MUSIC Connections in C++ and Python

The C++ interface

The C++ interface is the lowest-level interface and what you would use to implement a MUSIC interface in simulators. But it is not a complicated API, so you can easily use it for your own applications that connect to a MUSIC-enabled simulation.

Let’s take a look at a pair of programs that send and receive spikes. These can be used as inputs or outputs to the NEST models we created above with no change to the code. C++ code tends to be somewhat longwinded so we only show the relevant parts here. The C++ interface is divided into a setup phase and a runtime phase. You can see the setup below.

MPI::Intracomm comm;

int main(int argc, char **argv)
    MUSIC::Setup* setup = new MUSIC::Setup (argc, argv);
    comm = setup->communicator();

    double simt;                // read simulation time from the
    setup->config ("simtime", &simt);       // MUSIC configuration file

    MUSIC::EventOutputPort *outdata =       // set output port

    int nProcs = comm.Get_size();       // Number of mpi processes
    int rank = comm.Get_rank();         // I am this process

    int width = 0;              // Get number of channels
    if (outdata->hasWidth()) {          // from the MUSIC configuration
    width = outdata->width();
    // divide output channels evenly among MPI processes
    int nLocal = width / nProcs;    // Number of channels per process
    int rest = width % nProcs;
    int firstId = nLocal * rank;    // index of lowest ID
    if (rank < rest) {
    firstId += rank;
    nLocal += 1;
    } else
    firstId += rest;

    MUSIC::LinearIndex outindex(firstId, nLocal);   // Create local index
    outdata->map(&outindex, MUSIC::Index::GLOBAL);  // apply index to port

    [ ... continued below ... ]

At lines 5-6 we initialize MUSIC and MPI. The communicator is common to all processes running under MUSIC, and you’d use it instead of COMM_WORLD for your MPI processing.

Lines 7 and 8 illustrate something we haven’t discussed so far. We can set and read free parameters in the MUSIC configuration file. We can for instance use that to set the simulation time like we do here; although this is of limited use with a NEST simulation as you can’t read these configuration parameters from within NEST.

We set up an event output port and name it on line 11 and 12, then get the number of MPI processes and our process rank for later use. In lines 17-19 we read the number of channels specified for this port in the configuration file. We don’t need to set the channels explicitly beforehand like we do in the NEST interface.

We need to tell MUSIC which channels should be processed by what MPI processes. Lines 22-29 are the standard way to create a linear index map from channels to MPI processes. It divides the set of channels into equal-sized chunks, one per MPI process. If channels don’t divide evenly into processes, the lower-numbered ranks each get an extra channel. firstId is the index of the lowest-numbered channel for the current MPI process, and nLocal is the number of channels allocated to it.

On lines 31 and 32 we create the index map and then apply it to the output port we created in line 11. The Index::GLOBAL parameter says that each rank will refer to its channels by its global ID number. We could have used Index::LOCAL and each rank would refer to their own channels starting with 0. The linear index is the simplest way to map channels, but there is a permutation index type that lets you do arbitrary mappings if you want to.

The map method actually has one more optional argument: the maxBuffered argument. Normally MUSIC decides on its own how much event data to buffer on the receiving side before actually transmitting it. It depends on the connection structure, the amount of data that is generated and other things. But if you want, you can set this explicitly:

outdata->map(&outindex, MUSIC::Index::GLOBAL, maxBuffered)

With a maxBuffered value of 1, for instance, MUSIC will send emitted spike events every cycle. With a value of 2 it would send data every other cycle. This parameter can be necessary if the receiving side is time-sensitive (perhaps the input controls some kind of physical hardware), and the data needs to arrive as soon as possible.

[ ... continued from above ... ]

// Start runtime phase
MUSIC::Runtime runtime = MUSIC::Runtime(setup, TICK);
double tickt =  runtime.time();

while (tickt < simt) {
for (int idx = firstId; idx<(firstId+nLocal); idx++) {
    // send poisson spikes to every channel.
    send_poisson(outdata, RATE*(idx+1), tickt, idx);
runtime.tick();         // Give control to MUSIC
tickt = runtime.time();
runtime.finalize();         // clean up and end


double frand(double rate) {return -(1./rate)*log(random()/double(RAND_MAX));}

void send_poisson(MUSIC::EventOutputPort* outport,
          double rate, double tickt, int index) {
    double t = frand(rate);
    while (t<TICK) {
    outport -> insertEvent(tickt+t, MUSIC::GlobalIndex(index));
    t = t + frand(rate);

The runtime phase is short. On line 4 we create the MUSIC runtime object, and let it consume the setup. In the runtime loop on lines 7-14 we output data, then give control to MUSIC by its tick() function so it can communicate, until the simulation time exceeds the end time.

runtime.time() on lines 5 and 13 gives us the current time according to MUSIC. In lines 8-10 we loop through the channel indexes corresponding to our own rank (that we calculated during setup), and call a function defined from line 20 onwards that generates a poisson spike train with the rate we request.

The actual event insertion happens on line 24, and we give it the time and the global index of the channel we target. The loop on line 8 loops through only the indexes that belong to this rank, but that is only for performance. We could loop through all channels and send events to all of them if we wanted; MUSIC will silently ignore any events targeting a channel that does not belong to the current rank.

runtime.tick() gives control to MUSIC. Any inserted events will be sent to their destination, and any new incoming events will be received and available once the method returns. Be aware that this call is blocking and could take an arbitrary amount of time, if MUSIC has to wait for another simulation to catch up. If you have other time-critical communications you will need to put them in a different thread.

Once we reach the end of the simulation we call runtime.finalize(). Music will shut down the communications and clean up after itself before exiting.

MPI::Intracomm comm;
FILE *fout;

struct eventtype  {
    double t;
    int id;
std::queue <eventtype> in_q;

class InHandler : public MUSIC::EventHandlerGlobalIndex {
    void operator () (double t, MUSIC::GlobalIndex id) {
        struct eventtype ev = {t, (int)id};

int main(int argc, char **argv)
    MUSIC::Setup* setup = new MUSIC::Setup (argc, argv);
    comm = setup->communicator();

    double simt;
    setup->config ("simtime", &simt);

    MUSIC::EventInputPort *indata =

    InHandler inhandler;

    [ ... get processes, rank and channel width as in send.cpp ... ]

    char *fname;
    int dummy = asprintf(&fname, "output-%d.spk", rank);
    fout = fopen(fname, "w");

    [ ... calculate channel allocation as in send.cpp ... ]

    MUSIC::LinearIndex inindex(firstId, nLocal);
    indata->map(&inindex, &inhandler, IN_LATENCY);

The setup phase for the reveiving application is mostly the same as the sending one. The main difference is that we receive events through a callback function that we provide. During communication, MUSIC will call that function once for every incoming event, and that function stores those events until MUSIC is done and we can process them.

For storage we define a structure to hold time stamp and ID pairs on lines 4-7, and a queue of such structs on line 8. Lines 10-14 defines our callback function. The meat of it is lines 13-14, where we create a new event struct instance with the time stamp and ID we received, then push the structure onto our queue.

The actual setup code follows the same pattern as before: we create a setup object, get ourself a communicator, read any config file parameters and create a named input port. We also declare an instance of our callback event handler on line 29. We get our process and rank information and calculate our per-rank channel allocation in the exact same way as before.

The map for an input port that we create on line 40 needs two additional parameters that the output port map did not. We give it a reference to our callback function that we defined earlier. When events appear on the port, they get passed to the callback function. It also has an optional latency parameter. This is the same latency that we set with the separate SetAcceptableLatency() function in the NEST example earlier, and it works the same way. Just remember that the MUSIC unit of time is seconds, not milliseconds.

int main(int argc, char **argv)
    MUSIC::Runtime runtime = MUSIC::Runtime(setup, TICK);
    double tickt = runtime.time();

    while (tickt < simt) {
    runtime.tick();     // Give control to MUSIC
    tickt = runtime.time();
    while (!in_q.empty()) {
        struct eventtype ev = in_q.front();
        fprintf (fout, "%d\t%.4f\n",, ev.t);

The runtime is short. As before, we create a runtime object that consumes the setup, then we loop until the MUSIC time exceeds our simulation time. We call runtime.tick() each time through the loop on line 8 and we process received events after the call to tick(). If you had a process with both sending and receiving ports, you would submit the sending data before the tick() call, and process the receiving data after it in the same loop.

The in_q input queue we defined earlier holds any new input events. We take the first element on line 10, then process it — we write it out to a file — and finally pop it off the queue. When the queue is empty we’re done and go back around the main loop again.

Lastly we call runtime.finalize() as before.

Building the Code

We have to build our C++ code. The example code is already set up for the GNU Autotools, just to show how to do this for a MUSIC project. There’s only two build-related files we need to care about (all the rest are autogenerated), and

AC_INIT(simple, 1.0)
AM_INIT_AUTOMAKE([1.11 -Wall subdir-objects no-define foreign])
dnl # set OpenMPI compiler wrapper
AC_CHECK_LIB([music], [_init])

The first three lines set the project name and version, the minimum version of autotools we require and a list of options for Automake. Line 4 sets the current language, and line 5 that we want a config.h file.

Line 7 tells autoconf to use the mpicxx MPI wrapper as the C++ compiler. Lines 8-9 tells it to test for the existence of the music library, and look for the music.hh include file.

bin_PROGRAMS = send recv
send_SOURCES = send.cpp
recv_SOURCES = recv.cpp has only three lines: bin_PROGRAMS lists the binaries we want to build. send_SOURCES and recv_SOURCES lists the source files each one needs.

Your project should already be set up, but if you start from nothing, you need to generate the rest of the build files. You’ll need the Autotools installed for that. The easiest way to generate all build files is to use autoreconf:

autoreconf --install --force

Then you can build with the usual sequence of commands:


Try the Code

We can run these programs just like we did with the NEST example, using a Music configuration file:


  from.p_out -> to.p_in [2]

The structure is just the same as before. We have added a simtime parameter for the two applications to read, and the binaries are our two new programs. We run this the same way:

mpirun -np 4 music

You can change the simulation time by changing the simtime parameter at the top of the file. Also, these apps are made to deal with any number of channels, so you can change [2] to anything you like. If you have more channels than MPI processes for the recv app you will get more than one channel recorded per output file, just as the channel allocation code specified. If you have more MPI processes than input channels, some output files will be empty.

You can connect these with the NEST models that we wrote earlier. Copy them into the same directory. Then, in the config file, change the binary parameter in [from] from binary=./send to binary=./ You get two sets of output files. Concatenate them as before, and compare:            recv

2   26.100         1    0.0261
1   27.800         0    0.0278
2   54.200         1    0.0542
1   57.600         0    0.0576
2   82.300         1    0.0823
1   87.400         0    0.0874
2   110.40         1    0.1104

Indeed, we get the expected result. The IDs from the python process on the left are the originating neurons; the IDs on the right is the MUSIC channel on the receiving side. And of course NEST deals in milliseconds while MUSIC uses seconds.

This section has covered most things you need in order to use it for straightforward user-level input and output applications. But there is a lot more to the MUSIC API, especially if you intend to implement it as a simulator interface, so you should consult the documentation for more details.