Abstract: A message flow environment permits a developer to define a debug session in which nodes in a defined message flow have a debug function. A debug controller is used to select nodes as breakpoints using data in shared memory accessible to the nodes in the defined message flow. Each node having a debug function will determine whether it is a breakpoint by accessing the shared memory. Where a node determines that it is a breakpoint, the node will communicate debug information to the debug controller. An independent daemon runs to permit a developer to define breakpoints from the set of debug nodes. The nodes having a debug function maintain a stack in the shared memory which is sent to the debug controller by each breakpoint node when reached by a message.