ROS2 parameter service: where is it?

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There's not a single global parameter server in ROS 2, rather having a centralized (single point of failure) for parameters, in ROS 2 all parameters are owned by a node. You could have a special node that acts like the parameter server in ROS 1, i.e. accepts all parameters set on it and can have parameters initialized by yaml file. I made a demo that does exactly that in this pr recently:

https://github.com/ros2/demos/pull/333

We don't have a node that you can run as part of something like roscore from ROS 1 in ROS 2, but we may in the future. In the meantime, I'd recommend (re)-evaluating each of your parameters to see if they actually need to be global, and if not just use them locally in your node. There are some cases where global makes sense, and in those cases you can either duplicate the parameter in each node (e.g. this is what we do for use_sim_time and it works well if multiple nodes are just reading the parameter) or you can setup a node like the one in my demo above and read/write on it from all of your nodes.