[Bro] Trouble passing a message between two local endpoints.

David Banas capn.freako at gmail.com
Tue Aug 11 11:32:25 PDT 2015

Hi Jon,

Thanks for your reply!

I implemented your recommended changes, recompiled, and re-ran, but am
getting the same result:

davids-air-2:broker-haskell dbanas$ ./a.out
There are 0 messages.

Any thoughts?


On Aug 10, 2015, at 8:10 AM, Siwek, Jon <jsiwek at illinois.edu> wrote:

On Aug 10, 2015, at 8:22 AM, David Banas <capn.freako at gmail.com> wrote:

Does anyone see what I’m doing wrong?

There’s a few race conditions to be aware of:

1) For the sample code you made, you want to establish/create the message
queues attached to an endpoint before initiating peerings/connections with
other endpoints.  The point here is to allow topic
advertisements/subscriptions to be established before actually sending any

2) If you request a message to be sent before the connection w/ a peer is
actually established, it may just get dropped because it’s seen that no one
is interested in that message.  In your example, you can check the
outgoing/incoming connection status queues to wait for the connection to
establish.  Using the blocking “need” version of the function works fine as
a convenience in this case, but I’d think it more typical in real code to
use the non-blocking “want” version and to have integrated queues into
event loop (e.g. select(), poll(), etc.)

3) Using the non-blocking “want” version of popping the message queue
doesn’t give any time for the message to actually be sent and arrive at the
peer endpoint.  Either integrate into an event loop or just use the
blocking “need” version to wait for the message to arrive.

Here’s an example of revising your code w/ those 3 suggestions:

#include <stdio.h>
#include "broker.h"

int main (int argc, char* argv[]) {
   broker_endpoint* ep1 = broker_endpoint_create_with_flags("ep1", 3);
   broker_endpoint* ep2 = broker_endpoint_create_with_flags("ep2", 3);
   broker_string*       bs = broker_string_create("");
   broker_message_queue* q = broker_message_queue_create(bs, ep2);

   broker_peering*    p = broker_endpoint_peer_locally(ep2, ep1);

   const broker_outgoing_connection_status_queue* ocsq =

   broker_string*  msg_str = broker_string_create("Hello, World!\n");
   broker_string*    topic = broker_string_create("test");
   broker_data*        msg = broker_data_from_string(msg_str);

   broker_vector* vec = broker_vector_create();
   int            res = broker_vector_insert(vec, msg, 0L);
                  res = broker_endpoint_send(ep1, topic, vec);

   broker_deque_of_message* msg_list = broker_message_queue_need_pop(q);
   size_t                   num_msgs =

   printf("There are %ld messages.\n", num_msgs);

Hope that helps.

- Jon
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