ROS2 middleware message sequence number?

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My advice is to think twice before you design a node that relies on message sequence numbers.

In a distributed network system like ROS where multiple nodes can publish to a topic, it is very expensive to coordinate the sequence number generation (and even if you decide the cost is worth it, then it still might fail due to network problems or node restarts.) I'll also point out you have no way to prevent multiple publishers from existing in the system.

You'll have a more robust system if you find some other mechanism to check whatever it is you want to check.