Add dbx library: PostgreSQL cluster with master/replica routing, retry, health checking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

health.go

@@ -0,0 +1,90 @@
+package dbx
+
+import (
+	"context"
+	"time"
+)
+
+// healthChecker periodically pings nodes and updates their health state.
+type healthChecker struct {
+	nodes   []*Node
+	cfg     HealthCheckConfig
+	logger  Logger
+	metrics *MetricsHook
+	stop    chan struct{}
+	done    chan struct{}
+}
+
+func newHealthChecker(nodes []*Node, cfg HealthCheckConfig, logger Logger, metrics *MetricsHook) *healthChecker {
+	return &healthChecker{
+		nodes:   nodes,
+		cfg:     cfg,
+		logger:  logger,
+		metrics: metrics,
+		stop:    make(chan struct{}),
+		done:    make(chan struct{}),
+	}
+}
+
+func (h *healthChecker) start() {
+	go h.loop()
+}
+
+func (h *healthChecker) loop() {
+	defer close(h.done)
+
+	ticker := time.NewTicker(h.cfg.Interval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-h.stop:
+			return
+		case <-ticker.C:
+			h.checkAll()
+		}
+	}
+}
+
+func (h *healthChecker) checkAll() {
+	for _, node := range h.nodes {
+		h.checkNode(node)
+	}
+}
+
+func (h *healthChecker) checkNode(n *Node) {
+	ctx, cancel := context.WithTimeout(context.Background(), h.cfg.Timeout)
+	defer cancel()
+
+	err := n.pool.Ping(ctx)
+	wasHealthy := n.healthy.Load()
+
+	if err != nil {
+		n.healthy.Store(false)
+		if wasHealthy {
+			h.logger.Error(ctx, "dbx: node is down",
+				"node", n.name,
+				"error", err,
+			)
+			if h.metrics != nil && h.metrics.OnNodeDown != nil {
+				h.metrics.OnNodeDown(ctx, n.name, err)
+			}
+		}
+		return
+	}
+
+	n.healthy.Store(true)
+	if !wasHealthy {
+		h.logger.Info(ctx, "dbx: node is up",
+			"node", n.name,
+		)
+		if h.metrics != nil && h.metrics.OnNodeUp != nil {
+			h.metrics.OnNodeUp(ctx, n.name)
+		}
+	}
+}
+
+func (h *healthChecker) shutdown() {
+	close(h.stop)
+	<-h.done
+}