialize Follower", "error", err)
diff --git a/cl/singlenode/follower/follower.go b/cl/singlenode/follower/follower.go
index 12bd5bbb7..5abea967e 100644
--- a/cl/singlenode/follower/follower.go
+++ b/cl/singlenode/follower/follower.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net/http"
 	"sync"
 	"time"
 
@@ -13,12 +14,16 @@ import (
 )
 
 type Follower struct {
-	logger        *slog.Logger
-	sharedDB      payloadDB
-	syncBatchSize uint64
-	payloadCh     chan types.PayloadInfo
-	bbMutex       sync.RWMutex
-	bb            blockBuilder
+	logger                *slog.Logger
+	sharedDB              payloadDB
+	syncBatchSize         uint64
+	payloadCh             chan types.PayloadInfo
+	bbMutex               sync.RWMutex
+	bb                    blockBuilder
+	healthAddr            string
+	syncStopped           chan struct{}
+	handlePayloadsStopped chan struct{}
+	healthStopped         chan struct{}
 }
 
 const (
@@ -41,6 +46,7 @@ func NewFollower(
 	sharedDB payloadDB,
 	syncBatchSize uint64,
 	bb blockBuilder,
+	healthAddr string,
 ) (*Follower, error) {
 	if sharedDB == nil {
 		return nil, errors.New("payload repository not provided")
@@ -49,11 +55,15 @@ func NewFollower(
 		return nil, errors.New("sync batch size must be greater than 0")
 	}
 	return &Follower{
-		logger:        logger,
-		sharedDB:      sharedDB,
-		syncBatchSize: syncBatchSize,
-		payloadCh:     make(chan types.PayloadInfo),
-		bb:            bb,
+		logger:                logger,
+		sharedDB:              sharedDB,
+		syncBatchSize:         syncBatchSize,
+		payloadCh:             make(chan types.PayloadInfo),
+		bb:                    bb,
+		healthAddr:            healthAddr,
+		healthStopped:         make(chan struct{}),
+		syncStopped:           make(chan struct{}),
+		handlePayloadsStopped: make(chan struct{}),
 	}, nil
 }
 
@@ -61,11 +71,34 @@ func (f *Follower) Start(ctx context.Context) <-chan struct{} {
 
 	done := make(chan struct{})
 	eg, egCtx := errgroup.WithContext(ctx)
+
+	eg.Go(func() error {
+		defer close(f.healthStopped)
+		mux := http.NewServeMux()
+		mux.HandleFunc("/health", f.healthHandler)
+		server := &http.Server{Addr: f.healthAddr, Handler: mux}
+		f.logger.Info("Health endpoint listening", "address", f.healthAddr)
+
+		go func() {
+			<-egCtx.Done()
+			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+			defer cancel()
+			_ = server.Shutdown(ctx)
+		}()
+
+		if err := server.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			return err
+		}
+		return nil
+	})
+
 	eg.Go(func() error {
+		defer close(f.handlePayloadsStopped)
 		return f.handlePayloads(egCtx)
 	})
 
 	eg.Go(func() error {
+		defer close(f.syncStopped)
 		f.logger.Info("Starting sync from shared DB")
 		return f.syncFromSharedDB(egCtx)
 	})
@@ -80,6 +113,25 @@ func (f *Follower) Start(ctx context.Context) <-chan struct{} {
 	return done
 }
 
+func (f *Follower) healthHandler(w http.ResponseWriter, r *http.Request) {
+
+	select {
+	case <-f.healthStopped:
+		http.Error(w, "health loop has stopped", http.StatusServiceUnavailable)
+		return
+	case <-f.syncStopped:
+		http.Error(w, "sync from shared DB has stopped", http.StatusServiceUnavailable)
+		return
+	case <-f.handlePayloadsStopped:
+		http.Error(w, "handle payloads loop has stopped", http.StatusServiceUnavailable)
+		return
+	default:
+	}
+
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte("OK"))
+}
+
 func (f *Follower) syncFromSharedDB(ctx context.Context) error {
 	if f.getExecutionHead() == nil {
 		if err := f.setExecutionHeadFromRPC(ctx); err != nil {