Problems with multiple "persistent" publishers on the same topic at subscriber startup

[FabioBagni]

Hi, we're having trouble trying to implement "persistent" publishers and subscribers.

To do that we are initializing publishers with publisherOptions.historyCapacity = 10 (or any size > 0), and subscribers with subscriberOptions.historyRequest = 1.

With these options, the subscriber receives the latest published message at its creation.

The problem is that if we have multiple publishers with these options publishing on the same topic, the subscriber receives the latest published message from each publisher.

So, is there a way to understand which one of the received messages is the latest?

[elBoberido]

Unfortunately not. With multiple publisher it is quite difficult to determine a right order because everybody might define it in a different way.

You could circumvent the issue somehow by using a UserHeader with a timestamp and have a local cache which orders the samples according to your needs.

The problem with multiple publishers and history is that there are inherent races. Let's assume you have three publisher, A, B and C:

• A loans a sample and is interrupted right before getting the timestamp
• the system is under heavy load and A is interrupted for a long time
• B loans a sample, gets the timestamp and is interrupted right before publishing the data
• the system is still under heavy load and B is also interrupted for a long time
• C loans a sample, gets the timestamp and publishes the data
• A continues, gets the timestamp and publishes its data
• B continues and publishes its data

-> although A has the oldest data, it has the newest timestamp

[elBoberido]

@FabioBagni Are your questions answered and can this be closed?

[FabioBagni]

Yes the answers are very clear, thanks.

I solved it writing timestamp inside the payload and manually discarding everything but the latest.