Mysql分布式事务，前提条件

MySQL中只有当隔离级别设置为Serializable的时候才能使用分布式事务。

执行两个命令确认环境

show variables like 'innodb_support_xa';

show variables like '%tx_iso%';

异常信息

根据官方分布式示例

public function test()
    {
        Db::transactionXa(function () {
            $updateTime = time();
            Db::connect('cdpf_driver')->table('cdpf_driver_user_login_his')->where(["id" => 3128])->update(["login_time" => $updateTime]);
            Db::connect('cheduo')->table('users')->where(["id" => 2])->update(["updated_at" => date("Y-m-d H:i:s", $updateTime)]);
        }, [Db::connect('cdpf_driver'),Db::connect('cheduo')]);
}

如果是同一个服务，执行会出现
异常提示:

SQLSTATE[XAE08]: <>: 1440 XAER_DUPID: The XID already exists

解决办法：

方案一：单独编写，不要用thinkphp封装好的

public function test()
    {
        $xid = uniqid('xa');
        $updateTime = time();

        // 第一个数据库连接
        $cdpfDriverXid = "cdpf_driver_{$xid}";
        $cdpfDriverDb = Db::connect('cdpf_driver');
        $cdpfDriverConnect = $cdpfDriverDb->connect();

        // 第二个数据库连接
        $cheduoXid = "cheduo_{$xid}";
        $cheduoDb = Db::connect('cheduo');
        $cheduoConnect = $cheduoDb->connect();
        try {

            // 第一个事务开始
            $cdpfDriverConnect->exec("XA begin '$cdpfDriverXid'");
            $cdpfDriverDb->table('cdpf_driver_user_login_his')->where(["id" => 3128])->update(["login_time" => $updateTime]);
            $cdpfDriverConnect->exec("XA END '$cdpfDriverXid'");
            // 到这里挂起

            // 第二个事务开始
            $cheduoConnect->exec("XA begin '$cheduoXid'");
            $cheduoDb->table('users')->where(["id" => 2])->update(["updated_at" => date("Y-m-d H:i:s", $updateTime)]);
            $cheduoConnect->exec("XA END '$cheduoXid'");
            // 到这里挂起

            // 预执行与提交
            $cdpfDriverConnect->exec("XA PREPARE '$cdpfDriverXid'");
            $cdpfDriverConnect->exec("XA COMMIT '$cdpfDriverXid'");

            $cheduoConnect->exec("XA PREPARE '$cheduoXid'");
            $cheduoConnect->exec("XA COMMIT '$cheduoXid'");
        } catch (\Exception $ex) {
            echo $ex->getMessage();

            // 注意：其实如果语句本身存在问题，并没有走到 XA END。这里回滚会抛出异常
            $cdpfDriverConnect->exec("XA ROLLBACK '$cdpfDriverXid'");
            $cheduoConnect->exec("XA ROLLBACK '$cheduoXid'");
        }
        return "success";
    }

方案二，改动源码

namespace think\db;

/**
 * 数据库连接基础类
 * @property PDO[] $links
 * @property PDO   $linkID
 * @property PDO   $linkRead
 * @property PDO   $linkWrite
 */
abstract class PDOConnection extends Connection
{
	/**
     * 执行数据库Xa事务
     * @access public
     * @param  callable $callback 数据操作方法回调
     * @param  array    $dbs      多个查询对象或者连接对象
     * @return mixed
     * @throws PDOException
     * @throws \Exception
     * @throws \Throwable
     */
    public function transactionXa(callable $callback, array $dbs = [])
    {
        $xid = uniqid('xa');

        if (empty($dbs)) {
            $dbs[] = $this;
        }

        foreach ($dbs as $key => $db) {
            if ($db instanceof BaseQuery) {
                $db = $db->getConnection();

                $dbs[$key] = $db;
            }

            $db->startTransXa($db->getConfig('hostname').'_'.$db->getConfig('database').'_'.$xid);
        }

        try {
            $result = null;
            if (is_callable($callback)) {
                $result = $callback($this);
            }

            foreach ($dbs as $db) {
                $db->prepareXa($db->getConfig('hostname').'_'.$db->getConfig('database').'_'.$xid);
            }

            foreach ($dbs as $db) {
                $db->commitXa($db->getConfig('hostname').'_'.$db->getConfig('database').'_'.$xid);
            }

            return $result;
        } catch (\Exception | \Throwable $e) {
            foreach ($dbs as $db) {
                $db->rollbackXa($db->getConfig('hostname').'_'.$db->getConfig('database').'_'.$xid);
            }
            throw $e;
        }
    }
}

下面来分析原因：
其实从错误提醒这里很好得知
分布式唯一ID已经存在了

跟踪源码

namespace think\db;

/**
 * 数据库连接基础类
 * @property PDO[] $links
 * @property PDO   $linkID
 * @property PDO   $linkRead
 * @property PDO   $linkWrite
 */
abstract class PDOConnection extends Connection
{
	/**
     * 执行数据库Xa事务
     * @access public
     * @param  callable $callback 数据操作方法回调
     * @param  array    $dbs      多个查询对象或者连接对象
     * @return mixed
     * @throws PDOException
     * @throws \Exception
     * @throws \Throwable
     */
    public function transactionXa(callable $callback, array $dbs = [])
    {
        $xid = uniqid('xa');

        if (empty($dbs)) {
            $dbs[] = $this;
        }

        foreach ($dbs as $key => $db) {
            if ($db instanceof BaseQuery) {
                $db = $db->getConnection();

                $dbs[$key] = $db;
            }
			
			// 问题就在这里，同一个数据库服务器，一个事务ID还没执行完毕
			// 又循环生成同样的事务ID，导致异常
            $db->startTransXa($xid);
        }

        try {
            $result = null;
            if (is_callable($callback)) {
                $result = $callback($this);
            }

            foreach ($dbs as $db) {
                $db->prepareXa($xid);
            }

            foreach ($dbs as $db) {
                $db->commitXa($xid);
            }

            return $result;
        } catch (\Exception | \Throwable $e) {
            foreach ($dbs as $db) {
                $db->rollbackXa($xid);
            }
            throw $e;
        }
    }

}