src/Db/Transaction.php
public function setIsolationLevel($level)
    {
        if (!$this->getIsActive()) {
            throw new Exception('Failed to set isolation level: transaction was inactive.');
        }
        Reaction::debug('Setting transaction isolation level to ' . $level);
        return $this->getConnection()->setTransactionIsolationLevel($level);
    }
public function setIsolationLevel($level)
    {
        if (!$this->getIsActive()) {
            throw new Exception('Failed to set isolation level: transaction was inactive.');
        }
        Reaction::debug('Setting transaction isolation level to ' . $level);
        return $this->getConnection()->setTransactionIsolationLevel($level);
    }
Sets the transaction isolation level for this transaction.

This method can be used to set the isolation level while the transaction is already active.
However this is not supported by all DBMS so you might rather specify the isolation level directly
when calling [[begin()]].
@param string $level The transaction isolation level to use for this transaction.
This can be one of [[READ_UNCOMMITTED]], [[READ_COMMITTED]], [[REPEATABLE_READ]] and [[SERIALIZABLE]] but
also a string containing DBMS specific syntax to be used after `SET TRANSACTION ISOLATION LEVEL`.
@return ExtendedPromiseInterface|LazyPromiseInterface
@throws Exception if the transaction is not active
@see http://en.wikipedia.org/wiki/Isolation_%28database_systems%29#Isolation_levels