Synchronize data from OceanBase Database to a DataHub instance

This topic describes how to use OceanBase Migration Service (OMS) to synchronize data from OceanBase Database to a DataHub instance.

Prerequisites

• You have created a dedicated database user for data synchronization in the source OceanBase database and granted corresponding privileges to the user. For more information, see Create a database user.

• You have created data sources for the source and target databases. For more information, see Create a physical OceanBase data source and Create a DataHub data source.

Limitations

• OMS supports full synchronization of tables with unique keys only.

• DDL synchronization applies only to BLOB topics.

• During data synchronization, OMS allows you to drop a table before creating a new one. In other words, you can execute DROP TABLE and then execute CREATE TABLE. In OMS, you cannot create a new table by renaming a table. In other words, you cannot execute RENAME TABLE a TO a_tmp.

• OMS supports synchronization of data of the UTF8 and GBK character sets.

• Data source identifiers and user accounts must be globally unique in OMS.

• OMS supports the synchronization of only objects whose database name, table name, and column name are ASCII-encoded and do not contain special characters. The special characters are line breaks, spaces, and the following characters: . | " ' ` ( ) = ; / & \.

• OMS does not support a standby OceanBase database as the source.

DataHub has the following limitations:

• DataHub limits the size of a message based on the cloud environment, usually to 1 MB.

• DataHub sends messages in batches, with each batch sized no more than 4 MB. If a single message meets the conditions for sending, you can modify the batch.size parameter. By default, 20 messages are sent at a time within one second.

• For more information about the limitations and naming conventions of DataHub, see Limits of DataHub.

Considerations

• In a data synchronization task where the source is OceanBase Database and DDL synchronization is enabled, if a RENAME operation is performed on a table in the source database, we recommend that you restart the task to avoid data loss during incremental synchronization.

• If the source is OceanBase Database of a version in the range of V4.0.0 to V4.3.x, excluding V4.2.5 BP1, and you have selected Incremental Synchronization, you need to configure the STORED attribute for generated columns. For more information, see Generated column operations. Otherwise, information about generated columns will not be saved in incremental logs, which may lead to exceptions during incremental data synchronization.

• Take note of the following items when an updated row contains a LOB column:

  • If the LOB column is updated, do not use the value stored in the LOB column before the UPDATE or DELETE operation.

    The following data types are stored in LOB columns: JSON, GIS, XML, user-defined type (UDT), and TEXT such as LONGTEXT and MEDIUMTEXT.

  • If the LOB column is not updated, the value stored in the LOB column before and after the UPDATE or DELETE operation is NULL.

• If you select only Incremental Synchronization when you create a data synchronization task, OMS requires that the local incremental logs in the source database be retained for more than 48 hours.

  ​If you select Full Synchronization and Incremental Synchronization when you create the data synchronization task, OMS requires that the local incremental logs of the source database be retained for at least seven days. Otherwise, the data synchronization task may fail or the data in the source and target databases may be inconsistent because OMS cannot obtain incremental logs.

• At present, the data migration task does not support tables without a non-null unique key. To avoid duplicate data in case of task restart and other exceptions, we recommend that you configure a non-null unique key for each table.

• The following table lists the data types supported by DataHub. These data types apply only to Tuple topics.

  Type	Description	Value range
  BIGINT	An 8-byte signed integer.	-9223372036854775807 to 9223372036854775807
  DOUBLE	An 8-byte double-precision floating-point number.	-1.0 _10^308 to 1.0_10^308
  BOOLEAN	A boolean value.	True or False true or false 0 or 1
  TIMESTAMP	A timestamp.	It is accurate to microseconds.
  STRING	A string that supports only UTF-8 encoding.	A single STRING column supports a maximum of 2 MB.
  INTEGER	A 4-byte integer.	-2147483648 to 2147483647
  FLOAT	A 4-byte single-precision floating-point number.	-3.40292347_10^38 to 3.40292347_10^38
  DECIMAL	A numeric value.	- 10^38 +1 to 10^38 - 1

Supported DDL operations for synchronization

• ALTER TABLE

• CREATE INDEX

• DROP INDEX

• TRUNCATE TABLE

  In delayed deletion, the same transaction contains two identical TRUNCATE TABLE statements. In this case, idempotence is implemented for downstream consumption.

Data type mappings

A task that synchronizes data to a DataHub instance supports only the following data types: INTEGER, BIGINT, TIMESTAMP, FLOAT, DOUBLE, DECIMAL, STRING, and BOOLEAN.

• If you create a topic of another type when you set topic mapping, data synchronization will fail.

• The following table describes the default mapping rules, which are the most appropriate. If you change the mapping, an error may occur.

Data type mappings between MySQL-compatible tenants of OceanBase Database and DataHub instances

Data type mappings between Oracle-compatible tenants of OceanBase Database and DataHub instances

Supplemental properties

If you manually create a topic, add the following properties to the DataHub schema before you start a data synchronization task. If OMS automatically creates a topic and synchronizes the schema, OMS automatically adds the following properties.

Procedure

1. Create a data synchronization task.

   

   1. Log in to the OMS console.

   2. In the left-side navigation pane, click Data Synchronization.

   3. On the Data Synchronization page, click New Task in the upper-right corner.

2. On the New Task page, specify the name of the synchronization task.

   We recommend that you set it to a combination of digits and letters. It must not contain any spaces and cannot exceed 64 characters in length.

   Notice

   The task name must be a unique identifier in the OMS system.

3. On the Select Source and Target page, configure the parameters.

   

   Parameter	Description
   Source	If you have created an OceanBase data source, which can be a physical data source, a public cloud data source, or a standalone data source, select it from the drop-down list. Otherwise, click New Data Source in the drop-down list and create one in the dialog box that appears on the right. For more information about the parameters, see Create a physical OceanBase data source, Create a public cloud OceanBase data source, or Create a standalone OceanBase data source.
   Target	If you have created a DataHub data source, select it from the drop-down list. Otherwise, click New Data Source in the drop-down list and create one in the dialog box that appears on the right. For more information about the parameters, see Create a DataHub data source.
   Tag (Optional)	Click the text box and select a tag from the drop-down list. You can also click Manage Tags to create, modify, and delete tags. For more information, see Use tags to manage data synchronization tasks.

4. Click Next. In the Select Synchronization Type step, specify the synchronization types for the current data synchronization task.

   The supported synchronization types are Schema Synchronization, Full Synchronization, and Incremental Synchronization. Schema Synchronization creates a topic. Incremental Synchronization supports the DML Synchronization and DDL Synchronization options. The supported DML operations are INSERT, DELETE, and UPDATE. Select the options as needed. For more information, see Configure DDL/DML synchronization.

   Notice

   If you select DDL Synchronization here, you can select only topics of the BLOB type in the Select Synchronization Objects step.

5. (Optional) Click Next.

   If you have selected Incremental Synchronization without configuring the required parameters for the source OceanBase database, the More About Data Sources dialog box appears to prompt you to configure the parameters. For more information about the parameters, see Create a physical OceanBase data source, Create a public cloud OceanBase data source, or Create a standalone OceanBase data source.

   After you configure the parameters, click Test Connectivity. After the test succeeds, click Save.

6. Click Next. In the Select Synchronization Objects step, select the target topic type and objects.

   Available topic types are Tuple and BLOB. Tuple topics do not support DDL synchronization but support records similar to database records. Each record contains multiple columns. BLOB topics only support a binary block as a record. Data is Base64-encoded for transmission. For more information, visit the documentation center of DataHub.

   After you select the topic type for synchronization, you can select Specify Objects or Match by Rule to specify the synchronization objects. The following procedure describes how to specify synchronization objects by using the Specify Objects option. For information about the procedure for specifying synchronization objects by using the Match by Rule option, see the Configure matching rules for data migration or synchronization from a database to a Message Queue instance section in Configure matching rules.

   Note

   If you have selected DDL Synchronization in the Select Synchronization Type step, we recommend that you select Match by Rule to specify synchronization objects. This way, all new objects that meet the specified rules will be synchronized. If you select Specify Objects to specify synchronization objects, new or renamed objects will not be synchronized.

   1. In the Select Synchronization Objects step, select Specify Objects.

   2. In the left-side pane, select the objects to be synchronized.

   3. Click >.

      

   4. Select a mapping method.

      • To synchronize one Tuple table or one or more BLOB tables, select the required mapping method and click OK in the Map Object to Topic dialog box.

        

        If you did not select Schema Synchronization as the synchronization type, you can select only Existing Topics here. If you have selected Schema Synchronization when you specify the synchronization type, you can select only one mapping method to create or select a topic.

        For example, if you have selected Schema Synchronization, when you use both the Create Topic and Select Topic mapping methods or rename the topic, a precheck error will be returned due to option conflicts.

        Parameter	Description
        Create Topic	Enter the name of the new topic in the text box. The topic name can contain letters, digits, and underscores (_) and must start with a letter. It must not exceed 128 characters in length.
        Select Topic	OMS allows you to query DataHub topics. You can click Select Topic, and then find and select a topic for synchronization from the Existing Topics drop-down list. You can also enter the name of an existing topic and select it after it appears.
        Batch Generate Topics	The format for generating topics in batches is Topic_${Database Name}_${Table Name}.

        If you select Create Topic or Batch Generate Topics, after the schema migration succeeds, you can query the created topics in the DataHub instance. By default, the number of data shards is 2 and the data validity period is 7 days. These parameters cannot be modified. If the topics do not meet your business needs, you can create topics in the target database as needed.

      • To synchronize multiple Tuple tables, click OK in the dialog box that appears.

        

        If you have selected a Tuple topic and multiple tables without selecting Schema Synchronization, you must select an existing topic and click OK in the Map Object to Topic dialog box.

        

   5. Click OK.

      Note

      OMS automatically filters out unsupported tables. For information about the SQL statements for querying table objects, see SQL statements for querying table objects.

   OMS allows you to import objects by using text, set row filtering, sharding columns, and column filtering for target objects, as well as remove one or all objects. Objects in the target database are listed in the structure of Topic > Database > Table.

   Note

   When you select Match by Rule to specify synchronization objects, object renaming is implemented based on the syntax of the specified matching rules. In the operation area, you can only set filter conditions and select sharding columns and the columns to be synchronized. For more information, see Configure matching rules.

   

   Operation	Description
   Import objects	In the list on the right, click Import Objects in the upper-right corner. In the dialog box that appears, click OK. Notice This operation will overwrite previous selections. Proceed with caution. In the Import Synchronization Objects dialog box, import the objects to be synchronized. You can import CSV files to rename databases/tables and set row filtering conditions. For more information, see Download and import the settings of synchronization objects. Click Validate. After the validation succeeds, click OK.
   Change topics	When the topic type is set to BLOB, you can change the topic for objects in the target database. For more information, see Change the topic.
   Configure settings	OMS allows you to configure row-based filtering, select sharding columns, and select columns to be synchronized. In the list on the right, move the pointer over the object that you want to set. Click Settings. In the Settings dialog box, you can perform the following operations: In the Row Filters section, specify a standard SQL WHERE clause to filter data by row. For more information, see Use SQL conditions to filter data. Select the sharding columns that you want to use from the Sharding Columns drop-down list. You can select multiple columns as sharding columns. This parameter is optional. Unless otherwise specified, select the primary key as sharding columns. If the primary key is not load-balanced, select load-balanced columns with unique identifiers as sharding columns to avoid potential performance issues. Sharding columns can be used for the following purposes: Load balancing: Threads used for sending messages can be recognized based on the sharding columns if the target table supports concurrent writes. Orderliness: OMS ensures that messages are received in order if the values of the sharding columns are the same. The orderliness specifies the sequence of executing DML statements for a column. In the Select Columns section, select the columns to be synchronized. For more information, see Column filtering. Click OK.
   Remove one or all objects	OMS allows you to remove one or all objects to be synchronized to the target database during data mapping. To remove one synchronization object In the list on the right, move the pointer over the object that you want to remove, and click Remove to remove the synchronization object. Remove all synchronization objects In the list on the right, click Remove All in the upper-right corner. In the dialog box that appears, click OK to remove all synchronization objects.

7. Click Next. In the Synchronization Options step, configure the parameters.

   • Full synchronization

     The following parameters are displayed only if you have selected Full Synchronization in the Select Synchronization Type step.

     

     Parameter	Description
     Full Synchronization Rate Limit	You can choose whether to limit the full synchronization rate as needed. If you choose to limit it, you must specify the RPS and BPS. The RPS specifies the maximum rows of data synchronized to the target database per second during full synchronization, and the BPS specifies the maximum amount of data in bytes synchronized to the target database per second during full synchronization. Note The RPS and BPS values specified here are only for throttling. The actual full synchronization performance is subject to factors such as the settings of the source and target databases and the instance specifications.
     Full Synchronization Resource Configuration	You can select Small, Medium, or Large to use the corresponding default values of Read Concurrency, Write Concurrency, and Memory. You can also customize the resource configurations for full synchronization. By setting the resource configuration for the Full-Import component, you can limit the resource consumption of a task in the full synchronization stage. Notice In the case of custom configurations, the minimum value is 1, and only integers are supported.

   • Incremental synchronization

     The following parameters are displayed only if you have selected Incremental Synchronization in the Select Synchronization Type step.

     

     Parameter	Description
     Incremental Synchronization Rate Limit	You can choose whether to limit the incremental synchronization rate as needed. If you choose to limit it, you must specify the RPS and BPS. The RPS specifies the maximum rows of data synchronized to the target database per second during incremental synchronization, and the BPS specifies the maximum amount of data in bytes synchronized to the target database per second during incremental synchronization. Note The RPS and BPS values specified here are only for throttling. The actual incremental synchronization performance is subject to factors such as the settings of the source and target databases and the instance specifications.
     Incremental Log Pull Resource Configuration	You can select Small, Medium, or Large to use the corresponding default value of Memory. You can also customize the resource configurations for incremental log pull. By setting the resource configuration for the Store component, you can limit the resource consumption of a task in log pull in the incremental synchronization stage. Notice In the case of custom configurations, the minimum value is 1, and only integers are supported.
     Incremental Data Write Resource Configuration	You can select Small, Medium, or Large to use the corresponding default values of Write Concurrency and Memory. You can also customize the resource configurations for incremental data write. By setting the resource configuration for the Incr-Sync component, you can limit the resource consumption of a task in data writes in the incremental synchronization stage. Notice In the case of custom configurations, the minimum value is 1, and only integers are supported.
     Incremental Record Retention Duration	The duration that incremental parsed files are cached in OMS. A longer retention duration results in more disk space occupied by the Store component.
     Incremental Synchronization Start Timestamp	If you have selected Full Synchronization as the synchronization type, the default value of this parameter is the task startup time and cannot be modified. If you did not select Full Synchronization as the synchronization type, set this parameter to a certain point in time, which is the current system time by default. For more information, see Set an incremental synchronization timestamp.

   • Advanced options

     

     Parameter	Description
     Serialization Method	The message format for synchronizing data to a DataHub instance. Valid values: Default, Canal, Dataworks (version 2.0 supported), SharePlex, DefaultExtendColumnType, Debezium, DebeziumFlatten, and DebeziumSmt. For more information, see Data formats. Notice This parameter is available only when the topic type is set to BLOB in the Select Synchronization Type step. At present, only MySQL-compatible tenants of OceanBase Database support Debezium, DebeziumFlatten, and DebeziumSmt.
     Partitioning Rules	The rule for synchronizing data from the source database to a DataHub topic. Valid values: Hash and Table. We recommend that you select Table to ensure DDL and DML consumption consistency when downstream applications are consuming messages. Hash indicates that OMS uses a hash algorithm to select the shard of a DataHub topic based on the value of the primary key or sharding column. Table indicates that OMS delivers all data in a table to the same partition and uses the table name as the hash key. Notice If you select DDL Synchronization in the Select Synchronization Type step, the partitioning rule can be set only to Table.
     Business System Identification (Optional)	The identifier that identifies the source business system of data. This parameter is displayed only when Serialization Method is set to Dataworks. The business system identifier consists of 1 to 20 characters.

   If the parameter settings on the page cannot meet your requirements, you can click Parameter Configuration in the lower part of the page to configure more specific settings. You can also reference an existing task or component template.

   

8. Click Precheck.

   During the precheck, OMS checks the column name and column type, and checks whether the values are null. OMS does not check the value length or default value. If an error is returned during the precheck, you can perform the following operations:

   • Identify and troubleshoot the problem and then perform the precheck again.

   • Click Skip in the Actions column of a failed precheck item. In the dialog box that prompts the consequences of the operation, click OK.

9. Click Start Task. If you do not need to start the task now, click Save to go to the details page of the task. You can start the task later as needed.

   OMS allows you to modify the synchronization objects when the data synchronization task is running. For more information, see View and modify synchronization objects. After the data synchronization task is started, it will be executed based on the selected synchronization types. For more information, see the View synchronization details section in View details of a data synchronization task.