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Bidirectional Replication
Learn how to use bidirectional replication of TiCDC.

Bidirectional Replication

Starting from v6.5.0, TiCDC supports bi-directional replication among two TiDB clusters. Based on this feature, you can create a multi-active TiDB solution using TiCDC.

This section describes how to use bi-directional replication taking two TiDB clusters as an example.

Deploy bi-directional replication

TiCDC only replicates incremental data changes that occur after a specified timestamp to the downstream cluster. Before starting the bi-directional replication, you need to take the following steps:

  1. (Optional) According to your needs, import the data of the two TiDB clusters into each other using the data export tool Dumpling and data import tool TiDB Lightning.

  2. Deploy two TiCDC clusters between the two TiDB clusters. The cluster topology is as follows. The arrows in the diagram indicate the directions of data flow.

    TiCDC bidirectional replication

  3. Specify the starting time point of data replication for the upstream and downstream clusters.

    1. Check the time point of the upstream and downstream clusters. In the case of two TiDB clusters, make sure that data in the two clusters are consistent at certain time points. For example, the data of TiDB A at ts=1 and the data of TiDB B at ts=2 are consistent.

    2. When you create the changefeed, set the --start-ts of the changefeed for the upstream cluster to the corresponding tso. That is, if the upstream cluster is TiDB A, set --start-ts=1; if the upstream cluster is TiDB B, set --start-ts=2.

  4. In the configuration file specified by the --config parameter, add the following configuration:

    # Whether to enable the bi-directional replication mode
bdr-mode = true

  5. (Optional) If you need to track the data source, set a unique data source ID for each cluster using the tidb_source_id system variable.

After the configuration takes effect, the clusters can perform bi-directional replication.

Execute DDL

Bi-directional replication does not support replicating DDL statements.

If you need to execute DDL statements, take the following steps:

  1. Pause the write operations in the tables that need to execute DDL in all clusters. If the DDL statement is adding a non-unique index, skip this step.
  2. After the write operations of the corresponding tables in all clusters have been replicated to other clusters, manually execute all DDL statements in each TiDB cluster.
  3. After the DDL statements are executed, resume the write operations.

Note that a DDL statement that adds non-unique index does not break bi-directional replication, so you do not need to pause the write operations in the corresponding table.

Stop bi-directional replication

After the application has stopped writing data, you can insert a special record into each cluster. By checking the two special records, you can make sure that data in two clusters are consistent.

After the check is completed, you can stop the changefeed to stop bi-directional replication.

Limitations

  • For the limitations of DDL, see Execute DDL.

  • Bi-directional replication clusters cannot detect write conflicts, which might cause undefined behaviors. Therefore, you must ensure that there are no write conflicts from the application side.

  • Bi-directional replication supports more than two clusters, but does not support multiple clusters in cascading mode, that is, a cyclic replication like TiDB A -> TiDB B -> TiDB C -> TiDB A. In such a topology, if one cluster fails, the whole data replication will be affected. Therefore, to enable bi-directional replication among multiple clusters, you need to connect each cluster with every other clusters, for example, TiDB A <-> TiDB B, TiDB B <-> TiDB C, TiDB C <-> TiDB A.