zmq_ctx_set_monitor(3)
======================


NAME
----

zmq_ctx_set_monitor - register a monitoring callback


SYNOPSIS
--------
*int zmq_ctx_set_monitor (void '*context', zmq_monitor_fn '*monitor');*


DESCRIPTION
-----------
The _zmq_ctx_set_monitor()_ function shall register a callback function
specified by the 'monitor' argument. This is an event sink for changes in per
socket connection and mailbox (work in progress) states.

.The _zmq_ctx_set_monitor()_ callback function is expected to have this
prototype:

----
typedef void (zmq_monitor_fn) (void *s, int event, zmq_event_data_t *data);
----

The callback is global (per context), with the socket that triggered the event
passed to the handler as well. Each event also populates a 'zmq_event_data_t'
union with additional metadata which can be used for correlation.

CAUTION: _zmq_ctx_set_monitor()_ is intended for monitoring infrastructure /
operations concerns only - NOT BUSINESS LOGIC. An event is a representation of
something that happened - you cannot change the past, but only react to them.
The implementation is also only concerned with a single session. No state of
peers, other sessions etc. are tracked - this will only pollute internals and
is the responsibility of application authors to either implement or correlate
in another datastore. Monitor events are exceptional conditions and are thus
not directly in the messaging critical path. However, still be careful with
what you're doing in the callback function as excess time spent in the handler
will block the socket's application thread.

Only tcp and ipc specific transport events are supported in this initial
implementation.

Supported events are:


ZMQ_EVENT_CONNECTED: connection established
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CONNECTED' event triggers when a connection has been established
to a remote peer. This can happen either synchronous or asynchronous.

.Callback metadata:
----
data.connected.addr // peer address
data.connected.fd // socket descriptor
----

ZMQ_EVENT_CONNECT_DELAYED: synchronous connect failed, it's being polled
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CONNECT_DELAYED' event triggers when an immediate connection
attempt is delayed and it's completion's being polled for.

.Callback metadata:
----
data.connect_delayed.addr // peer address
data.connect_delayed.err // errno
----

ZMQ_EVENT_CONNECT_RETRIED: asynchronous connect / reconnection attempt
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CONNECT_RETRIED' event triggers when a connection attempt
is being handled by reconnect timer. The reconnect interval's recomputed
for each attempt.

.Callback metadata:
----
data.connect_retried.addr // peer address
data.connect_retried.interval // computed reconnect interval
----

ZMQ_EVENT_LISTENING: socket bound to an address, ready to accept connections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_LISTENING' event triggers when a socket's successfully bound
to a an interface.

.Callback metadata:
----
data.listening.addr //  listen address
data.listening.fd // socket descriptor
----

ZMQ_EVENT_BIND_FAILED: socket could not bind to an address
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_BIND_FAILED' event triggers when a socket could not bind to
a given interface.

.Callback metadata:
----
data.bind_failed.addr // listen address
data.bind_failed.err // errno
----

ZMQ_EVENT_ACCEPTED: connection accepted to bound interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_ACCEPTED' event triggers when a connection from a remote peer
has been established with a socket's listen address.

.Callback metadata:
----
data.accepted.addr // listen address
data.accepted.fd // socket descriptor
----

ZMQ_EVENT_ACCEPT_FAILED: could not accept client connection
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_ACCEPT_FAILED' event triggers when a connection attempt to
a socket's bound address fails.

.Callback metadata:
----
data.accept_failed.addr // listen address
data.accept_failed.err // errno
----

ZMQ_EVENT_CLOSED: connection closed
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CLOSED' event triggers when a connection's underlying descriptor
has been closed. 

.Callback metadata:
----
data.closed.addr // address
data.closed.fd // socket descriptor
----

ZMQ_EVENT_CLOSE_FAILED: connection couldn't be closed
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CLOSE_FAILED' event triggers when a descriptor could not be
released back to the OS.

.Callback metadata:
----
data.close_failed.addr // address
data.close_failed.err // errno
----

ZMQ_EVENT_DISCONNECTED: broken session
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_DISCONNECTED' event triggers when the stream engine (tcp and ipc
specific) detects a corrupted / broken session.

.Callback metadata:
----
data.disconnected.addr // address
data.disconnected.fd // socket descriptor
----

RETURN VALUE
------------
The _zmq_ctx_set_monitor()_ function returns a value of 0 or greater if
successful. Otherwise it returns `-1` and sets 'errno' to one of the values
defined below.


ERRORS
------
*EINVAL*::
The requested callback function _monitor_ is invalid.


EXAMPLE
-------
.Observing a 'PUB' socket's connection state 
----
void socket_monitor (void *s, int event_, zmq_event_data_t *data_)
{
    switch (event_) {
    case ZMQ_EVENT_LISTENING:
        printf ("Socket bound to %s, socket descriptor is %d\n",
                 data.listening.addr, data.listening.fd);
        break;
    case ZMQ_EVENT_ACCEPTED:
        printf ("Accepted connection to %s, socket descriptor is %d\n",
                data.accepted.addr, data.accepted.fd);
        break;
    }
}

void *context = zmq_ctx_new ();
int rc = zmq_ctx_set_monitor (context, socket_monitor);
assert (rc == 0);
void *pub = zmq_socket (context, ZMQ_PUB);
assert (pub);
void *sub = zmq_socket (context, ZMQ_SUB);
assert (pub);
rc = zmq_bind (pub, "tcp://127.0.0.1:5560");
assert (rc == 0);
rc = zmq_connect (sub, "tcp://127.0.0.1:5560");
assert (rc == 0);

// Allow a window for socket events as connect can be async
zmq_sleep (1);

rc = zmq_close (pub);
assert (rc == 0);
rc = zmq_close (sub);
assert (rc == 0);

zmq_term (context);
----


SEE ALSO
--------
linkzmq:zmq[7]


AUTHORS
-------
This 0MQ manual page was written by Lourens Naudé <lourens@methodmissing.com>