Message Federates

Message Federates provide the interfaces for registering endpoints and sending and receiving messages through those endpoints. Endpoints can be configured through API calls or through file configurations

API calls

Endpoints can be declared through MessageFederate methods or through Endpoint objects. These are defined in MessageFederate.hpp and Endpoints.hpp. For the MessageFederate api the register calls return and endpoint_id_t value that must be used whenever the endpoint is referenced. The Endpoint object api contains those calls in a separate object.

file configuration

File based configuration looks primarily at an “endpoints” JSON array

//this should be a valid JSON file (except comments are not recognized in standard JSON)
{ //example JSON configuration file for a message federate all arguments are optional
    "name":"messageFed", // the name of the federate
    //possible flags
    "observer":false,  // indicator that the federate does not send anything
    "rollback": false, // indicator that the federate can use rollback -NOTE: not used at present
    "only_update_on_change":false, //indicator that the federate should only indicate updated values on change
    "only_transmit_on_change":false,  //indicator that the federate should only publish if the value changed
    "source_only":false,  //indicator that the federate is only a source and is not expected to receive anything
    "uninterruptible":false, //indicator that the federate should only return requested times
    "coreType":"test", //the type of the core "test","zmq","udp","ipc","tcp","mpi"
    "coreName":"the name of the core",  //this matters most for ipc and test cores, can be empty
    "coreInit":"1", // the initialization string for the core in the form of a command line arguments
    "maxIterations":10, //the maximum number of iterations for a time step
    "period":  1.0, //the period with which federate may return time
    "offset": 0.0, // the offset shift in the period
    "timeDelta":0.0, // the minimum time between subsequent return times
    "outputDelay":0, //the propagation delay for federates to send data
    "inputDelay":0, //the input delay for external data to propagate to federates

//endpoints used in the federate
"endpoints":[
{
    "name":"ept1", // the name of the publication
    "type":"genmessage", // the type assocaited with a endpoint endpoint types have limited usefullness at present (optional)
    "global":true //set to true to make the key global (default is false in which case the publication is prepended with the federate name)
},
{
    "name":"ept2", // the name of the publication
    "type":"message2", // the type assocaited with a endpoint (optional)
 //the fact that there is no global value creates a local endpoint with global name messageFed/ept2
    "knownDestinations":"ept1", //this value can be an array of strings or just a single one it names key paths
 //knownDestinations can be used to optimize the communication pathways inside of HELICS
    "subscriptions":"fed2/sub1"  //subscribe an endpoint to a particular publication  this means that an endpoint will get a message whenever anything is published to that particular key
 //the message will be raw data so it would have to be translated to be useful. this can also be a JSON array to subscribe to multiple publications
}
]
}

See the comments in the file for more information. Endpoints can subscribe to publications in which case a message is delivered for every value published.