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

Question

I'm testing ZeroMQ for PHP. My goal is to send messages to a Python script. Everything works fine if i launch my transmission script from PHP cli

php /path/to/myscript.php

while it fails if it's a web request. I've tried executing the server script from PHP cli as above (which seems to be the more logical way) and with a web request.

I've got a Centos 7 server with PHP 7.2 and ZeroMQ 1.1.3 installed through PECL install.

I even tried launching the above command with shell_exec/exec inside the client script but it doesn't work. Connection works fine, but it doesn't send nor receive.

Client code:

$context = new ZMQContext();

//  Socket to talk to server
echo "Connecting to hello world server...\n";
$requester = new ZMQSocket($context, ZMQ::SOCKET_REQ);
$currentObject = $requester->connect("tcp://localhost:5555");


for ($request_nbr = 0; $request_nbr != 10; $request_nbr++) {
    printf ("Sending request %d...\n", $request_nbr);
    $risSend = $requester->send("Hello", ZMQ::MODE_NOBLOCK);
    print_r($risSend);
    $reply = $requester->recv();
    printf ("Received reply %d: [%s]\n", $request_nbr, $reply);
}

Server Code:

$context = new ZMQContext(1);

//  Socket to talk to clients
$responder = new ZMQSocket($context, ZMQ::SOCKET_REP);
$responder->bind("tcp://*:5555");

while (true) {
    //  Wait for next request from client
    $request = $responder->recv();

    printf ("Received request: [%s]\n", $request);

    //  Send reply back to client
    $responder->send("World");
}

The browser gets stuck, without any error. Even using a timeout it reaches the limit and exits but I can't get any error message.

score 1 · Answer 1 · edited Oct 20 '19 at 10:06

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 made\n";
      }
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 details\n";
        } 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 made\n";
      }
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 details\n";
        } 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 made\n";
        }
    } 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 details\n";
        } 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() ) )

Unfortunately php's ZMQSocket Object doesn't have a poll() method. ( https://www.php.net/manual/en/class.zmqsocket.php ) I added $requester->setSockOpt(ZMQ::SOCKOPT_IMMEDIATE, 1); (the same for responder) but still, it works from PHP cli and not using a web request — dgtal, Oct 10 '19 at 14:05
**0)** high time to make the .poll()-method added to the language-binding, isn't it? **1)** did you test the pair of simplex-channels: PUSH/PULL + PULL/PUSH to avoid an influence of the dFSA-blocking issues in REQ/REP and to prove the Web-side works with having due access rights to use all the needed O/S resources ( sockets etc )? — user3666197, Oct 10 '19 at 14:30
Done both. Used ZQMPoll ( https://www.php.net/manual/en/zmqpoll.poll.php ) and the most basic PUSH/PULL i could. Same exact results. — dgtal, Oct 10 '19 at 16:13
May you clarify the meanings?Both what?I've expressed an advice to 0) extend the many years old-and-"rusty" php-language binding, so as to have a .poll()-method ( documented state does not contain even the const ZMQ_IMMEDIATE ) + to do 1) - a setup, where messages may flow freely in **upstream** ( python-side **PUSH/PULL** on web-side ) and **downstream** ( python-side **PULL/PUSH** from web-side ) to prove { PASS | FAIL }-state of message delivery, independently of the fragile dFSA-REQ/REP-code above. **Would you mind to clarify what did you try** with code **and what were the results?** Thx — user3666197, Oct 10 '19 at 16:22
0) I used ZMQPoll ( https://www.php.net/manual/en/class.zmqpoll.php ) 1) I changed my code to use push pull in PHP. And the result is identical. I don't get any message or error from the client side nor the server side. Regarding the use of python, i will try it but i'm not sure it can help me understand what's the problem. — dgtal, Oct 11 '19 at 07:00
BTW, the full code (i hope it's correct, i'm new to ZMQ): Client/Push: https://pastebin.com/AEEXaYmv Server/Pull: https://pastebin.com/wvUXN9xa — dgtal, Oct 11 '19 at 07:15

score 1 · Accepted Answer · answered Oct 14 '19 at 15:24

Ok, i finally found the solution. Thanks to @user3666197 i managed to get an error. And it was a "permission denied" error.

With CentOS (and probably all linux sytems with SELinux) you gotta add a rule for the port used by ZMQ for the webserver.

Example:

semanage port -a -t http_port_t -p tcp 5555

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

2 Answers2