Need help in understanding MaxAckPending behaviour

[apmcodes]

From the MaxAckPending doc below, does acknowledgement means only ACK and not NAK?

MaxAckPending implements a simple form of one-to-many flow control. It sets the maximum number of messages without an **acknowledgement** that can be outstanding, once this limit is reached message delivery will be suspended.

Currently, we have a stream with WorkQueue rentention policy, when the push consumer NAK (NAKWithDelay of 4 hour) a msg, the Outstanding Acks increases (say 5 out of maximum 1,000). Eventhough there is no inflight msg (not currently being processed by any process), but nats counts the NAKed msg towards outstanding. So this decreases the expected inflight msg limit. Is my understanding of the behavioutr correct? If so how can we have a steady 1000 inflight messages despite many NAKed messages (those will be waiting to be redelivered after 4 hours).

Thank you for any clarification.

[kozlovic]

The MaxAckPending is the maximum number of messages for which an acknowledgement is pending, that is, not yet received by the server. NAK is simply reseting the message's AckWait to whatever value you have specified, which means that this message is going to stay in a delivered but non-ack'ed state for that long, and yes, it counts toward the maximum outstanding acknowledgments.

[apmcodes]

Thanks for the reply, it nicely clarifies the working of message acknowledgment.

So, for long NAK delay (AckWait) its better to increase the MaxAckPending to process new messages if many messages are being NAked.

[kozlovic]

Not really, because then you going to have possibly lots of messages being delivered to a consumer, which can put strain on the system, etc.. If you see a pattern where a lot of messages get NAK'ed, then maybe the design is not optimum. Maybe those messages should be moved to a different stream or something?

[jnmoyne]

What I would do is use a pull consumer instead of a push. That way you can leave/set Max Acks Pending to something very high, but you are not relying on it do to flow control and instead the flow control is happening automatically because your clients use Fetch() (so for example if you have 10 clients each does Fetch(100) and the max 'in-flight' is 1000 (and you can also use MaxPullWaiting).