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Manage Table Schemas of Tables to be Migrated

This document describes how to manage the schema of the table in DM during migration using dmctl.

Implementation principles

When you migrate tables using DM, DM performs the following operations on the table schema:

  • For full export and import, DM directly exports the upstream table schema of the current time to SQL files and applies the table schema to the downstream.

  • For incremental replication, the whole data link contains the following table schemas, which might be the same or different:

    schema

    • The upstream table schema at the current time, identified as schema-U.

    • The table schema of the binlog event currently being consumed by DM, identified as schema-B. This schema corresponds to the upstream table schema at a historical time.

    • The table schema currently maintained in DM (the schema tracker component), identified as schema-I.

    • The table schema in the downstream TiDB cluster, identified as schema-D.

      In most cases, the four table schemas above are the same.

When the upstream database performs a DDL operation to change the table schema, schema-U is changed. By applying the DDL operation to the internal schema tracker component and the downstream TiDB cluster, DM updates schema-I and schema-D in an orderly manner to keep them consistent with schema-U. Therefore, DM can then normally consume the binlog event corresponding to the schema-B table schema. That is, after the DDL operation is successfully migrated, schema-U, schema-B, schema-I, and schema-D are still consistent.

However, during the migration with optimistic mode sharding DDL support enabled, the schema-D of the downstream table might be inconsistent with the schema-B and schema-I of some upstream sharded tables. In such cases, DM still keeps schema-I and schema-B consistent to ensure that the binlog event corresponding to DML can be parsed normally.

In addition, in some scenarios (such as when the downstream table has more columns than the upstream table), schema-D might be inconsistent with schema-B and schema-I.

To support the scenarios mentioned above and handle other migration interruptions caused by schema inconsistency, DM provides the binlog-schema command to obtain, modify, and delete the schema-I table schema maintained in DM.

Note

The binlog-schema command is supported only in DM v6.0 or later versions. For earlier versions, you must use the operate-schema command.

Command

help binlog-schema
manage or show table schema in schema tracker

Usage:
  dmctl binlog-schema [command]

Available Commands:
  delete      delete table schema structure
  list        show table schema structure
  update      update tables schema structure

Flags:
  -h, --help   help for binlog-schema

Global Flags:
  -s, --source strings   MySQL Source ID.

Use "dmctl binlog-schema [command] --help" for more information about a command.
Note
  • Because a table schema might change during data migration, to obtain a predictable table schema, currently the binlog-schema command can be used only when the data migration task is in the Paused state.
  • To avoid data loss due to mishandling, it is strongly recommended to get and backup the table schema firstly before you modify the schema.

Parameters

  • delete: Deletes the table schema.
  • list: Lists the table schema.
  • update: Updates the table schema.
  • -s or --source:
    • Required.
    • Specifies the MySQL source that the operation is applied to.

Usage example

Get the table schema

To get the table schema, run the binlog-schema list command:

help binlog-schema list
show table schema structure

Usage:
  dmctl binlog-schema list <task-name> <database> <table> [flags]

Flags:
  -h, --help   help for list

Global Flags:
  -s, --source strings   MySQL Source ID.

If you want to get the table schema of the `db_single`.`t1` table corresponding to the mysql-replica-01 MySQL source in the db_single task, run the following command:

binlog-schema list -s mysql-replica-01 task_single db_single t1
{
    "result": true,
    "msg": "",
    "sources": [
        {
            "result": true,
            "msg": "CREATE TABLE `t1` ( `c1` int(11) NOT NULL, `c2` int(11) DEFAULT NULL, PRIMARY KEY (`c1`)) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_bin",
            "source": "mysql-replica-01",
            "worker": "127.0.0.1:8262"
        }
    ]
}

Update the table schema

To update the table schema, run the binlog-schema update command:

help binlog-schema update
update tables schema structure

Usage:
  dmctl binlog-schema update <task-name> <database> <table> [schema-file] [flags]

Flags:
      --flush         flush the table info and checkpoint immediately (default true)
      --from-source   use the schema from upstream database as the schema of the specified tables
      --from-target   use the schema from downstream database as the schema of the specified tables
  -h, --help          help for update
      --sync          sync the table info to master to resolve shard ddl lock, only for optimistic mode now (default true)

Global Flags:
  -s, --source strings   MySQL Source ID.

If you want to set the table schema of the `db_single`.`t1` table corresponding to the mysql-replica-01 MySQL source in the db_single task as follows:

CREATE TABLE `t1` (
    `c1` int(11) NOT NULL,
    `c2` bigint(11) DEFAULT NULL,
    PRIMARY KEY (`c1`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_bin

Save the CREATE TABLE statement above as a file (for example, db_single.t1-schema.sql), and run the following command:

operate-schema set -s mysql-replica-01 task_single -d db_single -t t1 db_single.t1-schema.sql
{
    "result": true,
    "msg": "",
    "sources": [
        {
            "result": true,
            "msg": "",
            "source": "mysql-replica-01",
            "worker": "127.0.0.1:8262"
        }
    ]
}

Delete the table schema

To delete the table schema, run the binlog-schema delete command:

help binlog-schema delete
delete table schema structure

Usage:
  dmctl binlog-schema delete <task-name> <database> <table> [flags]

Flags:
  -h, --help   help for delete

Global Flags:
  -s, --source strings   MySQL Source ID.
Note

After the table schema maintained in DM is deleted, if a DDL/DML statement related to this table needs to be migrated to the downstream, DM will try to get the table schema from the following three sources in an orderly manner:

  • The table_info field in the checkpoint table
  • The meta information in the optimistic sharding DDL
  • The corresponding table in the downstream TiDB

If you want to delete the table schema of the `db_single`.`t1` table corresponding to the mysql-replica-01 MySQL source in the db_single task, run the following command:

binlog-schema delete -s mysql-replica-01 task_single db_single t1
{
    "result": true,
    "msg": "",
    "sources": [
        {
            "result": true,
            "msg": "",
            "source": "mysql-replica-01",
            "worker": "127.0.0.1:8262"
        }
    ]
}
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