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In this page, there three nodes, robot_state, motion_interface and joint_trajectory_action. The robot_state just publishes three topics without subscribing anyone topics while motoin_interface is completely opposite. Does the robot_state node decode the message received from robot controller using simple message? And the motion_*_interface subscribes the message form topics and encode them using simple message then send to robot controller? Can I understand the process of these two nodes like this?

yes, that is correct.

Does joint_trajectory_action node act as an action server? I noted that this node subscribed topics like goal, cancel and states and publishes topics like feedback, status etc.

Yes.

where is the action client?

That could be any compatible client. One example could be MoveIt, or a custom node that you write yourself.