A) When the user's request enters the system, a globally unique traceld will be generated on the first layer of the calling network. and C) Spanld can locate the position of a request in the system call, and who its upstream and downstream dependencies are respectively. and D) Traceld is used to distinguish the sequence of calls between different services of the system, spanld is used to concatenate all the paths that a certain request traverses in the system.

B) The tracelD will be called with each layer and passed back continuously, so through traceld, the path of a user request in the system can be connected in series.