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Learning and Using ØMQ

Home Page:http://zguide.zeromq.org

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Lazy pirate example: wrong polling?

altmind opened this issue · comments

commented

I'm checking an examples for REQ/REP sockets, and trying to make some sense from lazy pirate examples. Take a look at php sample http://zguide.zeromq.org/php:chapter4#toc3 http://zguide.zeromq.org/php:lpclient :

$context = new ZMQContext();
$client = client_socket($context);

$sequence = 0;
$retries_left = REQUEST_RETRIES;
$read = $write = array();

while ($retries_left) {
    //  We send a request, then we work to get a reply
    $client->send(++$sequence);

    $expect_reply = true;
    while ($expect_reply) {
        //  Poll socket for a reply, with timeout
        $poll = new ZMQPoll();
        $poll->add($client, ZMQ::POLL_IN);
        $events = $poll->poll($read, $write, REQUEST_TIMEOUT);

        //  If we got a reply, process it
        if ($events > 0) {
            //  We got a reply from the server, must match sequence
            $reply = $client->recv();
            if (intval($reply) == $sequence) {
                printf ("I: server replied OK (%s)%s", $reply, PHP_EOL);
                $retries_left = REQUEST_RETRIES;
                $expect_reply = false;
            } else {
                printf ("E: malformed reply from server: %s%s", $reply, PHP_EOL);
            }
        } elseif (--$retries_left == 0) {
            echo "E: server seems to be offline, abandoning", PHP_EOL;
            break;
        } else {
            echo "W: no response from server, retrying…", PHP_EOL;
            //  Old socket will be confused; close it and open a new one
            $client = client_socket($context);
            //  Send request again, on new socket
            $client->send($sequence);
        }
    }
}

The block in question is the last else. My understanding that we can get there only when poll returned 0 pollin events, which can happen when not-final poll try for REQUEST_TIMEOUT and got nothing in the socket(yet). This looks like a normal situation - the poll data have just not returned yet - we should do $retries_left more attempts.

Why do we close the socket and send the message again in the inner while? sending messages again should only happen when we did $retries_left poll attempts. Re-sending the request is handled in the outer while cycle, its unnecessary to kill the socket after 1st poll without data.

commented