ests. 
      Registers allow for some level of reliability,
      <em>i.e.</em> registers can ensure with certain probability 
             that providers are available at the time,
             when a request is made.
     Therefore, registries may request heartbeats from providers.
     Heartbeats are negotiated on registration.
     Note that registries do not send heartbeats back to providers.
     Providers have to use other strategies to make sure
     that the registry to which they have registered i