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Chapter 9. Message delivery

9.1. Sending messages
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To send a message, override the on_sendable event handler and call the sender::send() method. The sendable event fires when the proton::sender has enough credit to send at least one message.

Example: Sending messages

struct example_handler : public proton::messaging_handler {
    void on_container_start(proton::container& cont) override {
        proton::connection conn = cont.connect("amqp://example.com");
        conn.open_sender("jobs");
    }

    void on_sendable(proton::sender& snd) override {
        proton::message msg {"job-1"};
        snd.send(msg);
    }
};
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9.2. Tracking sent messages
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When a message is sent, the sender can keep a reference to the tracker object representing the transfer. The receiver accepts or rejects each message that is delivered. The sender is notified of the outcome for each tracked delivery.

To monitor the outcome of a sent message, override the on_tracker_accept and on_tracker_reject event handlers and map the delivery state update to the tracker returned from send().

Example: Tracking sent messages

void on_sendable(proton::sender& snd) override {
    proton::message msg {"job-1"};
    proton::tracker trk = snd.send(msg);
}

void on_tracker_accept(proton::tracker& trk) override {
    std::cout << "Delivery for " << trk << " is accepted\n";
}

void on_tracker_reject(proton::tracker& trk) override {
    std::cout << "Delivery for " << trk << " is rejected\n";
}
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The tracker object has a tag() method for accessing the unique identifier for each delivery. The delivery tag can be used to store in-flight messages for resending after a connection failure.

9.3. Receiving messages
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To receive messages, create a receiver and override the on_message event handler.

Example: Receiving messages

struct example_handler : public proton::messaging_handler {
    void on_container_start(proton::container& cont) override {
        proton::connection conn = cont.connect("amqp://example.com");
        conn.open_receiver("jobs");
    }

    void on_message(proton::delivery& dlv, proton::message& msg) override {
        std::cout << "Received message '" << msg.body() << "'\n";
    }
};
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The delivery object has a tag() method for accessing the unique identifier for each delivery.

9.4. Acknowledging received messages
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To explicitly accept or reject a delivery, use the delivery::accept() or delivery::reject() methods in the on_message event handler.

Example: Acknowledging received messages

void on_message(proton::delivery& dlv, proton::message& msg) override {
    try {
        process_message(msg);
        dlv.accept();
    } catch (std::exception& e) {
        dlv.reject();
    }
}
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By default, if you do not explicitly acknowledge a delivery, then the library accepts it after on_message returns. To disable this behavior, set the auto_accept receiver option to false.

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