org.postgresql.util.PSQLException:错误:由于事务之间的读/写依赖性,无法序列化访问

Question

更新:我最终设法在最小的设置中重现这个,我作为一个单独的问题发布.

从同一个PostgreSQL实例和表并行运行的两个不同应用程序进行JDBC插入时,我遇到了以下异常:

 org.postgresql.util.PSQLException: ERROR: could not serialize access due to read/write dependencies among transactions
 [java] ERROR>  Detail: Reason code: Canceled on identification as a pivot, during write.
 [java] ERROR>  Hint: The transaction might succeed if retried.

尝试执行以下语句时发生异常:

public int logRepositoryOperationStart(String repoIvoid, MetadataPrefix prefix, RepositoryOperation operation, int pid, String command, String from_XMLGregCal) throws SQLException {
    Connection        conn = null;
    PreparedStatement ps   = null;
    try {
        conn = getConnection();
        conn.commit();
        String SQL = "INSERT INTO vo_business.repositoryoperation(ivoid, metadataprefix, operation, i, pid, command, from_xmlgregcal, start_sse)  "+
                     "(SELECT ?, ?, ?, COALESCE(MAX(i)+1,0), ?, ?, ?, ? FROM vo_business.repositoryoperation                                      "+
                     "WHERE ivoid=? AND metadataprefix=? AND operation=?)                                                                         ";
        ps = conn.prepareStatement(SQL);
        ps.setString(1, repoIvoid);
        ps.setString(2, prefix.value());
        ps.setString(3, operation.value());
        ps.setInt   (4, pid);
        ps.setString(5, command);
        ps.setString(6, from_XMLGregCal);
        ps.setInt   (7, Util.castToIntWithChecks(TimeUnit.SECONDS.convert(System.currentTimeMillis(), TimeUnit.MILLISECONDS)));
        ps.setString(8, repoIvoid);
        ps.setString(9, prefix.value());
        ps.setString(10, operation.value());
        if (ps.executeUpdate() != 1) { // line 217
            conn.rollback();
            throw new RuntimeException();
        }
        conn.commit();
        return getMaxI(conn, repoIvoid, prefix, operation);
    } catch (SQLException e) {
        conn.rollback();
        throw e;
    } finally {
        DbUtils.closeQuietly(conn, ps, (ResultSet) null);
    }

}

..在line-217上面标有线.我在最后提供了实际的堆栈跟踪.

Connection conn对象的事务隔离级别设置为SERIALIZABLE执行getConnection():

protected Connection getConnection() throws SQLException {
    Connection conn = ds.getConnection();
    conn.setAutoCommit(false);
    conn.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
    return conn;
}

很可能另一个应用程序也试图同时在同一个表上写,虽然它确实提供了一个不同的operation字段,所以我看不出有任何混淆可能发生.而且,这是一个单一的原子插入,所以我看不到访问序列化是如何发挥作用的.

这是什么样的错误,我应该如何解决此问题？我应该查看事务隔离级别,整个表与行特定的锁(如果PostgreSQL中有这样的概念),等等？我应该重试(提示说"如果重试,交易可能会成功.").我将尝试在SSCCE中重现它,但我只是发布它,以防它有明显的原因/解决方案

 [java] ERROR>org.postgresql.util.PSQLException: ERROR: could not serialize access due to read/write dependencies among transactions
 [java] ERROR>  Detail: Reason code: Canceled on identification as a pivot, during write.
 [java] ERROR>  Hint: The transaction might succeed if retried.
 [java] ERROR>  at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2102)
 [java] ERROR>  at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1835)
 [java] ERROR>  at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:257)
 [java] ERROR>  at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:500)
 [java] ERROR>  at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:388)
 [java] ERROR>  at org.postgresql.jdbc2.AbstractJdbc2Statement.executeUpdate(AbstractJdbc2Statement.java:334)
 [java] ERROR>  at org.apache.commons.dbcp.DelegatingPreparedStatement.executeUpdate(DelegatingPreparedStatement.java:105)
 [java] ERROR>  at org.apache.commons.dbcp.DelegatingPreparedStatement.executeUpdate(DelegatingPreparedStatement.java:105)
 [java] ERROR>  at _int.esa.esavo.dbbusiness.DBBusiness.logRepositoryOperationStart(DBBusiness.java:217)
 [java] ERROR>  at _int.esa.esavo.harvesting.H.main(H.java:278)

Answer 1

每当您请求SERIALIZABLE隔离时,DB将尝试使并发查询集看起来按照它们生成的结果进行串行执行.这并不总是可行的,例如当两个交易具有相互依赖性时.在这种情况下,PostgreSQL将中止其中一个带有序列化失败错误的事务,告诉您应该重试它.

使用的代码SERIALIZABLE必须始终准备好重新尝试事务.它必须检查SQLSTATE和,对于序列化失败,重复该事务.

请参阅事务隔离文档.

在这种情况下,我认为你的主要误解可能是:

这是一个单一的原子插入

因为它不是那种,它是一种INSERT ... SELECT触及vo_business.repositoryoperation阅读和写作的东西.这足以与另一个执行相同操作的事务创建潜在依赖关系,或者以另一种方式读取和写入表.

另外,出于效率原因,可序列化隔离代码在某些情况下可以去生成保持块级依赖性信息.因此,它可能不一定是触及相同行的事务,只是相同的存储块,尤其是在负载下.

如果不确定它是否安全,PostgreSQL更愿意中止可序列化的事务.证明系统有局限性.所以你也有可能找到一个愚弄它的案例.

要确切地知道我需要并排查看这两笔交易,但这里有一个证明insert ... select可以与自己发生冲突的证明.打开三个psql会话并运行:

session0: CREATE TABLE serialdemo(x integer, y integer);

session0: BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;

session0: LOCK TABLE serialdemo IN ACCESS EXCLUSIVE MODE;

session1: BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;

session2: BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;

session1: INSERT INTO serialdemo (x, y)
          SELECT 1, 2
          WHERE NOT EXISTS (SELECT 1 FROM serialdemo WHERE x = 1);

session2: INSERT INTO serialdemo (x, y)
          SELECT 1, 2
          WHERE NOT EXISTS (SELECT 1 FROM serialdemo WHERE x = 1);

session0: ROLLBACK;

session1: COMMIT;

session2: COMMIT;

session1将提交正常.session2将失败:

ERROR:  could not serialize access due to read/write dependencies among transactions
DETAIL:  Reason code: Canceled on identification as a pivot, during commit attempt.
HINT:  The transaction might succeed if retried.

它与您的情况不同,序列化失败并不能证明您的语句可能相互冲突,但它表明a insert ... select不像您想象的那样原子.