FEAT: Discovery Video Service Subscription

discovery/internal/services/sqs.go

@@ -0,0 +1,83 @@
+// Package services holds this service's AWS clients. Discovery reaches exactly
+// one AWS API — the SQS queue subscribed to the catalogue topic — so unlike
+// cms there is no S3 or MediaConvert here, and nothing database-related.
+package services
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/aws/aws-sdk-go-v2/service/sqs"
+)
+
+var SQSClient SQS
+
+// QueueMessage is one message taken off the queue. ReceiptHandle is what
+// identifies it for deletion — it belongs to this delivery, not to the
+// message, so it cannot be held across receives.
+type QueueMessage struct {
+	Body          string
+	ReceiptHandle string
+}
+
+type SQS interface {
+	ReceiveMessages(ctx context.Context) ([]QueueMessage, error)
+	DeleteMessage(ctx context.Context, receiptHandle string) error
+}
+
+type SQSConcrete struct {
+	SQSClient *sqs.Client
+	QueueURL  string
+}
+
+// receiveWaitSeconds turns every receive into a long poll: the call parks on
+// the server until a message arrives or this elapses, so an event is picked up
+// within milliseconds of being published while an idle consumer costs one
+// request every twenty seconds rather than spinning. 20 is the AWS maximum.
+const receiveWaitSeconds = 20
+
+func (svc SQSConcrete) ReceiveMessages(ctx context.Context) ([]QueueMessage, error) {
+	output, err := svc.SQSClient.ReceiveMessage(ctx, &sqs.ReceiveMessageInput{
+		QueueUrl:            &svc.QueueURL,
+		MaxNumberOfMessages: 10,
+		WaitTimeSeconds:     receiveWaitSeconds,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	messages := make([]QueueMessage, 0, len(output.Messages))
+	for _, message := range output.Messages {
+		if message.Body == nil || message.ReceiptHandle == nil {
+			continue
+		}
+		messages = append(messages, QueueMessage{
+			Body:          *message.Body,
+			ReceiptHandle: *message.ReceiptHandle,
+		})
+	}
+
+	return messages, nil
+}
+
+// DeleteMessage acknowledges a message. Until this is called the message is
+// merely invisible, and it returns to the queue when its visibility timeout
+// expires — which is how a handler that failed gets another attempt.
+func (svc SQSConcrete) DeleteMessage(ctx context.Context, receiptHandle string) error {
+	_, err := svc.SQSClient.DeleteMessage(ctx, &sqs.DeleteMessageInput{
+		QueueUrl:      &svc.QueueURL,
+		ReceiptHandle: &receiptHandle,
+	})
+	return err
+}
+
+// AssertSuccessfulConnection verifies the queue is reachable and readable with
+// the configured credentials. It panics on failure, since a service whose
+// consumer cannot start would serve a catalogue that silently stops growing.
+func (svc SQSConcrete) AssertSuccessfulConnection(ctx context.Context) {
+	if _, err := svc.SQSClient.GetQueueAttributes(ctx, &sqs.GetQueueAttributesInput{
+		QueueUrl: &svc.QueueURL,
+	}); err != nil {
+		panic(fmt.Errorf("sqs: cannot connect to queue %q: %w", svc.QueueURL, err))
+	}
+}