DRS supports now Real-Time Synchronization from DDM into RDS for MySQL

• DDM instances

• RDS for MySQL (5.5, 5.6, 5.7, and 8.0)

Restrictions on the source database

• The database names and table names of the source sharding middleware cannot contain non-ASCII characters or the following characters: .'<>/\

Enable skip-name-resolve for the source sharded database to reduce the possibility of connection timeout.

Restrictions on usage

General

• Do not modify or delete the usernames, passwords, and permissions of the source and destination databases or change the ports of the source and destination databases.
• During the synchronization, do not change the sharding key of a table on the source DDM instance, or change an unsharded or broadcast table to a sharded table, or change a sharded table to an unsharded or broadcast table.

Full synchronization
During full data synchronization, a lot of binlogs are generated in the destination database, occupying too much storage space. Therefore, during full data synchronization, only the latest five binlogs are retained in the destination database by default. After the full synchronization is complete, the retention period of binlogs in the destination database is restored to the value you configure. If you want to keep the binlog retention period of the destination database to be the value you specify due to service requirements, you need to submit a service ticket.

Incremental synchronization

• Do not perform data restoration on the source database.
• Some DDL operations are supported.
  • D​R​O​P​_​D​A​T​A​B​A​S​E, D​R​O​P​_​T​A​B​L​E, T​R​U​N​C​A​T​E​_​T​A​B​L​E, C​R​E​A​T​E​_​V​I​E​W and D​R​O​P​_​V​I​E​W are not supported.
  • Online DDL is not supported.
  • Tables can be created. For example:
    cre​a​te ta​ble `d​dl_t​est` (i​​d in​t, c1 va​rch​ar(25), pri​mary ke​y(​id));
    cre​​ate ta​​ble `d​dl_te​st_g​ho` l​i​ke `d​dl_t​est`;
  • Tables can be renamed. Both the source and destination tables must be selected. For example:
    ren​ame tab​le `d​dl_t​est` t​o `d​dl_te​st_n​ew`;
  • Columns in a table can be added and modified, but cannot be deleted. For example:
    al​t​er ta​b​le `d​dl_tes​t` a​dd co​lumn `c​2` varc​har(​25);
    al​​ter ta​b​le `d​dl_te​st` m​odify col​umn `c​1` var​ch​ar(50);
    alt​e​r ta​b​le `d​dl_tes​t` a​lter c​1 s​et def​ault '*​**​';
  • Table indexes can be modified. For example:
    alt​e​r ta​b​le `d​dl_t​est` dro​p prim​ary k​ey;
    alt​​er ta​b​le `dd​l_te​st` ad​d pri​mary key​(id);
    a​lt​er t​ab​le `dd​l_te​st` a​dd ind​ex  `d​dl_t​est_u​k`(id​);
    a​lt​er t​ab​le `d​dl_t​est` dr​op inde​x `d​dl_te​st_u​k`;
  • In table-level synchronization, you can add columns, modify columns, and add primary keys and normal indexes.
  • During database-level synchronization, you can create tables, rename tables, add columns, modify columns, and add primary keys and normal indexes.
  • The name of a table, column, or index to be added or modified cannot exceed 63 characters. Otherwise, the task fails.
  • If a primary key is added to a table that does not have a primary key in the source database, the DDL operation must contain the first column. Otherwise, the task fails.

• Perform DDL operations on the same table or column during off-peak hours at an interval of more than 1 minute.
• The DDL operation of renaming an unselected table is filtered out during the synchronization. As a result, the task may fail or data may be inconsistent.
  • If you rename table A to the name of table B and tables A and B are selected for synchronization, this RENAME statement will not be filtered out.
  • If you rename table A to the name of table B but table B is not synchronized, this RENAME statement will be filtered out.
  • You are not advised to perform the rename operation in the many-to-one synchronization scenario. Otherwise, the task may fail or data may be inconsistent.

Troubleshooting
If any problem occurs during task creation, startup, full synchronization, incremental synchronization, or completion, rectify the fault by referring to Troubleshooting.

Other restrictions

• If the data types are incompatible, the synchronization may fail.
• If a physically generated column in a table is generated based on a time type, the data in the column may be inconsistent.
• Resumable upload is supported. However, data may be repeatedly inserted into a non-transactional table that does not have a primary key when the server system breaks down.
• If the source database contains a duplicate primary key or unique key, the data synchronized to the destination database will be less than that in the source database. Therefore, you must check and correct the data before starting the synchronization task.
• If the source database contains non-standard floating-point data and the data can be written in loose mode but cannot be written in strict mode, there may be data inconsistency during synchronization.

If a table in the source database contains system columns _ddm_lock and _slot, data in these columns will be filtered out during full synchronization and incremental synchronization, resulting in data loss.

• Before creating a DRS task, if concurrency control rules of SQL statements are configured for the destination database, the DRS task may fail.
• The destination table can contain more columns than the source table. However, the following failures must be avoided:
  • Assume that extra columns on the destination cannot be null or have default values. If newly inserted data records are synchronized from the source to the destination, the extra columns will become null, which does not meet the requirements of the destination and will cause the task to fail.
  • Assume that extra columns on the destination must be fixed at a default value and have a unique constraint. If newly inserted data records are synchronized from the source to the destination, the extra columns will contain multiple default values. That does not meet the unique constraint of the destination and will cause the task to fail.
• After a task is created, the destination database cannot be set to read-only.
• After a task is created, you cannot add schemas to the source database or modify the old schema to associate with the new RDS DB instance. Otherwise, data cannot be synchronized or the task fails.

When editing a task, do not import a lot of data to the newly-added table. You are advised to edit the task during off-peak hours.