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eclipse-cyclonedds / cyclonedds

Eclipse Cyclone DDS project
https://projects.eclipse.org/projects/iot.cyclonedds
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Cyclonedds has lost some messages #2118

Open Gummum opened 3 weeks ago

Gummum commented 3 weeks ago

When I use dds_get_matched_subscriptions to detect subscribers, I start sending messages. But some frames from the beginning will be lost.

Gummum commented 3 weeks ago

Could it be that there is a period of time between the discovery of subscriber and subscriber availability? Can you tell me if there is a state of judgment?

eboasson commented 3 weeks ago

That should never happen for a reliable, keep-all, volatile writer-reader pair. The deal in Cyclone is that everything written by writer W after reader R is listed as a matched reader should make it to R, and nothing that was sent before R was matched.

Now if it is a keep-last writer or reader and you're writing quickly (but not so quickly that you don't overwrite any data in steady state) it is possible that the handshake causes a sample to be overwritten. If it is not reliable, then that handshake isn't there so then the loss would only be because of packet loss.

For transient-local, clearly there is the historical data to account for as well. It takes some time to fetch and deliver everything in-order and that increases the window for overwriting data in the writer history. But otherwise it is quite like volatile.

Do you happen to have a small reproducer?

Gummum commented 3 weeks ago

So the implication is that there is no such time.

Could it be that there is a period of time between the discovery of subscriber and subscriber availability?

I wonder if there's a bug that needs to be fixed if the mechanism is not working this way.

I tested this on my embedded device. My testing method was to create a pub process on the device and start my sub process to receive data after a certain period of time (approximately 3-5 seconds).

The result could be that the pub dds_write succeeded 10 times, but sub only received 7 messages.

How can I help you?! I really want to participate in the maintenance of the project, and I am very interested in dds!