Nodejs-pubsub: Duplicate messages exactly every 15 minutes

[ctavan]

Environment details

• OS: Kubernetes Engine
• Node.js version: 6.3.0
• npm version: –
• @google-cloud/pubsub version: 0.16.2

Steps to reproduce

This is most likely related to the discussion in https://github.com/googleapis/nodejs-pubsub/issues/2#issuecomment-356423284 (and following comments in that thread). However since the discussion there somewhat faded out I want to report my findings in a new issue.

My subscriber is consuming messages at a rate of roughly 500/s and it is receiving small batches of duplicate messages exactly every 15 minutes. Those batches typically contain between 100 and 400 duplicate messages. Here's a plot of the number of duplicates over time:

Most of the duplicates are being delivered to my subscriber within less then a second. Here's a histogram of the durations between redeliveries in milliseconds:

As you can see, the batches of duplicates coincide with spikes in Stackdriver graphs on StreamingPull Operations and StreamingPull Acknowledge Requests (please note that Stackdriver shows Berlin time while the above graph shows UTC, hence 1h time difference):

From the comments in the other thread I did not really understand whether the behavior we see is actually expected. What's the reason for this to happen precisely every 15 minutes?

Even though the absolute number of duplicates is well below 1%, this still looks pretty odd, unexpected and unnecessary. I'd love to understand better what's causing this issue and how it could potentially be fixed.

/cc @kir-titievsky @callmehiphop @rossj

[callmehiphop]

I believe the lifespan of each stream created via streamingPull is 15 minutes, in my own testing I've also seen messages get redelivered when one stream closes and another opens. That being said, I believe the stream is just reaching the end of its lifespan and when a new stream is opened to take its place, the PubSub service redelivers the messages.

@kir-titievsky does that sound like the expected behavior? Is there something on the client we should be doing differently to prevent these redeliveries?

[rossj]

From my reading of the code, the Node subscription client assumes that it will not receive messages with ack ids that it is already leasing. This is evident based on how it tracks and clears ack ids in the internal inventory array, with no checking of an existing duplicate ack id.

IMHO the current client is broken in this regard, as after receiving a duplicate message / ack id, calling ack() or nack() on either message instance will result in the subscription client ack / nacking the id, while at the same time attempting to further lease it until ack() / nack() is called on the duplicate message instance. Even worse, if a duplicate message is received and the inventory is at it's set maximum size, the ack id will be nacked right away, even though it's also still being leased.

I've monkey patched my own subscriptions on('message') handler to completely ignore duplicate ack ids that are already being leased, and this has been working well for me, with my downstream code receiving no duplicates. I also haven't had any problems maintaining rather long leases of about an hour.

````javascript
const openConnectionOld = sub.openConnection_;
sub.openConnection_ = function () {
openConnectionOld.call(this, ...arguments);
const pool = this.connectionPool;

pool.removeAllListeners('message');
pool.on('message', (message) => {
    // If we already have the ackId leased, ignore
    if (this.inventory_.lease.indexOf(message.ackId) >= 0) {
        console.log(`Already leasing ack ID ${message.ackId}. Ignoring...`);
        return;
    }

    if (!this.hasMaxMessages_()) {
        this.emit('message', this.leaseMessage_(message));
        return;
    }

    if (!pool.isPaused) {
        pool.pause();
    }

    message.nack();
});

};
````

I could submit a PR, but I'm not sure if the duplicate messages are indicative of some other, upstream problem. Perhaps the ack id inventory would be better implemented using a Set instead of an array with indexOf checks.

[callmehiphop]

@rossj I agree with you, however the client was built with specifications from the PubSub team and this particular issue has been brought up several times before. I think we want input from the PubSub team on what the correct solution to this issue actually is.

[callmehiphop]

@kir-titievsky just a friendly ping :)

[kir-titievsky]

@ctavan @rossj This should get better this week. The behavior had to do with how treated messages that were sent to a client but not yet acknowledged when a streamingPull connection was broken down and rebuilt. A feature rolling out right now (and expected to be completely out tomorrow) should ensure that nothing special -- like duplicate delivery-- happens when connections are rebuilt.

@ctavan Might you check in on your metrics in two days and tell me if you still see this?

[ctavan]

Thanks @kir-titievsky, we'll check again in a few days!

Any chance that the changes might affect #73 as well?

[callmehiphop]

@ctavan have you been experiencing this issue lately?

[ctavan]

I'm currently out of office. I'll try to reproduce as soon as possible, but it might take me a few more days.

[callmehiphop]

No worries!

On Thu, Mar 22, 2018, 5:45 AM Christoph Tavan [e-mail]
wrote:

I'm currently out of office. I'll try to reproduce as soon as possible,
but it might take me a few more days.

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[ctavan]

@callmehiphop @kir-titievsky I have finally found the time to check this again.

First of all I re-ran the test using @google-cloud/pubsub@^0.16.4 and I saw a similar behavior. Just that the duplicates now no longer appeared in strict 15-minute intervals but in more random intervals that were however still strict multiples of 1 minute!

I have then upgraded to @google-cloud/pubsub@^0.18.0 and I've been running the consumer for more than one hour without a single duplicate since the upgrade.

So even though I cannot confirm that there was a server-side fix for the issue (since it still persists with the old version of the node client) I will still go ahead and close this issue since it is gone since the latest refactoring of this client.

Thanks for your support!

[callmehiphop]

@ctavan glad to hear it, thanks for all your help!