谷歌PubSub :带有拉功能的显式NACK？

谷歌Pub/Sub是一个灵活、可靠的消息队列服务，它支持发布-订阅模式

启用流控制：通过设置maxMessages参数，您可以限制每次拉取操作中获取的消息数量。这有助于实现显式NACK的需求，因为您可以在处理消息后，通过再次调用pull()方法来获取新的消息。

from google.cloud import pubsub_v1

subscriber = pubsub_v1.SubscriberClient()
subscription_path = subscriber.subscription_path("your-project-id", "your-subscription-id")

def callback(message):
    print(f"Received message: {message}")
    message.ack()

streaming_pull_future = subscriber.subscribe(subscription_path, callback=callback)
streaming_pull_future.result()

处理失败的消息：在您的callback函数中，可以通过message.nack()方法显式地拒绝并重新排队一个消息。这将导致Pub/Sub重新投递该消息给订阅者。请注意，还需要确保消息处理逻辑能够处理重试次数和频率。

def callback(message):
    try:
        print(f"Received message: {message}")
        # 处理消息
        message.ack()
    except Exception as e:
        print(f"Error processing message: {e}")
        message.nack()  # 显式地拒绝并重新排队消息

设置重试策略：在创建订阅时，您可以设置dead_letter_policy，以便在消息达到最大重试次数后，将其发送到死信队列。这样，您可以可以对死信队列中的消息进行单独处理，例如记录或手动修复问题。

from google.api_core.exceptions import NotFound
from google.cloud.pubsub_v1 import SubscriberClient
from google.cloud.pubsub_v1.subscriber.message import Message

subscriber = SubscriberClient()
subscription_path = subscriber.subscription_path("your-project-id", "your-subscription-id")

# 创建死信队列的订阅
try:
    dlq_subscription = subscriber.create_subscription(
        request={"name": "projects/your-project-id/subscriptions/your-dead-letter-subscription-id"}
    )
except NotFound:
    dlq_subscription = subscriber.get_subscription(request={"subscription": "projects/your-project-id/subscriptions/your-dead-letter-subscription-id"})

policy = {
    "dead_letter_policy": {
        "dead_letter_topic": dlq_subscription.name,
        "max_delivery_attempts": 5,
    }
}

subscriber.modify_subscription(request={"subscription": subscription_path, "dead_letter_policy": policy})

通过结合这些方法，您可以实现带有拉功能的显式NACK，以便在处理消息时更好地控制消息的重试和重新排队。