ZeroMQ send/receive on PHP works if script launched from CLI/shell but not from APACHE/web request – CentOS

OBSERVATION : The browser gets stuck, without any error.

This is pretty legal state. For it to happen, it is quite enough to “miss” the arrival of the first REQ-side-already dispatched request and due to a pleasure do depend on a distributed-Finite-State-Automaton, we fall into an unsalvageable dead-lock, where the REQ-side waits for an answer, that will never arrive (see next) and the REP-side waits for a request, that will never arrive (see the REQ-side already waiting ) and such a state remains forever that.

A best next step:

_{In case one has never worked with ZeroMQ,
or have never met the concept of the art of Zen-of-Zero,
one may here enjoy to first look at “ZeroMQ Principles in less than Five Seconds“ before diving into further details}

Start
with unconditionally working archetypes – a pair of PUSH / PULL simplex-channels, that do not require a dFSA-two-step of REQ-REP-REQ-REP-REQ-REP-...-{deadlock} _{… a principally unavoidable terminal state, about which one is just never sure when it happens, but it will … at some later time :o)}

Next,
may increase a robustness of the message-flow, using zmq_setsockopt( ZMQ_IMMEDIATE, 1 ) that avoids moving messages onto incomplete connections between / among peers.

Always
prefer non-blocking forms of .recv()-methods, best with a pre-test of a message-presence with a .poll()-method. Poller-class, while available in many language-bindings is not always as handy and as flexible as using explicit .poll()-method directly on a Socket-instance.

Also feel free to read more about fine-tuning the ZeroMQ tools and other implications of the Art of the Zen-of-Zero here.

A Server-side mock-up: _{As a { PASS | FAIL }-proof of .send()---.recv()-delivery chain works?}

<?php                                      /* Create new PUSH-ing end */
$aCTX   = new ZMQContext();
try {                                      /* Try: things may turn wreck havoc */

      $PUSHer = $aCTX->getSocket(, ZMQ::SOCKET_PUSH );
      echo "POSACK'd: .getSocket() was maden";
      }
catch ( ZMQSocketException $e ){
      echo "  NACK'd: I told you ...n";   /* Handle with care ... */
      if ( $e->getCode() === ZMQ::ERR_... ) {
            echo " - Got ERR_..., read ZeroMQ API documentation for detailsn";
        } else {
            die( " - Get ERR: " . $e->getMessage() );
        }
      }
try {                                      /* Try: things may turn wreck havoc */
      $PUSHer->bind( "tcp://A.B.C.D:NNN" ); /* IP address to .connect() */
      echo "POSACK'd: .bind() was maden";
      }
catch ( ZMQSocketException $e ){
      echo "  NACK'd: I told you ...n";   /* Handle with care ... */
      if ( $e->getCode() === ZMQ::ERR_... ) {
            echo " - Got ERR_..., read ZeroMQ API documentation for detailsn";
        } else {
            die( " - Get ERR: " . $e->getMessage() );
        }
      }

$retries = 1234567;

do {                                       /* Start a loop */
    try {                                  /* Try: to PUSH.send() */
            echo "Trying to send a message #" . ( 1234568 - $retries ) . "n";
            $PUSHer->send( "This is a message", ZMQ::MODE_DONTWAIT );
            echo "POSACK'd: PUSHer.send() was maden";
        }
    } catch ( ZMQSocketException $e ) {
        echo "  NACK'd: I told you ...n"; /* Handle with care ... */
        if ( $e->getCode() === ZMQ::ERR_... ) {
            echo " - Got ERR_..., read ZeroMQ API documentation for detailsn";
        } else {                           /* For all ERR_... states */
            die( " - Got ERR_...: " . $e->getMessage() );
        }
    }
 /* --------------------------------------------------------------------
    Here one may add an attempt to .recv( $PULLer, ZMQ::MODE_DONTWAIT );
             and test for a non-empty string returned
    -------------------------------------------------------------------- */
    usleep( 1 );                           /* Sleep a bit between operations */
} while ( --$retries );
?>

Client-side mock-up, to test the PUSH-er lives and .send()-s

import time, datetime, zmq; print( "Thissssss Sssssssssssssssssssssssssssssssssssssssnake uses ZeroMQ ver:{0:}".format( zmq.__version__ ) )

aCtx = zmq.Context()
aPull= aCtx.Socket( zmq.PULL )
aPull.setsockopt(   zmq.LINGER, 0 )         # always ... be explicit
aPull_address2c = "tcp://A.B.C.D:NNN"

M0 = "{0:} try a .connect( {1:} ), if it gets to PUSH-er side"
M1 = "{0:} try a .recv(), if it gets any message"
M2 = "{0:} got a .recv()->[[[ {1:} ]]]"
M3 = "{0:} EXC'd           will gracefully release resources and terminate..."
M4 = "{0:} did"

try:
    print( M0.format( datetime.datetime.isoformat( datetime.datetime.now() ),
                      aPull_address2c
                      )
           )
    aPull.connect( aPull_address2c );

    while True:
        print( M1.format( datetime.datetime.isoformat( datetime.datetime.now() ) )
        m = aPull.recv( zmq.NOBLOCK )       # always ... avoid blocking waits
        if ( len( m ) > 0 ):
             print( M2.format( datetime.datetime.isoformat( datetime.datetime.now() ),
                               str( m )     # always ... fused to str()
                               )
                    )
             time.sleep( 5 )
        else:
             time.sleep( 1 )

        pass

        ################################################################
        # Here one may add an attempt to aPush.send( M4, zmq.NOBLOCK )
        #          and test if the reverse path aPush->$PULLer goes well
        ################################################################

except:
    print( M3.format( datetime.datetime.isoformat( datetime.datetime.now() ) )

finally:
    aPull.close()                           # always ... be explicit
    aCtx.term()                             # always ... be explicit

    print( M4.format( datetime.datetime.isoformat( datetime.datetime.now() ) )