(DEPRECATED) Subscribe with concurrency control

Code sample

Go

Before trying this sample, follow the Go setup instructions in the Pub/Sub quickstart using client libraries. For more information, see the Pub/Sub Go API reference documentation.

To authenticate to Pub/Sub, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

import (
	"context"
	"fmt"
	"io"
	"sync/atomic"
	"time"

	"cloud.google.com/go/pubsub"
)

func pullMsgsConcurrencyControl(w io.Writer, projectID, subID string) error {
	// projectID := "my-project-id"
	// subID := "my-sub"
	ctx := context.Background()
	client, err := pubsub.NewClient(ctx, projectID)
	if err != nil {
		return fmt.Errorf("pubsub.NewClient: %w", err)
	}
	defer client.Close()

	sub := client.Subscription(subID)
	// Must set ReceiveSettings.Synchronous to false (or leave as default) to enable
	// concurrency pulling of messages. Otherwise, NumGoroutines will be set to 1.
	sub.ReceiveSettings.Synchronous = false
	// NumGoroutines determines the number of goroutines sub.Receive will spawn to pull
	// messages.
	sub.ReceiveSettings.NumGoroutines = 16
	// MaxOutstandingMessages limits the number of concurrent handlers of messages.
	// In this case, up to 8 unacked messages can be handled concurrently.
	// Note, even in synchronous mode, messages pulled in a batch can still be handled
	// concurrently.
	sub.ReceiveSettings.MaxOutstandingMessages = 8

	// Receive messages for 10 seconds, which simplifies testing.
	// Comment this out in production, since `Receive` should
	// be used as a long running operation.
	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	var received int32

	// Receive blocks until the context is cancelled or an error occurs.
	err = sub.Receive(ctx, func(_ context.Context, msg *pubsub.Message) {
		atomic.AddInt32(&received, 1)
		msg.Ack()
	})
	if err != nil {
		return fmt.Errorf("sub.Receive returned error: %w", err)
	}
	fmt.Fprintf(w, "Received %d messages\n", received)

	return nil
}

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