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Data Replication Features
Learn about the data replication features provided by the Data Migration tool.
reference
/docs/tools/dm/data-synchronization-features/

Data Replication Features

This document describes the data replication features provided by the Data Migration tool and explains the configuration of corresponding parameters.

Table routing

The table routing feature enables DM to replicate a certain table of the upstream MySQL or MariaDB instance to the specified table in the downstream.

Note:

  • Configuring multiple different routing rules for a single table is not supported.
  • The match rule of schema needs to be configured separately, which is used to replicate create/drop schema xx, as shown in rule-2 of the parameter configuration.

Parameter configuration

routes:
  rule-1:
    schema-pattern: "test_*"
    table-pattern: "t_*"
    target-schema: "test"
    target-table: "t"
  rule-2:
    schema-pattern: "test_*"
    target-schema: "test"

Parameter explanation

DM replicates the upstream MySQL or MariaDB instance table that matches the schema-pattern/table-pattern rule provided by Table selector to the downstream target-schema/target-table.

Usage examples

This sections shows the usage examples in different scenarios.

Merge sharded schemas and tables

Assuming in the scenario of sharded schemas and tables, you want to replicate the test_{1,2,3...}.t_{1,2,3...} tables in two upstream MySQL instances to the test.t table in the downstream TiDB instance.

To replicate the upstream instances to the downstream test.t, you must create two routing rules:

  • rule-1 is used to replicate DML or DDL statements of the table that matches schema-pattern: "test_*" and table-pattern: "t_*" to the downstream test.t.
  • rule-2 is used to replicate DDL statements of the schema that matches schema-pattern: "test_*", such as create/drop schema xx.

Note:

  • If the downstream schema: test already exists and will not be deleted, you can omit rule-2.
  • If the downstream schema: test does not exist and only rule-1 is configured, then it reports the schema test doesn't exist error during replication.
  rule-1:
    schema-pattern: "test_*"
    table-pattern: "t_*"
    target-schema: "test"
    target-table: "t"
  rule-2:
    schema-pattern: "test_*"
    target-schema: "test"

Merge sharded schemas

Assuming in the scenario of sharded schemas, you want to replicate the test_{1,2,3...}.t_{1,2,3...} tables in the two upstream MySQL instances to the test.t_{1,2,3...} tables in the downstream TiDB instance.

To replicate the upstream schemas to the downstream test.t_[1,2,3], you only need to create one routing rule.

  rule-1:
    schema-pattern: "test_*"
    target-schema: "test"

Incorrect table routing

Assuming that the following two routing rules are configured and test_1_bak.t_1_bak matches both rule-1 and rule-2, an error is reported because the table routing configuration violates the number limitation.

  rule-1:
    schema-pattern: "test_*"
    table-pattern: "t_*"
    target-schema: "test"
    target-table: "t"
  rule-2:
    schema-pattern: "test_1_bak"
    table-pattern: "t_1_bak"
    target-schema: "test"
    target-table: "t_bak"

Black and white table lists

The black and white lists filtering rule of the upstream database instance tables is similar to MySQL replication-rules-db/tables, which can be used to filter or only replicate all operations of some databases or some tables.

Parameter configuration

black-white-list:
  rule-1:
    do-dbs: ["~^test.*"]         # Starting with "~" indicates it is a regular expression.
​    ignore-dbs: ["mysql"]
    do-tables:
    - db-name: "~^test.*"
      tbl-name: "~^t.*"
    - db-name: "test"
      tbl-name: "t"
    ignore-tables:
    - db-name: "test"
      tbl-name: "log"

Parameter explanation

  • do-dbs: white lists of the schemas to be replicated
  • ignore-dbs: black lists of the schemas to be replicated
  • do-tables: white lists of the tables to be replicated
  • ignore-tables: black lists of the tables to be replicated
  • In black and white lists, starting with the "~" character indicates it is a regular expression.

Filtering process

The filtering process is as follows:

  1. Filter at the schema level:

    • If do-dbs is not empty, judge whether a matched schema exists in do-dbs.

      • If yes, continue to filter at the table level.
      • If not, filter test.t.

    • If do-dbs is empty and ignore-dbs is not empty, judge whether a matched schema exits in ignore-dbs.

      • If yes, filter test.t.
      • If not, continue to filter at the table level.

    • If both do-dbs and ignore-dbs are empty, continue to filter at the table level.

  2. Filter at the table level:

    1. If do-tables is not empty, judge whether a matched table exists in do-tables.

      • If yes, replicate test.t.
      • If not, filter test.t.

    2. If ignore-tables is not empty, judge whether a matched table exists in ignore-tables.

      • If yes, filter test.t.
      • If not, replicate test.t.

    3. If both do-tables and ignore-tables are empty, replicate test.t.

Note:

To judge whether the schema test is filtered, you only need to filter at the schema level.

Usage example

Assume that the upstream MySQL instances include the following tables:

`logs`.`messages_2016`
`logs`.`messages_2017`
`logs`.`messages_2018`
`forum`.`users`
`forum`.`messages`
`forum_backup_2016`.`messages`
`forum_backup_2017`.`messages`
`forum_backup_2018`.`messages`

The configuration is as follows:

black-white-list:
  bw-rule:
    do-dbs: ["forum_backup_2018", "forum"]
    ignore-dbs: ["~^forum_backup_"]
    do-tables:
    - db-name: "logs"
      tbl-name: "~_2018$"
    - db-name: "~^forum.*"
​      tbl-name: "messages"
    ignore-tables:
    - db-name: "~.*"
​      tbl-name: "^messages.*"

After using the bw-rule rule:

Table Whether to filter Why filter
logs.messages_2016 Yes The schema logs fails to match any do-dbs.
logs.messages_2017 Yes The schema logs fails to match any do-dbs.
logs.messages_2018 Yes The schema logs fails to match any do-dbs.
forum_backup_2016.messages Yes The schema forum_backup_2016 fails to match any do-dbs.
forum_backup_2017.messages Yes The schema forum_backup_2017 fails to match any do-dbs.
forum.users Yes 1. The schema forum matches do-dbs and continues to filter at the table level.
2. The schema and table fail to match any of do-tables and ignore-tables and do-tables is not empty.
forum.messages No 1. The schema forum matches do-dbs and continues to filter at the table level.
2. The table messages is in the db-name: "~^forum.*",tbl-name: "messages" of do-tables.
forum_backup_2018.messages No 1. The schema forum_backup_2018 matches do-dbs and continues to filter at the table level.
2. The schema and table match the db-name: "~^forum.*",tbl-name: "messages" of do-tables.

Binlog event filter

Binlog event filter is a more fine-grained filtering rule than the black and white lists filtering rule. You can use statements like INSERT or TRUNCATE TABLE to specify the binlog events of schema/table that you need to replicate or filter out.

Note:

If a same table matches multiple rules, these rules are applied in order and the black list has priority over the white list. This means if both the Ignore and Do rules are applied to a single table, the Ignore rule takes effect.

Parameter configuration

filters:
  rule-1:
    schema-pattern: "test_*"
    ​table-pattern: "t_*"
    ​events: ["truncate table", "drop table"]
    sql-pattern: ["^DROP\\s+PROCEDURE", "^CREATE\\s+PROCEDURE"]
    ​action: Ignore

Parameter explanation

  • schema-pattern/table-pattern: the binlog events or DDL SQL statements of upstream MySQL or MariaDB instance tables that match schema-pattern/table-pattern are filtered by the rules below.

  • events: the binlog event array.

    Events Type Description
    all Includes all the events below
    all dml Includes all DML events below
    all ddl Includes all DDL events below
    none Includes none of the events below
    none ddl Includes none of the DDL events below
    none dml Includes none of the DML events below
    insert DML The INSERT DML event
    update DML The UPDATE DML event
    delete DML The DELETE DML event
    create database DDL The CREATE DATABASE DDL event
    drop database DDL The DROP DATABASE DDL event
    create table DDL The CREATE TABLE DDL event
    create index DDL The CREATE INDEX DDL event
    drop table DDL The DROP TABLE DDL event
    truncate table DDL The TRUNCATE TABLE DDL event
    rename table DDL The RENAME TABLE DDL event
    drop index DDL The DROP INDEX DDL event
    alter table DDL The ALTER TABLE DDL event

  • sql-pattern: it is used to filter specified DDL SQL statements. The matching rule supports using a regular expression. For example, "^DROP\\s+PROCEDURE".

  • action: the string (Do/Ignore). Based on the following rules, it judges whether to filter. If either of the two rules is satisfied, the binlog will be filtered; otherwise, the binlog will not be filtered.

    • Do: the white list. The binlog will be filtered in either of the following two conditions:
      • The type of the event is not in the event list of the rule.
      • The SQL statement of the event cannot be matched by sql-pattern of the rule.
    • Ignore: the black list. The binlog will be filtered in either of the following two conditions:
      • The type of the event is in the event list of the rule.
      • The SQL statement of the event can be matched by sql-pattern of the rule.

Usage examples

This sections shows the usage examples in the scenario of sharding (sharded schemas and tables).

Filter all sharding deletion operations

To filter out all deletion operations, configure the following two filtering rules:

  • filter-table-rule filters out the truncate table, drop table and delete statement operations of all tables that match the test_*.t_* pattern.
  • filter-schema-rule filters out the drop database operation of all schemas that match the test_* pattern.
filters:
  filter-table-rule:
    schema-pattern: "test_*"
    table-pattern: "t_*"
    events: ["truncate table", "drop table", "delete"]
    action: Ignore
  filter-schema-rule:
    schema-pattern: "test_*"
    events: ["drop database"]
    action: Ignore

Only replicate sharding DML statements

To only replicate sharding DML statements, configure the following two filtering rules:

  • do-table-rule only replicates the create table, insert, update and delete statements of all tables that match the test_*.t_* pattern.
  • do-schema-rule only replicates the create database statement of all schemas that match the test_* pattern.

Note:

The reason why the create database/table statement is replicated is that you can replicate DML statements only after the schema and table are created.

filters:
  do-table-rule:
    schema-pattern: "test_*"
    table-pattern: "t_*"
    events: ["create table", "all dml"]
    action: Do
  do-schema-rule:
    schema-pattern: "test_*"
    events: ["create database"]
    action: Do

Filter out the SQL statements that TiDB does not support

To filter out the PROCEDURE statements that TiDB does not support, configure the following filter-procedure-rule:

filters:
  filter-procedure-rule:
    schema-pattern: "test_*"
    table-pattern: "t_*"
    sql-pattern: ["^DROP\\s+PROCEDURE", "^CREATE\\s+PROCEDURE"]
    action: Ignore

filter-procedure-rule filters out the ^CREATE\\s+PROCEDURE and ^DROP\\s+PROCEDURE statements of all tables that match the test_*.t_* pattern.

Filter out the SQL statements that the TiDB parser does not support

For the SQL statements that the TiDB parser does not support, DM cannot parse them and get the schema/table information. So you must use the global filtering rule: schema-pattern: "*".

Note:

To avoid unexpectedly filtering out data that need to be replicated, you must configure the global filtering rule as strictly as possible.

To filter out the PARTITION statements that the TiDB parser does not support, configure the following filtering rule:

filters:
  filter-partition-rule:
    schema-pattern: "*"
    sql-pattern: ["ALTER\\s+TABLE[\\s\\S]*ADD\\s+PARTITION", "ALTER\\s+TABLE[\\s\\S]*DROP\\s+PARTITION"]
    action: Ignore

Column mapping

The column mapping feature supports modifying the value of table columns. You can execute different modification operations on the specified column according to different expressions. Currently, only the built-in expressions provided by DM are supported.

Note:

  • It does not support modifying the column type and the table schema.
  • It does not support configuring multiple different column mapping rules for a same table.

Parameter configuration

column-mappings:
  rule-1:
​    schema-pattern: "test_*"
​    table-pattern: "t_*"
​    expression: "partition id"
​    source-column: "id"
​    target-column: "id"
​    arguments: ["1", "test", "t", "_"]
  rule-2:
​    schema-pattern: "test_*"
​    table-pattern: "t_*"
​    expression: "partition id"
​    source-column: "id"
​    target-column: "id"
​    arguments: ["2", "test", "t", "_"]

Parameter explanation

  • schema-pattern/table-pattern: to execute column value modifying operations on the upstream MySQL or MariaDB instance tables that match the schema-pattern/table-pattern filtering rule.
  • source-column, target-column: to modify the value of the source-column column according to specified expression and assign the new value to target-column.
  • expression: the expression used to modify data. Currently, only the partition id built-in expression is supported.

The partition id expression

partition id is used to resolve the conflicts of auto-increment primary keys of sharded tables.

partition id restrictions

Note the following restrictions:

  • The partition id expression only supports the bigint type of auto-increment primary key.
  • If the schema prefix is not empty, the schema name format must be schema prefix or schema prefix + separator + number (the schema ID). For example, it supports s and s_1, but does not support s_a.
  • If the table prefix is not empty, the table name format must be table prefix or table prefix + separator + number (the table ID).
  • If the schema/table name does not contain the … + separator + number part, the corresponding ID is considered as 0.
  • Restrictions on sharding size:
    • It supports 16 MySQL or MariaDB instances at most (0 <= instance ID <= 15).
    • Each instance supports 128 schemas at most (0 <= schema ID <= 127).
    • Each schema of each instance supports 256 tables at most (0 <= table ID <= 255).
    • The ID range of the auto-increment primary key is "0 <= ID <= 17592186044415".
    • The {instance ID, schema ID, table ID} group must be unique.
  • Currently, the partition id expression is a customized feature. If you want to modify this feature, contact the corresponding developers.

partition id arguments configuration

Configure the following three or four arguments in order:

  • instance_id: the ID of the upstream sharded MySQL or MariaDB instance (0 <= instance ID <= 15)
  • schema prefix: used to parse the schema name and get the schema ID
  • table prefix: used to parse the table name and get the table ID
  • The separator: used to separate between the prefix and the IDs, and can be omitted to use an empty string as separator

Any of instance_id, schema prefix and table prefix can be set to an empty string ("") to indicate that the corresponding parts will not be encoded into the partition ID.

partition id expression rules

partition id fills the beginning bit of the auto-increment primary key ID with the argument number, and computes an int64 (MySQL bigint) type of value. The specific rules are as follows:

instance_id schema prefix table prefix Encoding
☑ defined ☑ defined ☑ defined [S: 1 bit] [I: 4 bits] [D: 7 bits] [T: 8 bits] [P: 44 bits]
☐ empty ☑ defined ☑ defined [S: 1 bit] [D: 7 bits] [T: 8 bits] [P: 48 bits]
☑ defined ☐ empty ☑ defined [S: 1 bit] [I: 4 bits] [T: 8 bits] [P: 51 bits]
☑ defined ☑ defined ☐ empty [S: 1 bit] [I: 4 bits] [D: 7 bits] [P: 52 bits]
☐ empty ☐ empty ☑ defined [S: 1 bit] [T: 8 bits] [P: 55 bits]
☐ empty ☑ defined ☐ empty [S: 1 bit] [D: 7 bits] [P: 56 bits]
☑ defined ☐ empty ☐ empty [S: 1 bit] [I: 4 bits] [P: 59 bits]
  • S: the sign bit, reserved
  • I: the instance ID, 4 bits by default if set
  • D: the schema ID, 7 bits by default if set
  • T: the table ID, 8 bits by default if set
  • P: the auto-increment primary key ID, occupying the rest of bits (≥44 bits)

Usage example

Assuming in the sharding scenario where all tables have the auto-increment primary key, you want to replicate two upstream MySQL instances test_{1,2,3...}.t_{1,2,3...} to the downstream TiDB instances test.t.

Configure the following two rules:

column-mappings:
  rule-1:
​    schema-pattern: "test_*"
​    table-pattern: "t_*"
​    expression: "partition id"
​    source-column: "id"
​    target-column: "id"
​    arguments: ["1", "test", "t", "_"]
  rule-2:
​    schema-pattern: "test_*"
​    table-pattern: "t_*"
​    expression: "partition id"
​    source-column: "id"
​    target-column: "id"
​    arguments: ["2", "test", "t", "_"]
  • The column ID of the MySQL instance 1 table test_1.t_1 is converted from 1 to 1 << (64-1-4) | 1 << (64-1-4 -7) | 1 << 44 | 1 = 580981944116838401.
  • The row ID of the MySQL instance 2 table test_1.t_2 is converted from 2 to 2 << (64-1-4) | 1 << (64-1-4 -7) | 2 << 44 | 2 = 1157460288606306306.

Replication delay monitoring

The heartbeat feature supports calculating the real-time replication delay between each replication task and MySQL or MariaDB based on real replication data.

Note:

  • The estimation accuracy of the replication delay is at the second level.
  • The heartbeat related binlog will not be replicated into the downstream, which is discarded after calculating the replication delay.

System privileges

If the heartbeat feature is enabled, the upstream MySQL or MariaDB instances must provide the following privileges:

  • SELECT
  • INSERT
  • CREATE (databases, tables)

Parameter configuration

In the task configuration file, enable the heartbeat feature:

enable-heartbeat: true

Principles introduction

  • DM-worker creates the dm_heartbeat (currently unconfigurable) schema in the corresponding upstream MySQL or MariaDB.
  • DM-worker creates the heartbeat (currently unconfigurable) table in the corresponding upstream MySQL or MariaDB.
  • DM-worker uses replace statement to update the current TS_master timestamp every second (currently unconfigurable) in the corresponding upstream MySQL or MariaDB dm_heartbeat.heartbeat tables.
  • DM-worker updates the TS_slave_task replication time after each replication task obtains the dm_heartbeat.heartbeat binlog.
  • DM-worker queries the current TS_master timestamp in the corresponding upstream MySQL or MariaDB dm_heartbeat.heartbeat tables every 10 seconds, and calculates task_lag = TS_master - TS_slave_task for each task.

See the replicate lag in the binlog replication processing unit of DM monitoring metrics.