Where is roscore in ROS2?

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There is no roscore in ROS 2. It used a decentralized discovery mechanism instead.

Please take some time to read up about ROS 2. There are a lot of resources available on the main ROS 2 page. https://index.ros.org/doc/ros2/

I recommend reading through the documents linked from the Learn More Section.

It has design articles, differences from ROS 1 and ROS 2, and links to dozens of talks about ROS 2 that you can listen to from ROSCon as far back as 2014.

We're happy to have you keep asking questions but please try to be a little bit more complete in how you ask your question. Link to related documentation that you're looking for clarification to or where you're starting. As well as any other questions and answers that you've already read to try to find your answer.

We have a guide to asking questions here: https://wiki.ros.org/Support

There is no roscore because there is no master anymore. Ros2 uses peer to peer communication which creates basically a mesh network, so nodes can talk directly to other nodes without relying on a master.