Client-Server Interface

Due to security limitation of gdb (see gdb manual) mdb implements its own client-server interface. This section explains the plan for this interface.

Launch Mode

The first stage of debugging with mdb is to run the launch command. mdb launch is the part that actually runs the target binary using the MPI launcher (mpirun, mpiexec, srun etc.).

mdb launch starts two child processes; (a) the first process runs the MPI launcher, in this example mpirun, and (b) the second is an mdb exchange server which will be responsible for authenticating communication between the gdb subprocesses and the mdb attach process.
The call to mpirun in the previous step will launch nproc mdb server processes which will provide a thin wrapper around gdb, essentially providing encrypted command input and output to each rank.
mdb exchange server will perform a TLS handshake with each mdb server process. This sets up the initial encrypted communication channel.
The last step in launch mode is for each mdb server process to communicate some basic information with the mdb exchange server, such as, IP address (or HOSTNAME), port number and MPI rank.

Attach Mode

Now that the debug processes are launched we can connect using the mdb attach command.

Running mdb attach will trigger a TLS handshake with the mdb exchange server on the specified host (--host [hostname]).
Once authenticated, mdb can forward command IO to and from the mdb exchange server.
mdb server has already authenticated the connection with thin mdb server processes. It forwards the relevant commands to each rank.
Each rank responds depending on the command issued.
mdb exchange server sends encrypted output back to mdb attach, where it will be unencrypted automatically using TLS/SSL sockets.

Communication Protocol

This is still a work in progress but some things are already certain:

messages will be sent as json.
each message will include a version number.
each message will have a destination (e.g., exchange server or mdb server).