Implemented a builtin NATS sink.

sinks/nats.go

@@ -0,0 +1,97 @@
+package sinks
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"sync"
+	"time"
+
+	"gitea.maximumdirect.net/ejr/feedkit/config"
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+	"github.com/nats-io/nats.go"
+)
+
+type NATSSink struct {
+	name     string
+	url      string
+	exchange string
+
+	mu   sync.Mutex
+	conn *nats.Conn
+}
+
+func NewNATSSinkFromConfig(cfg config.SinkConfig) (Sink, error) {
+	url, err := requireStringParam(cfg, "url")
+	if err != nil {
+		return nil, err
+	}
+	ex, err := requireStringParam(cfg, "exchange")
+	if err != nil {
+		return nil, err
+	}
+	return &NATSSink{name: cfg.Name, url: url, exchange: ex}, nil
+}
+
+func (r *NATSSink) Name() string { return r.name }
+
+func (r *NATSSink) Consume(ctx context.Context, e event.Event) error {
+	// Boundary validation: if something upstream violated invariants,
+	// surface it loudly rather than printing partial nonsense.
+	if err := e.Validate(); err != nil {
+		return fmt.Errorf("NATS sink: invalid event: %w", err)
+	}
+
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+
+	conn, err := r.connect(ctx)
+	if err != nil {
+		return fmt.Errorf("NATS sink: connect: %w", err)
+	}
+
+	b, err := json.Marshal(e)
+	if err != nil {
+		return fmt.Errorf("NATS sink: marshal event: %w", err)
+	}
+
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+	if err := conn.Publish(r.exchange, b); err != nil {
+		return fmt.Errorf("NATS sink: publish: %w", err)
+	}
+	return nil
+}
+
+func (r *NATSSink) connect(ctx context.Context) (*nats.Conn, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if r.conn != nil && r.conn.Status() != nats.CLOSED {
+		return r.conn, nil
+	}
+
+	opts := []nats.Option{
+		nats.Name(fmt.Sprintf("feedkit sink %s", r.name)),
+	}
+	if deadline, ok := ctx.Deadline(); ok {
+		timeout := time.Until(deadline)
+		if timeout <= 0 {
+			return nil, ctx.Err()
+		}
+		opts = append(opts, nats.Timeout(timeout))
+	}
+
+	conn, err := nats.Connect(r.url, opts...)
+	if err != nil {
+		return nil, err
+	}
+	r.conn = conn
+	return conn, nil
+}