FEAT: Discovery Video Service Subscription

cms/internal/services/catalogue.go

@@ -0,0 +1,75 @@
+package services
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/aws/aws-sdk-go-v2/service/sns"
+)
+
+var CatalogueClient Catalogue
+
+// CatalogueVideo is what cms announces when a video becomes ready. It is a
+// published contract — every subscriber of the catalogue topic decodes it —
+// so the JSON names here are as much an API as the ones in internal/api, and
+// changing one is a change every subscriber sees.
+//
+// Categories are named rather than numbered on purpose: the ids are cms's
+// own, and a consumer holding a name needs nothing from cms to make sense
+// of it.
+type CatalogueVideo struct {
+	VideoID     string   `json:"videoId"`
+	Title       string   `json:"title"`
+	PlaybackURL string   `json:"playbackUrl"`
+	Categories  []string `json:"categories"`
+}
+
+type Catalogue interface {
+	AnnounceVideo(ctx context.Context, video CatalogueVideo) error
+}
+
+type CatalogueConcrete struct {
+	SNSClient *sns.Client
+	TopicARN  string
+}
+
+// AnnounceVideo publishes one video to the catalogue topic, which fans it out
+// to whatever has subscribed. Delivery is at-least-once: the caller
+// republishes when a later step fails, so a subscriber has to tolerate seeing
+// the same video twice.
+//
+// A topic delivers only to the subscriptions that exist at the moment it
+// publishes, so a subscriber that wants the backlog needs its queue in place
+// before the announcement, not after.
+func (svc CatalogueConcrete) AnnounceVideo(ctx context.Context, video CatalogueVideo) error {
+	// A nil slice marshals to null, which would make "has no categories" and
+	// "categories unknown" indistinguishable to a reader. An empty list says
+	// the first, which is what an unfiled video means.
+	if video.Categories == nil {
+		video.Categories = []string{}
+	}
+
+	body, err := json.Marshal(video)
+	if err != nil {
+		return err
+	}
+
+	_, err = svc.SNSClient.Publish(ctx, &sns.PublishInput{
+		TopicArn: aws.String(svc.TopicARN),
+		Message:  aws.String(string(body)),
+	})
+	return err
+}
+
+// AssertSuccessfulConnection verifies the topic exists and is readable with
+// the configured credentials. It panics on failure: a service that cannot
+// announce would transcode videos the catalogue never hears about.
+func (svc CatalogueConcrete) AssertSuccessfulConnection(ctx context.Context) {
+	if _, err := svc.SNSClient.GetTopicAttributes(ctx, &sns.GetTopicAttributesInput{
+		TopicArn: aws.String(svc.TopicARN),
+	}); err != nil {
+		panic(fmt.Errorf("sns: cannot connect to catalogue topic %q: %w", svc.TopicARN, err))
+	}
+}