# 死信队列 死信交换机（Dead-Letter-Exchange），当消息在一个队列中变成死信之后，它能被发送到另一个交换机中，这个交换机就是DLX，绑定DLX的队列就称之为死信队列 消息变成死信一般是由于下面三种情况： * 消息被拒绝（Basic.Reject或Basic.Nack），并在调用时设置requeue参数为false * 消息过期 * 队列达到最大长度通过在channel.queueDeclare方法中设置x-dead-letter-exchange参数来为这个队列添加DLX ``` $connection = new AMQPStreamConnection('127.0.0.1', 5672, 'why', 'why'); $channel = $connection->channel(); //测试死信的上游交换机 $channel->exchange_declare('dlxBeforeExchange', 'direct', false, true, false, false, false, []); //死信的交换机 $channel->exchange_declare('dlxExchange', 'direct', false, true, false, false, false, []); //测试死信队列的上游队列 $dlx_table = new \PhpAmqpLib\Wire\AMQPTable(); $dlx_table->set('x-dead-letter-exchange', 'dlxExchange' ); $dlx_table->set('x-dead-letter-routing-key', 'dlxKey' ); $channel->queue_declare('dlxBeforeQueue', false, true, false, false, false, $dlx_table); $channel->queue_bind('dlxBeforeQueue', 'dlxBeforeExchange', 'dlxBeforeKey'); //死信队列 $channel->queue_declare('dlxQueue', false, true, false, false, false); $channel->queue_bind('dlxQueue', 'dlxExchange', 'dlxKey'); $head = array_merge(array('content_type' => 'text/plain', 'delivery_mode' => AMQPMessage::DELIVERY_MODE_PERSISTENT), ['expiration' => '60000']); $msg = new AMQPMessage('hello why', $head); $res = $channel->basic_publish($msg, 'dlxBeforeExchange', 'dlxBeforeKey'); ```