> Just to make this point clear: The "write-to-odb-read-via-hotlink" was never meant to guarantee the receiving side to see each change. If changes happen too often, updates might get lost. If one relies on the 
> sequence of updates, one should use direct RPC calls to the frontend or use a midas buffer and encode updates in events.

I recommend that people use the jrpc mechanism that does an RPC directly from javascript into the frontend.
It passes 2 strings as arguments (command and data value). Arbitrary objects can be passed by encoding
the data in json (use mjson.h to decode it in the frontend). A string is returned back to javascript (again, encode
arbitrary data as json, use the mjson.h library).

Call sequence:
javascript -> (http) -> mhttpd -> (MIDAS RPC call) -> frontend -> (write, read, frob hardware) -> frontend -> (MIDAS RPC reply) -> mhttpd -> (http reply) -> javascript

Example of all this is in example.html and fetest.cxx:

javascript side code: mjsonrpc_call("jrpc", { "client_name":"fetest", "cmd":"xxx", "args":"xxx" })

frontend side code:

INT rpc_callback(INT index, void *prpc_param[])
{
   const char* cmd  = CSTRING(0);
   const char* args = CSTRING(1);
   char* return_buf = CSTRING(2);
   int   return_max_length = CINT(3);
   cm_msg(MINFO, "rpc_callback", "--------> rpc_callback: index %d, max_length %d, cmd [%s], args [%s]", index, return_max_length, cmd, args);
   ... do stuff ... put result into string "tmp"
   strlcpy(return_buf, tmp, return_max_length);
   return RPC_SUCCESS;
}

... somewhere in frontend_init(), register the RPC:

#ifdef RPC_JRPC
   status = cm_register_function(RPC_JRPC, rpc_callback);
   assert(status == SUCCESS);
#endif

K.O.