pub-lmt decreasing causing a BACK_PRESSURE event

[alexkoon]

We have observed the pub-lmt value go down when a remote consumer unsubscribes from the stream id. This causes a BACK_PRESSURE event on the producer. Not sure what causes this.

We have a server (A) that runs a daemon, and a producer. This publishes to a stream with a UDP channel. Server (B) runs a daemon and a consumer is subscribed to the stream. At a subsequent point in time the subscriber no longer requires the data and unsubscribes from the stream id.

We run a aeron stat periodically (every 5s) and dump the results.

When the BACK_PRESSURE event occurs on the producer on Server A, we checked and debugged the consumer and can see no reason for the BACK_PRESSURE (it does not hold up the consumption of data). We checked the aeron-stat log on Server A and around the time (as we are on 5s periodic logging) of the consumer unsubscribing, we observe that the pub-lmt value that was steadily increasing throughout suddenly clamps down to the same value as the pub-pos. After that event, the pub-lmt resumes to increasing (there are other subscribers on other servers for the same streamid requiring data).

Not sure what causes the back pressure as none of the consumers seem to be holding up any data.