死信队列：绑定了死信交换机的队列，处理变成死信的消息。死信队列也是和正常队列没有区别，只是在声明的时候，配置 x-dead-letter-exchange参数。

3.1 当消息被拒绝

当消息被拒绝时，消息进入死信队列，配置流程如下：

1.声明业务队列时，配置死信交换机和 Routing Key。

public function index()
{
    $connect = new AMQPStreamConnection('127.0.0.1', 5672, 'admin', 'admin2021');
    $channel = $connect->channel();
    $channel->exchange_declare('normal_exchange', AMQPExchangeType::DIRECT, false, true, false);

    // 声明业务队列的死信交换机
    $argument = new AMQPTable([
        'x-dead-letter-exchange'    => 'dl_exchange',        // 配置死信交换机
        'x-dead-letter-routing-key' => 'dl_route_key',       // 配置 Routing Key，路由到 dl_exchange
    ]);
    $channel->queue_declare('normal_queue', false, false, false, false, false, $argument);

    // 业务队列“normal_queue”绑定到业务交换机“normal_exchange”，Routing Key 为“normal_route_key”
    $channel->queue_bind('normal_queue', 'normal_exchange', 'normal_route_key');

    // 消费消息时，拒绝消息
    $callback = function ($msg) {
        $msg->delivery_info['channel']->basic_reject($msg->delivery_info['delivery_tag'], false);
    };
    $channel->basic_consume('normal_queue', '', false, false, false, false, $callback);

    print_r("consuming...\n");
    while ($channel->is_consuming()) {
        $channel->wait();
    }
    $channel->close();
    $connect->close();
}

2.为死信交换机配置死信队列。

public function index()
{
    $connect = new AMQPStreamConnection('127.0.0.1', 5672, 'admin', 'admin2021');
    $channel = $connect->channel();

    // 声明死信交换机
    $channel->exchange_declare('dl_exchange', AMQPExchangeType::DIRECT, false, true, false);

    // 声明死信队列
    $channel->queue_declare('dl_queue', false, false, false, false);

    // 死信队列绑定到死信交换机，Routing Key 为“dl_route_key”
    $channel->queue_bind('dl_queue', 'dl_exchange', 'dl_route_key');

    // 消费消息
    $callback = function ($msg) {
        print_r(">receive the message:".$msg->body."\n");
        $msg->delivery_info['channel']->basic_ack($msg->delivery_info['delivery_tag']);
    };
    $channel->basic_consume('dl_queue', '', false, false, false, false, $callback);

    print_r("consuming...\n");
    while ($channel->is_consuming()) {
        $channel->wait();
    }
    $channel->close();
    $connect->close();
}

当消息在业务队列normal_queue 的消息被拒绝时，它会按照x-dead-letter-exchange和x-dead-letter-routing-key重新将消息 publish 到死信交换机dl_exchange。此时，绑定了死信交换机的死信队列dl_queue就能订阅到消息。

3.2 当消息过期时

当消息过期时，消息进入死信队列，利用这一特性可以实现延迟队列，代码如下。

1.声明业务队列时，配置消息过期时间、死信交换机：

public function index()
{
    $connect = new AMQPStreamConnection('127.0.0.1', 5672, 'admin', 'admin2021');
    $channel = $connect->channel();
    $channel->exchange_declare('normal_exchange', AMQPExchangeType::DIRECT, false, true, false);

    // 声明队列
    $argument = new AMQPTable([
        'x-message-ttl'             => 10000,                // 10s,设置消息存活时间（TTL，Time To Live）
        'x-dead-letter-exchange'    => 'dl_exchange',        // 配置死信交换机
        'x-dead-letter-routing-key' => 'dl_route_key',       // 配置 Routing Key，路由到 dl_exchange
    ]);
    $channel->queue_declare('normal_queue', false, false, false, false, false, $argument);
    $channel->queue_bind('normal_queue', 'normal_exchange', 'normal_route_key');

    // 一次拉取一条消息
    $channel->basic_qos(0, 1, false);

    // 消费消息
    $callback = function ($msg) {
        sleep(5);  // 延时，所以处理两条消息后，后面的消息都已经过期
        $msg->delivery_info['channel']->basic_ack($msg->delivery_info['delivery_tag']);
    };
    $channel->basic_consume('normal_queue', '', false, false, false, false, $callback);

    print_r("consuming...\n");
    while ($channel->is_consuming()) {
        $channel->wait();
    }
    $channel->close();
    $connect->close();
}

2.为死信交换机配置死信队列。（同上）

3.3 当队列达到最大长度

当队列达到最大长度限制时，消息进入死信队列。

1.声明业务队列时，配置队列长度限制、死信交换机：

public function index()
{
    $connect = new AMQPStreamConnection('127.0.0.1', 5672, 'admin', 'admin2021');
    $channel = $connect->channel();
    $channel->exchange_declare('normal_exchange', AMQPExchangeType::DIRECT, false, true, false);

    $argument = new AMQPTable([
        'x-max-length'              => 5,
        'x-dead-letter-exchange'    => 'dl_exchange',        // 配置死信交换机
        'x-dead-letter-routing-key' => 'dl_route_key',       // 配置 Routing Key，路由到 dl_exchange
        'x-overflow'                => 'reject-publish-dlx'
    ]);
    $channel->queue_declare('normal_queue', false, false, false, false, false, $argument);
    $channel->queue_bind('normal_queue', 'normal_exchange', 'normal_route_key');

    // 消费消息
    $callback = function ($msg) {
        print_r(">receive the message:".$msg->body."\n");
        $msg->delivery_info['channel']->basic_ack($msg->delivery_info['delivery_tag']);
    };
    $channel->basic_consume('normal_queue', '', false, false, false, false, $callback);

    print_r("consuming...\n");
    while ($channel->is_consuming()) {
        $channel->wait();
    }
    $channel->close();
    $connect->close();
}

2.为死信交换机配置死信队列。（同上）

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