Changefeed DDL Replication
This document describes the rules and special cases of DDL replication in TiCDC.
DDL allow list
Currently, TiCDC uses an allow list to determine whether to replicate a DDL statement. Only the DDL statements in the allow list are replicated to the downstream. The DDL statements not in the allow list are not replicated.
In addition, TiCDC determines whether to replicate a DDL statement to the downstream based on whether the table has a valid index and whether the configuration item force-replicate is set to true. When force-replicate=true, the replication task attempts to forcibly replicate tables without a valid index.
The following is the allow list of DDL statements supported by TiCDC. The abbreviations in the table:
- Y: Replication to the downstream is supported in this condition.
- N: Replication to the downstream is not supported in this condition.
| DDL | A valid index exists | A valid index does not exist and force-replicate is false (default) | A valid index does not exist and force-replicate is set to true |
|---|---|---|---|
CREATE DATABASE | Y | Y | Y |
DROP DATABASE | Y | Y | Y |
ALTER DATABASE CHARACTER SET | Y | Y | Y |
CREATE INDEX | Y | Y | Y |
ADD INDEX | Y | Y | Y |
DROP INDEX | Y | N | Y |
ADD PRIMARY KEY | Y | Y | Y |
DROP PRIMARY KEY | Y | N | Y |
CREATE TABLE | Y | N | Y |
DROP TABLE | Y | N | Y |
ADD COLUMN | Y | N | Y |
DROP COLUMN | Y | N | Y |
TRUNCATE TABLE | Y | N | Y |
MODIFY COLUMN | Y | N | Y |
RENAME TABLE | Y | N | Y |
ALTER COLUMN DEFAULT VALUE | Y | N | Y |
ALTER TABLE COMMENT | Y | N | Y |
RENAME INDEX | Y | N | Y |
ADD PARTITION | Y | N | Y |
DROP PARTITION | Y | N | Y |
TRUNCATE PARTITION | Y | N | Y |
CREATE VIEW | Y | N | Y |
DROP VIEW | Y | N | Y |
ALTER TABLE CHARACTER SET | Y | N | Y |
RECOVER TABLE | Y | N | Y |
REBASE AUTO ID | Y | N | Y |
ALTER TABLE INDEX VISIBILITY | Y | N | Y |
EXCHANGE PARTITION | Y | N | Y |
REORGANIZE PARTITION | Y | N | Y |
ALTER TABLE TTL | Y | N | Y |
ALTER TABLE REMOVE TTL | Y | N | Y |
DDL replication considerations
Asynchronous execution of ADD INDEX and CREATE INDEX DDLs
When the downstream is TiDB, TiCDC executes ADD INDEX and CREATE INDEX DDL operations asynchronously to minimize the impact on changefeed replication latency. This means that, after replicating ADD INDEX and CREATE INDEX DDLs to the downstream TiDB for execution, TiCDC returns immediately without waiting for the completion of the DDL execution. This avoids blocking subsequent DML executions.
During the execution of the ADD INDEX or CREATE INDEX DDL operation in the downstream, when TiCDC executes the next DDL operation of the same table, this DDL operation might be blocked in the queueing state for a long time. This can cause TiCDC to repeatedly execute this DDL operation, and if retries take too long, it might lead to replication task failure. Starting from v8.4.0, if TiCDC has the SUPER permission of the downstream database, it periodically runs ADMIN SHOW DDL JOBS to check the status of asynchronously executed DDL tasks. TiCDC will wait for index creation to complete before proceeding with replication. Although this might increase replication latency, it avoids replication task failure.
DDL replication considerations for renaming tables
Due to the lack of some context during the replication process, TiCDC has some constraints on the replication of RENAME TABLE DDLs.
Rename a single table in a DDL statement
If a DDL statement renames a single table, TiCDC only replicates the DDL statement when the old table name matches the filter rule. The following is an example.
Assume that the configuration file of your changefeed is as follows:
[filter]
rules = ['test.t*']
TiCDC processes this type of DDL as follows:
| DDL | Whether to replicate | Reason for the handling |
|---|---|---|
RENAME TABLE test.t1 TO test.t2 | Replicate | test.t1 matches the filter rule |
RENAME TABLE test.t1 TO ignore.t1 | Replicate | test.t1 matches the filter rule |
RENAME TABLE ignore.t1 TO ignore.t2 | Ignore | ignore.t1 does not match the filter rule |
RENAME TABLE test.n1 TO test.t1 | Report an error and exit the replication | The old table name test.n1 does not match the filter rule, but the new table name test.t1 matches the filter rule. This operation is illegal. In this case, refer to the error message for handling. |
RENAME TABLE ignore.t1 TO test.t1 | Report an error and exit the replication | Same reason as above. |
Rename multiple tables in a DDL statement
If a DDL statement renames multiple tables, TiCDC replicates the DDL statement only when the old database name, old table names, and new database name all match the filter rule.
In addition, TiCDC does not support the RENAME TABLE DDL that swaps the table names. The following is an example.
Assume that the configuration file of your changefeed is as follows:
[filter]
rules = ['test.t*']
TiCDC processes this type of DDL as follows:
| DDL | Whether to replicate | Reason for the handling |
|---|---|---|
RENAME TABLE test.t1 TO test.t2, test.t3 TO test.t4 | Replicate | All database names and table names match the filter rule. |
RENAME TABLE test.t1 TO test.ignore1, test.t3 TO test.ignore2 | Replicate | The old database name, the old table names, and the new database name match the filter rule. |
RENAME TABLE test.t1 TO ignore.t1, test.t2 TO test.t22; | Report an error | The new database name ignore does not match the filter rule. |
RENAME TABLE test.t1 TO test.t4, test.t3 TO test.t1, test.t4 TO test.t3; | Report an error | The RENAME TABLE DDL swaps the names of test.t1 and test.t3 in one DDL statement, which TiCDC cannot handle correctly. In this case, refer to the error message for handling. |
DDL statement considerations
When executing cross-database DDL statements (such as CREATE TABLE db1.t1 LIKE t2) in the upstream, it is recommended that you explicitly specify all relevant database names in DDL statements (such as CREATE TABLE db1.t1 LIKE db2.t2). Otherwise, cross-database DDL statements might not be executed correctly in the downstream due to the lack of database name information.
Notes on using event filter rules to filter DDL events
If a filtered DDL statement involves table creation or deletion, TiCDC only filters out the DDL statement without affecting the replication behavior of DML statements. The following is an example.
Assume that the configuration file of your changefeed is as follows:
[filter]
rules = ['test.t*']
matcher = ["test.t1"] # This filter rule applies only to the t1 table in the test database.
ignore-event = ["create table", "drop table", "truncate table", "rename table"]
| DDL | DDL behavior | DML behavior | Explanation |
|---|---|---|---|
CREATE TABLE test.t1 (id INT, name VARCHAR(50)); | Ignore | Replicate | test.t1 matches the event filter rule, so the CREATE TABLE event is ignored. The replication of DML events remains unaffected. |
CREATE TABLE test.t2 (id INT, name VARCHAR(50)); | Replicate | Replicate | test.t2 does not match the event filter rule. |
CREATE TABLE test.ignore (id INT, name VARCHAR(50)); | Ignore | Ignore | test.ignore matches the event filter rule, so both DDL and DML events are ignored. |
DROP TABLE test.t1; | Ignore | - | test.t1 matches the event filter rule, so the DROP TABLE event is ignored. Because the table is deleted, TiCDC no longer replicates DML events for t1. |
TRUNCATE TABLE test.t1; | Ignore | Replicate | test.t1 matches the event filter rule, so the TRUNCATE TABLE event is ignored. The replication of DML events remains unaffected. |
RENAME TABLE test.t1 TO test.t2; | Ignore | Replicate | test.t1 matches the event filter rule, so the RENAME TABLE event is ignored. The replication of DML events remains unaffected. |
RENAME TABLE test.t1 TO test.ignore; | Ignore | Ignore | test.t1 matches the event filter rule, so the RENAME TABLE event is ignored. test.ignore matches the event filter rule, so both DDL and DML events are ignored. |