获取最近的全局检查点（GCP）时间。也就是说，你需要确定副本集群中 ndb_apply_status 表中的最新纪元，可以使用以下查询找到： Obtain the time of the most recent global checkpoint (GCP). That is, you need to determine the most recent epoch from the ndb_apply_status table on the replica cluster, which can be found using the following query:

mysqlR'> SELECT @latest:=MAX(epoch)
      ->        FROM mysql.ndb_apply_status;

在循环复制拓扑中，每个主机上运行一个源服务器和一个副本，当你使用 ndb_log_apply_status=1 时，NDB Cluster 的时期会写入副本的二进制日志中。这意味着 ndb_apply_status 表包含了该主机上的副本信息，以及作为该主机上运行的复制源服务器的副本的任何其他主机的信息。In a circular replication topology, with a source and a replica running on each host, when you are using ndb_log_apply_status=1, NDB Cluster epochs are written in the replicas' binary logs. This means that the ndb_apply_status table contains information for the replica on this host as well as for any other host which acts as a replica of the replication source server running on this host.

在这种情况下，您需要确定此副本上的最新纪元，而不考虑此副本二进制日志中任何其他副本的纪元，这些纪元未在用于设置此副本的 CHANGE REPLICATION SOURCE TO 语句的 IGNORE_SERVER_IDS 选项中列出。排除这些纪元的原因是，mysql.ndb_apply_status 表中其服务器 ID 与用于准备该副本源的 IGNORE_SERVER_IDS 语句中的 CHANGE REPLICATION SOURCE TO 列表匹配的行，也被认为来自本地服务器，此外还包括具有副本自身服务器 ID 的行。您可以通过 SHOW REPLICA STATUS 的输出将此列表检索为 Replicate_Ignore_Server_Ids。我们假设您已获取此列表，并将其替换为此处查询中的 ignore_server_ids，如同前一个查询版本一样，该查询将最大的纪元选择到名为 @latest 的变量中：In this case, you need to determine the latest epoch on this replica to the exclusion of any epochs from any other replicas in this replica's binary log that were not listed in the IGNORE_SERVER_IDS options of the CHANGE REPLICATION SOURCE TO statement used to set up this replica. The reason for excluding such epochs is that rows in the mysql.ndb_apply_status table whose server IDs have a match in the IGNORE_SERVER_IDS list from the CHANGE REPLICATION SOURCE TO statement used to prepare this replicas's source are also considered to be from local servers, in addition to those having the replica's own server ID. You can retrieve this list as Replicate_Ignore_Server_Ids from the output of SHOW REPLICA STATUS. We assume that you have obtained this list and are substituting it for ignore_server_ids in the query shown here, which like the previous version of the query, selects the greatest epoch into a variable named @latest:

mysqlR'> SELECT @latest:=MAX(epoch)
      ->        FROM mysql.ndb_apply_status
      ->        WHERE server_id NOT IN (ignore_server_ids);

在某些情况下，使用要包含的服务器 ID 列表以及在前一个查询的 WHERE 条件中使用 server_id IN server_id_list 可能更简单或更高效（或两者兼而有之）。In some cases, it may be simpler or more efficient (or both) to use a list of the server IDs to be included and server_id IN server_id_list in the WHERE condition of the preceding query.

使用第 1 步查询中获取的信息，从源集群的 ndb_binlog_index 表中获取相应的记录。 Using the information obtained from the query shown in Step 1, obtain the corresponding records from the ndb_binlog_index table on the source cluster.

您可以使用以下查询从源的 ndb_binlog_index 表中获取所需记录：You can use the following query to obtain the needed records from the ndb_binlog_index table on the source:

mysqlS'> SELECT
    ->     @file:=SUBSTRING_INDEX(next_file, '/', -1),
    ->     @pos:=next_position
    -> FROM mysql.ndb_binlog_index
    -> WHERE epoch = @latest;

这些是自主复制通道失败以来在源端保存的记录。我们在这里使用了一个用户变量@latest来表示步骤 1 中获取的值。当然，一个mysqld实例无法直接访问在另一个服务器实例上设置的用户变量。这些值必须手动或通过应用程序“插入”到第二个查询中。These are the records saved on the source since the failure of the primary replication channel. We have employed a user variable @latest here to represent the value obtained in Step 1. Of course, it is not possible for one mysqld instance to access user variables set on another server instance directly. These values must be “plugged in” to the second query manually or by an application.

现在可以通过在次级副本服务器上运行以下查询来同步次级通道：Now it is possible to synchronize the secondary channel by running the following query on the secondary replica server:

mysqlR'> CHANGE REPLICATION SOURCE TO
      ->     SOURCE_LOG_FILE='@file',
      ->     SOURCE_LOG_POS=@pos;

在这里我们再次使用了用户变量（在本例中为 @file 和 @pos）来表示在步骤 2 中获得并在步骤 3 中应用的值；在实际操作中，这些值必须手动插入，或使用能够访问两个服务器的应用程序来插入。Again we have employed user variables (in this case @file and @pos) to represent the values obtained in Step 2 and applied in Step 3; in practice these values must be inserted manually or using an application that can access both of the servers involved.

现在，您可以通过在辅助副本 mysqld 上发出相应命令来启动辅助通道的复制：You can now initiate replication on the secondary channel by issuing the appropriate command on the secondary replica mysqld:

mysqlR'> START REPLICA;