Why Does Google Pub/Sub Process Messages with the Same OrderingKey Concurrently Before Acknowledgment?

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I'm using Google Cloud Pub/Sub and trying to process messages with the same orderingKey sequentially as per the documentation. However, I’m observing that messages with the same orderingKey are delivered and processed in parallel even before message.ack() is called.

Configuration details and implementation steps:

Subscription: Created with --enable-message-ordering to enforce message order.
Message Publishing: Messages are published with the exact same orderingKey.
Subscriber: I'm using a Node.js subscriber with explicit asynchronous handling, where message.ack() is only called after a message is fully processed.

JAVASCRIPT
const { PubSub } = require("@google-cloud/pubsub");
const pubSubClient = new PubSub();

const publishMessage = async (orderingKey, data) => {
  const dataBuffer = Buffer.from(JSON.stringify(data));
  await pubSubClient
    .topic("YOUR_TOPIC_NAME")
    .publishMessage({
      data: dataBuffer,
      orderingKey: orderingKey,
    });
};

publishMessage("my-key", { orderId: "123" });

JAVASCRIPT
const subscription = pubSubClient.subscription("YOUR_SUBSCRIPTION_NAME");

subscription.on("message", async (message) => {
  try {
    console.log(`Message received: ${message.id}, orderingKey: ${message.attributes.orderingKey}`);
    const data = JSON.parse(message.data);

    // Simulate long processing
    await new Promise((resolve) => setTimeout(resolve, 5000));
    console.log(`Message processed: ${data.orderId}`);
  } catch (err) {
    console.error(`Error: ${err}`);
    return;
  } finally {
    console.log(`Calling message.ack() for: ${message.id}`);
    message.ack();
  }
});

Problem:

Even with this setup, messages with the same orderingKey are delivered and processed concurrently rather than sequentially. The next message is not held back until the previous one is acknowledged.

What I Want:

I need Google Pub/Sub to ensure that a subsequent message is only delivered after the prior one (with the same orderingKey) has been acknowledged, without adding extra local queuing logic.

Questions:

Why does Pub/Sub deliver messages with the same orderingKey concurrently in this case?
How can I configure my subscriber or system so that messages are processed strictly sequentially per orderingKey?

Additional Context:

Using Node.js for the subscriber.
Subscription is created with the --enable-message-ordering flag.
I prefer not to implement additional local logic (like a queue) for managing message order.

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is an explanation addressing both questions:

1. Why are messages with the same orderingKey delivered in parallel?

Although you created your subscription with ordering enabled and publish messages with the same orderingKey, Google Cloud Pub/Sub’s ordering guarantee applies only to the order in which messages are stored and made available for delivery. The service does not block the delivery of a subsequent message on that key until the previous one is acknowledged. In other words, the guarantee is that messages will be delivered in order when they are pulled—but if your subscriber is handling messages asynchronously (as in your Node.js example), the messages can be dispatched concurrently. This behavior is by design so that the system can optimize throughput; Pub/Sub will send the next message even if the previous one is still being processed.

2. How can you configure your subscriber or system to ensure strictly sequential processing?

Currently, there isn’t a configuration setting in Pub/Sub that automatically holds back delivering the next message with the same orderingKey until you call ack() for the previous one. With the subscriber’s async message handler (and default concurrency settings), multiple messages—even with the same orderingKey—can be outstanding at the same time.

To ensure strictly sequential processing without adding your own queuing logic, you have two general approaches:

Limit Concurrency Globally:
Adjust the subscriber’s flow control settings (e.g., set maxMessages to 1). In the Node.js client you could configure your subscriber like this:
```
JAVASCRIPT
const subscription = pubSubClient.subscription("YOUR_SUBSCRIPTION_NAME", {
  flowControl: {
    maxMessages: 1,
  },
});
```
This ensures that only one message is delivered at a time across the entire subscriber. Note that this approach limits your throughput for all messages, not just those with a specific orderingKey.
Implement Per-Ordering Key Queues:
Since Pub/Sub does not provide per-key concurrency control natively, another method is to add local logic (for example, maintaining a map of orderingKey to a processing queue) so that even if messages are fetched in parallel, your application processes them sequentially. Although you mentioned wanting to avoid additional local logic, this is currently the only way to get per-orderingKey sequential processing.

In summary, if you must have strictly sequential processing per orderingKey, you need to either limit overall concurrency (which may not be optimal for your use case) or implement additional logic to serialize processing per key.

This explains why you’re observing parallel message processing and outlines the options available to enforce sequential processing.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by AstroExplorer261 • 1 month ago

The guarantee that Pub/Sub ordering provides is not that the next message for a key will be delivered only after the previous one is acknowledged, but that the next message for a key will be delivered after the previous one has been sent to the user callback and any synchronous work has completed. There are some more details about this behavior for Node specifically in GitHub:

This is unfortunately a "works as intended", because the guarantee in the library is that you'll receive all of the messages in order within one subscriber RPC callback (meaning, the code will not return to the event loop until it's called the user callback for each message in order). But Node makes that guarantee somewhat less useful because of the asynchrony.

In similar cases, we've been recommending using a queue on the user side, just pushing all of the messages for that key into the queue and then executing their callbacks one at a time.

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Why Does Google Pub/Sub Process Messages with the Same OrderingKey Concurrently Before Acknowledgment?

2 Answers

1. Why are messages with the same orderingKey delivered in parallel?

2. How can you configure your subscriber or system to ensure strictly sequential processing?

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Why Does Google Cloud Pub/Sub Process Messages Concurrently for the Same OrderingKey Before Acknowledgment?