ticdc: add ddl detail to bidirectional replication doc (#13913) (#14029)

ticdc/ticdc-bidirectional-replication.md

@@ -38,16 +38,36 @@ After the configuration takes effect, the clusters can perform bi-directional re
 
 ## 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.
+After the bidirectional replication is enabled, TiCDC does not replicate any DDL statements. You need to execute DDL statements in the upstream and downstream clusters respectively.
+
+Note that some DDL statements might cause table structure changes or data change time sequence problems, which might lead to data inconsistency after the replication. Therefore, after enabling bidirectional replication, only the DDL statements in the following table can be executed without stopping the write operations of the application.
+
+| Event | Does it cause changefeed errors | Note |
+|---|---|---|
+| create database | Yes | After you manually execute the DDL statements in the upstream and downstream clusters, the errors can be automatically recovered. |
+| drop database | Yes | You need to manually restart the changefeed and specify `--overwrite-checkpoint-ts` as the `commitTs` of the DDL statement to recover the errors. |
+| create table | Yes | After you manually execute the DDL statements in the upstream and downstream clusters, the errors can be automatically recovered. |
+| drop table | Yes | You need to manually restart the changefeed and specify `--overwrite-checkpoint-ts` as the `commitTs` of the DDL statement to recover the errors. |
+| alter table comment | No |  |
+| rename index | No |  |
+| alter table index visibility | No |  |
+| add partition | Yes | After you manually execute the DDL statements in the upstream and downstream clusters, the errors can be automatically recovered. |
+| drop partition | No |  |
+| create view | No |  |
+| drop view | No |  |
+| alter column default value | No |  |
+| reorganize partition | Yes | After you manually execute the DDL statements in the upstream and downstream clusters, the errors can be automatically recovered. |
+| alter table ttl | No |  |
+| alter table remove ttl | No |  |
+| add **not unique** index | No |  |
+| drop **not unique** index | No |  |
+
+If you need to execute DDL statements that are not in the preceding table, take the following steps:
+
+1. Pause the write operations in the tables that need to execute DDL in all clusters.
 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.

ticdc/ticdc-filter.md

@@ -87,3 +87,40 @@ Description of configuration parameters:
 >
 > - When TiDB updates a value in the column of the clustered index, TiDB splits an `UPDATE` event into a `DELETE` event and an `INSERT` event. TiCDC does not identify such events as an `UPDATE` event and thus cannot correctly filter out such events.
 > - When you configure a SQL expression, make sure all tables that matches `matcher` contain all the columns specified in the SQL expression. Otherwise, the replication task cannot be created. In addition, if the table schema changes during the replication, which results in a table no longer containing a required column, the replication task fails and cannot be resumed automatically. In such a situation, you must manually modify the configuration and resume the task.
+
+## DDL allow list
+
+Currently, TiCDC uses an allow list to replicate DDL statements. Only the DDL statements in the allow list are replicated to the downstream. The DDL statements not in the allow list are not replicated to the downstream.
+
+The allow list of DDL statements supported by TiCDC is as follows:
+
+- create database
+- drop database
+- create table
+- drop table
+- add column
+- drop column
+- create index / add index
+- drop index
+- truncate table
+- modify column
+- rename table
+- alter column default value
+- alter table comment
+- rename index
+- add partition
+- drop partition
+- truncate partition
+- create view
+- drop view
+- alter table character set
+- alter database character set
+- recover table
+- add primary key
+- drop primary key
+- rebase auto id
+- alter table index visibility
+- exchange partition
+- reorganize partition
+- alter table ttl
+- alter table remove ttl