Where are the messages stored in ROS2 for external access ?

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I'm fairly sure that ROS2 doesn't store this information at all. It certainly won't be available in the filesystem.

If you are writing/modifying your own service provider node then you can interface this directly with your external tool. However if you're trying to access service call results from existing service provider nodes then I don't believe this is possible.

The messaging system is different however, this uses a one-to-many concept so you can make a new node that listens and monitors all messages passing through the system.