Synchronize data from an Oracle database to a DataHub instance

Alibaba Cloud DataHub is a streaming data processing platform for publishing, subscribing to, and distributing streaming data, enabling easy analysis and application based on streaming data.

Limitations

• OceanBase Migration Service (OMS) does not support incremental synchronization of a table if all the columns in the table that are of the LOB type.

• The Shard Columns parameter must be set for tables without a primary key in the Oracle database for successful synchronization to the DataHub instance.

• OMS cannot parse the actual values of the generated columns used in the Oracle database. Therefore, when data is synchronized to the DataHub instance, the corresponding values are NULL.

• 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 without special characters. The special characters are . | " ' ` ( ) = ; / & \n

• OMS does not support the synchronization of database objects, such as schemas, tables, and columns, whose name exceeds 30 bytes in length from an Oracle database of version 12c or later.

Considerations

• When the Oracle database is in standby database only or primary/standby databases mode, if the number of instances that run on the primary Oracle database differs from that on the standby database, incremental logs of some instances may not be pulled. You need to manually set the parameters of the Store component to specify the instances for which incremental logs are to be pulled from the standby database. The procedure is as follows:

  1. Stop the Store component as soon as it starts.

  2. On the Update Configuration page of the Store component, add the deliver2store.logminer.instance_threads parameter and specify the instances for which logs are to be pulled.

     Separate multiple threads with a vertical bar (|), for example, 1|2|3. For more information about how to update a store component, see Update a store component.

  3. Restart the Store component.

  4. Wait for five minutes, and then run the grep 'log entries from' connector/connector.log command to check the instances for which logs are pulled. The thread field indicates the instances for which logs are pulled.

• If you need to synchronize incremental data from an Oracle database, we recommend that you restrict the size of a single archive file in the Oracle database within 2 GB. An excessively large archive file may incur the following risks:

  • The log pulling time increases not in proportion to the size of a single archive file, but much more sharply.

  • When the Oracle database is in standby database only or primary/standby databases mode, the incremental data is pulled from the standby database. In this case, only archive files can be pulled. An archive file is pulled after it is generated. A larger archive file means a longer delay before the archive file is processed, and a longer time for processing the archive file.

  • In addition, a larger size of a single archive file means larger memory required by the Store component under the same data pulling concurrency.

• The archive files must be stored for more than two days in the Oracle database. Otherwise, in the case of a sharp increase in the number of archive files or an exception in the Store component, restoration may fail due to the lack of required archive files.

• If a DML operation is performed to exchange primary keys in the source Oracle database, errors occur when OMS parses logs. This causes data loss when data is synchronized to the destination. Here is a sample DML statement that exchanges primary keys:

  UPDATE test SET c1=(CASE WHEN c1=1 THEN 2 WHEN c1=2 THEN 1 END) WHERE c1 IN (1,2);
  

• If the clocks between nodes or between the client and the server are out of synchronization, the latency may be inaccurate during incremental synchronization.

  For example, if the clock is earlier than the standard time, the latency can be negative. If the clock is later than the standard time, the latency can be positive.

• When data transmission is resumed for a project, some data (transmitted within the last minute) may be duplicate in the DataHub instance. Therefore, data deduplication is required in downstream applications.

• If LogMiner generates invalid time data, such as 13621-11-11 11:32:08, during data synchronization, the Store component generates an error.

  In this case, you can perform the following operations: Choose OPS & Monitoring > Components > Store. On the page that appears, click Update for the target store, add the deliver2store.logminer.replace_invalid_date parameter, and set it to true. Then skip the data in the data synchronization project.

  If users are not sensitive to DATE data, you can set the deliver2store.logminer.replace_invalid_date parameter to TRUE to enable the reader to continue running. When the Store component generates data, it converts abnormal DATE data into the date when the logs were written to the disk.

Data type mappings

Check and modify the configurations of the source Oracle database

• Check the character set configurations

  OMS allows you to synchronize data from the source Oracle database based on the AL32UTF8, AL16UTF16, ZHS16GBK, or GB18030 character set.

• Check and modify the system configurations of the Oracle instance

  1. Enable archivelog and supplemental_log for the source Oracle database.

  2. In the Oracle database, perform the following operations as the sys user.

     Execute the following statement to check whether log_mode is set to archivelog and supplemental_log parameters are set to yes or implicit:

     select log_mode, supplemental_log_data_pk, supplemental_log_data_ui, supplemental_log_data_min from v$database;
     

     If not, use the following syntax to modify the configuration of the Oracle database:

     ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;
     ALTER DATABASE ADD SUPPLEMENTAL LOG DATA(PRIMARY KEY) COLUMNS;
     ALTER DATABASE ADD SUPPLEMENTAL LOG DATA(UNIQUE) COLUMNS;
     

  3. Restart the Oracle database.

Supplemental properties

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

Procedure

1. Create a data synchronization project.

   1. Log on to the OMS console.

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

   3. On the Data Synchronization page, click Create Synchronization Project in the upper-right corner.

2. On the Select Source and Destination page, configure the parameters.

   Parameter	Description
   Synchronization Project Name	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.
   Tag (Optional)	Click the field and select a target tag from the drop-down list. You can also click Manage Tags to create, modify, and delete tags. For more information, see Manage data synchronization projects by using tags.
   Source	If you have created an Oracle data source, select it from the drop-down list. If not, click New Data Source in the drop-down list to create one in the dialog box on the right side. For more information about the parameters, see Create an Oracle data source.
   Destination	If you have created a DataHub data source, select it from the drop-down list. If not, click New Data Source in the drop-down list to create one in the dialog box on the right side. For more information about parameters, see Create a DataHub data source.

3. Click Next. On the Select Synchronization Type page, select the synchronization type for the current data synchronization project.

   The supported synchronization types include Schema Synchronization and Incremental Synchronization. Schema Synchronization creates a topic. Incremental Synchronization supports only the DML Synchronization option. The supported DML operations are Insert, Delete, and Update. Select the options based on your business needs. For more information, see DML filtering.

4. Click Next. On the Select Synchronization Objects page, select the topic type and scope for synchronization.

   Available topic types are Tuple and BLOB. Tuple topics 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, perform the following operations:

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

      Notice

      The name of a table to be synchronized, as well as the names of columns in the table, must not contain Chinese characters.

   2. Click >.

   3. Select a mapping method.

      Notice

      When you set the topic type to Tuple without selecting Schema Synchronization, you can only synchronize a single table to a single topic.

      • To synchronize a single table, select the mapping method as needed in the Map Object to Topic dialog box and click OK.

        If you do not select Schema Synchronization when you set the synchronization type and configuration, 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 on the DataHub side. By default, the number of data shards is 2 and the data expiration time is 7 days. These parameters cannot be modified. If the topics do not meet your business needs, you can create topics in the destination database as needed.

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

        If you have selected the Tuple type without selecting Schema Synchronization, when you select multiple tables to synchronize and select one topic and click OK in the Map Object to Topic dialog box, the selected tables are displayed under the topic in the right pane but only one table can be synchronized. Click Next. A prompt appears, indicating that only one-to-one mapping is supported between Tuple topics and tables.

   4. Click OK.

   When you synchronize data from an Oracle database to a DataHub instance, OMS allows you to import objects from text data, set sharding columns for tables in the destination database, and remove a single object or all objects. Objects in the destination database are listed in the structure of Topic > Database > Table.

   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 configure synchronization objects by importing a CSV file. For more information, see Download and import the settings of synchronization objects. Click Validate. After the validation succeeds, click OK.
   Change the topic	When the topic type is set to BLOB, you can change the topic for objects in the destination. For more information, see Change the topic.
   Configure settings	In the list on the right, hover over the object that you want to set. Click Settings. Click the Shard Columns drop-down list and select the target sharding columns. You can select multiple fields 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 fields 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 destination 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	During data mapping, OMS allows you to remove one or more selected objects to be migrated or synchronized to the destination. Remove a single synchronization object In the list on the right of the selection section, hover over the target object, and click Remove. The synchronization object is removed. Remove all synchronization objects In the list on the right of the selection section, click Remove All in the upper-right corner. In the dialog box that appears, click OK to remove all synchronization objects.

5. Click Next. On the Synchronization Options page, specify the following parameters.

   • Incremental synchronization

     The following table describes the incremental synchronization parameters, which are displayed only when you have selected Incremental Synchronization on the Select Synchronization Type page.

     Parameter	Description
     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. Through resource configuration for the Store component, you can limit the resource consumption of a project in log pull in the incremental synchronization phase. 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. Through resource configuration for the Incr-Sync component, you can limit the resource consumption of a project in data writes in the incremental synchronization phase. Notice In the case of custom configurations, the minimum value is 1 and only integers are supported.
     Incremental Record Retention Time	The duration that incremental parsed files are cached in OMS. A longer retention period 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 project startup time and cannot be modified. If you do not select Full Synchronization as the synchronization type, set this parameter to a certain point of 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, and DefaultExtendColumnType. For more information, see Data formats. Notice This parameter is available only when the topic type is set to BLOB on the Select Synchronization Type page.
     Partitioning Rules	The rule for synchronizing data from the source database to a DataHub topic. Valid values: Hash and Table. 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.
     Business System Identification (Optional)	Identifies the source business system of data. 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 project or component template.

6. Click Precheck.

   During the precheck, OMS detects the connection with the destination data source. Take note of the following items if an error is returned during the precheck:

   • You can identify and troubleshoot the error and then perform the precheck again.

   • You can also click Skip in the Actions column of a failed precheck item. In the dialog box that appears, you can view the prompt for the consequences of the operation and click OK.

7. Click Start Project. If you do not need to start the project now, click Save to go to the details page of the data synchronization project. You can start the project later as needed.

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

   If the data synchronization project encounters a running exception due to a network failure or slow startup of processes, you can click Recover on the Synchronization Projects page or on the Details page of the synchronization project.