Configure two-way synchronization between PostgreSQL databases-Data Transmission Service(DTS)-阿里云帮助中心

Data Transmission Service (DTS) supports real-time two-way data synchronization between two PostgreSQL databases, including ApsaraDB RDS for PostgreSQL and self-managed PostgreSQL. Two-way synchronization is ideal for active geo-redundancy (cell-based architecture) and geo-disaster recovery. This topic describes how to configure two-way synchronization between ApsaraDB RDS for PostgreSQL instances. You can apply the same procedure to other PostgreSQL data sources.

Prerequisites

The source and destination ApsaraDB RDS for PostgreSQL instances are created. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
Note
For supported database versions, see Overview of synchronization scenarios.
The wal_level parameter is set to logical for both the source and destination ApsaraDB RDS for PostgreSQL instances. For more information, see Modify instance parameters.

Limitations

Type	Description
Limits on source and destination databases	If the tables to be synchronized do not have a primary key or unique constraint, you must enable the Exactly-Once write feature when configuring the task. Otherwise, data duplication may occur in the destination database. For more information, see Synchronize tables that do not have a primary key or unique constraint. The name of the database to be synchronized cannot contain hyphens (-), for example, dts-testdata. If you synchronize data at the table level and need to edit the objects, such as mapping table or column names, for more than 5,000 tables in a single data synchronization task, we recommend splitting them into multiple tasks or configuring a task to synchronize the entire database. Otherwise, the task may fail upon submission. DTS does not support synchronizing temporary tables, internal triggers (TRIGGER), or certain functions such as C language functions and internal functions for PROCEDURE and FUNCTION from the source database. DTS supports synchronizing some custom data types (TYPE is COMPOSITE, ENUM, or RANGE) and the following constraints: primary key, foreign key, unique, and CHECK. Write-ahead logging (WAL): You must enable WAL by setting the wal_level parameter to logical. For an incremental data synchronization task, DTS requires that the WAL of the source database be retained for more than 24 hours. For a task that performs both full and incremental data synchronization, the WAL must be retained for at least seven days. You can change the retention period to more than 24 hours after the full data synchronization is complete. If a task fails because DTS cannot obtain the WAL due to a shorter retention period, the failure may lead to data inconsistency or loss in extreme cases. Issues caused by an insufficient WAL retention period are not covered by the DTS Service Level Agreement (SLA). Operational limits on the source database To ensure smooth task execution and prevent logical subscription interruptions from a primary/secondary switchover, enable the Logical Replication Slot Failover feature on the ApsaraDB RDS for PostgreSQL instance. For more information, see Logical Replication Slot Failover. Due to the inherent limitations of logical subscriptions, if a single row change during incremental synchronization exceeds 256 MB, the data synchronization instance may fail and cannot be recovered. You must reconfigure the data synchronization instance. Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails. Note During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database. If the source database has long-running transactions and the instance is configured for incremental data synchronization, the write-ahead logging (WAL) data generated before these transactions are committed cannot be cleared. This can cause WAL data to accumulate and may lead to insufficient disk space on the source database. If you perform a major version upgrade on the source database while a synchronization instance is running, it will fail and must be reconfigured. DTS reports consumption checkpoints back to the source database approximately every 15 minutes. During this interval, the source database defers the cleanup of its Write-Ahead Logs (WAL), which causes an increase in the monitoring metric `logical_rep_latency_in_mb`. This is a normal occurrence related to checkpoint reporting latency and does not indicate an actual synchronization delay in DTS. Please refer to the DTS console for the accurate synchronization status.
Other limits	A single data synchronization task can synchronize only one database. To synchronize multiple databases, you must configure a separate task for each. DTS does not support synchronizing TimescaleDB plug-in tables, tables with cross-schema inheritance, or tables with expression-based unique indexes. Schemas created by installing plug-ins are not supported for synchronization. You cannot retrieve information about these schemas in the console when you configure a task. If a two-way synchronization instance in the China (Chengdu) or China (Shanghai) region includes a full data synchronization task, DTS creates a full data verification task for that instance by default. For this verification task, Full Data Verification is set to Verify based on the number of table rows. If you have already configured a full data verification task, your configuration takes precedence. If a table to be synchronized contains a SERIAL type column, the source database automatically creates a sequence for that column. Therefore, when you configure Source Objects, if you select Schema Synchronization for Synchronization Types, we recommend that you also select Sequence or synchronize the entire schema. Otherwise, the synchronization instance may fail. In the following three scenarios, you must run the `ALTER TABLE schema.table REPLICA IDENTITY FULL;` command on the source tables before writing data to them to ensure data consistency. Do not perform table-locking operations while this command is running to prevent deadlocks. If you skip this check during the precheck, DTS automatically runs this command during instance initialization. When the instance runs for the first time. When you select objects at the schema level, and a new table is created in the schema or a table is rebuilt by using the RENAME command. When you use the feature to modify synchronization objects. Note Replace `schema` and `table` in the command with the names of the schema and table to be synchronized. Perform this operation during off-peak hours. DTS verifies data content but does not support verifying metadata such as sequences. You must verify such metadata yourself. After you switch your business to the destination database, sequences do not automatically continue from the maximum value of the corresponding sequences in the source database. Before the switchover, query the maximum value of the corresponding sequence in the source database and use it as the initial value for the corresponding sequence in the destination database. Run the following command to query the sequence values in the source database: `do language plpgsql $$ declare nsp name; rel name; val int8; begin for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S' loop execute format($_$select last_value from %I.%I$_$, nsp, rel) into val; raise notice '%', format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1); end loop; end; $$;` Note The SQL statements output by the preceding command include all sequences from the source database. Run them in the destination database based on your business requirements. DTS creates the following temporary tables in the source database to obtain DDL statements for incremental data, the schema of incremental tables, and heartbeat information. Do not delete these temporary tables during synchronization. Otherwise, the DTS task may fail. The temporary tables are automatically deleted after the DTS instance is released. `public.dts_pg_class`, `public.dts_pg_attribute`, `public.dts_pg_type`, `public.dts_pg_enum`, `public.dts_postgres_heartbeat`, `public.dts_ddl_command`, `public.dts_args_session`, and `public.aliyun_dts_instance`. To ensure the accuracy of the displayed synchronization latency, DTS adds a heartbeat table named dts_postgres_heartbeat to the source database. During data synchronization, DTS creates a logical replication slot with the prefix `dts_sync_` on the source database to read incremental logs. If the data synchronization fails or the synchronization instance is released, DTS will attempt to proactively clean up this replication slot. Note If you change the password of the source database account or delete the IP whitelist for DTS during data synchronization, the replication slot will not be cleaned up automatically. Manually clean up the replication slot in the source database to prevent it from consuming disk space and making the source database unavailable. If the source database fails over, log on to the standby database to perform the cleanup manually. Run the SQL statement `SELECT * FROM pg_replication_slots;` to view all replication slots in the source database. In the results, the DTS replication slot is the record where the slot_name value starts with `dts_sync_` and the active value is `true`. Before you start data synchronization, evaluate the performance of the source and destination databases. Perform the synchronization during off-peak hours. Full data synchronization consumes read and write resources on both databases, which can increase their load. Full data synchronization performs concurrent INSERT operations, which can cause table fragmentation in the destination database. As a result, the tablespace of the destination instance will be larger than that of the source instance after full data synchronization is complete. For table-level data synchronization, if no data from sources other than DTS is written to the destination database, you can use DMS to perform online DDL changes. For more information, see Change schemas without locking tables. During DTS synchronization, do not write data to the destination database from other sources. This can cause data inconsistency between the source and destination databases. For example, if you use DMS to perform online DDL changes while data is being written to the destination database from other sources, data loss may occur. For full or incremental data synchronization tasks, if the source tables to be synchronized contain foreign keys, triggers, or event triggers, DTS temporarily sets the `session_replication_role` parameter to `replica` at the session level. If the destination database account does not have the required permissions, you must manually set the `session_replication_role` parameter to `replica` in the destination database. During this period, if cascade update or delete operations occur in the source database while `session_replication_role` is set to `replica`, data inconsistency may occur. After the DTS task is released, change the `session_replication_role` parameter back to `origin`. A two-way synchronization task includes forward and reverse synchronization tasks. When you configure or reset the task, if the destination object of one task matches the synchronization object of the other task: Allow only one task to synchronize full and incremental data. The other task supports only incremental synchronization. Data from the source of the current task synchronizes only to the destination of the current task. It does not serve as source data for the other task. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters. When you synchronize partitioned tables, you must include both the parent table and its child partitions as synchronization objects. Otherwise, data in the partitioned table may become inconsistent. Important The parent table of a PostgreSQL partitioned table does not store data directly. All data is stored in the child partitions. A data synchronization task must include the parent table and all its child partitions. Otherwise, data from the child partitions may be missed, causing data inconsistency between the source and destination. Synchronization of partitioned tables and inheritance tables (parent-child tables) across different databases is not supported. Ensure that the partitioned tables and all their partitions, and the parent tables and all their child tables, are in the same database.
Special cases	If the source is an ApsaraDB RDS for PostgreSQL instance: During data synchronization, do not modify the endpoint or zone of the ApsaraDB RDS for PostgreSQL instance. Otherwise, the data synchronization task fails. If the source is a self-managed PostgreSQL database: Ensure that the values of the max_wal_senders and max_replication_slots parameters are both greater than the sum of the number of used replication slots in the database and the number of DTS instances that you want to create with this database as the source. If the source is a Cloud SQL for PostgreSQL instance, you must enter a database account that has the `cloudsqlsuperuser` permission in the Database Account field. When you select the objects to be synchronized, you must select objects that this account is authorized to manage, or grant the OWNER permission on the objects to this account. For example, run the `GRANT <owner_of_objects> TO <source_account_for_task>` command to allow the account to perform operations as the owner of the objects. Note An account that has the `cloudsqlsuperuser` permission cannot manage data owned by other accounts that also have the `cloudsqlsuperuser` permission.

Billing

Synchronization type	Pricing
Schema synchronization and full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

Supported synchronization topologies

DTS supports two-way synchronization between two PostgreSQL databases only. Multi-node two-way synchronization topologies are not supported.

Conflict detection

To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated in only one of the database instances in the two-way synchronization. If data records are updated on both database instances, DTS the system will apply the conflict resolution policy configured in the task.

DTS checks and fixes conflicts to maximize the stability of two-way synchronization tasks. DTS can detect the following types of conflicts:

Uniqueness conflicts caused by INSERT operations

In two-way synchronization, if records with the same primary key are inserted into both database instances simultaneously (or in close succession), a uniqueness constraint conflict will be triggered. When the INSERT statement is synchronized to the peer instance, it will fail because a record with the same primary key value already exists.
Mismatched records in UPDATE operations
- If the records to be updated do not exist in the destination instance, DTS converts the UPDATE operation into an INSERT operation. However, uniqueness conflicts may occur.
- The record to be updated by an UPDATE operation causes a primary key or unique key conflict.
Non-existent records to be deleted

The records to be deleted do not exist in the destination instance. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you specify.

Important

Due to time differences and latency, DTS cannot guarantee 100% conflict prevention. To ensure consistency, update records with the same primary or unique key on only one database instance at a time.
DTS provides various conflict resolution strategies for the aforementioned data conflicts, which you can select while configuring two-way data synchronization.

Step 5: Configure objects to synchronize

In the Configure Objects step, set the following parameters:

Parameter	Description
Synchronization Types	Incremental Data Synchronization is selected by default. Also select Schema Synchronization and Full Data Synchronization to sync historical data as the baseline for incremental sync. Selecting Schema Synchronization causes DTS to sync table schemas, including foreign keys.
Synchronization Topology	Select One-way Synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors (default): the precheck fails if the source and destination have tables with the same name. Use object name mapping to rename conflicting tables in the destination if needed. Ignore Errors and Proceed: skips the name conflict check. During full sync, existing destination records with matching primary or unique key values are kept. During incremental sync, they are overwritten. If schemas differ, initialization may partially fail. Use with caution.
Capitalization of Object Names in Destination Instance	Controls how database, table, and column names are capitalized in the destination. Default is DTS default policy. See Specify the capitalization of object names.
Source Objects	Select columns, tables, or schemas and click the arrow icon to move them to Selected Objects.
Selected Objects	Right-click an object to rename it (see Map the name of a single object), filter by SQL operation, or add WHERE conditions (see Set filter conditions). To rename multiple objects at once, click Batch Edit. Note that renaming an object may break synchronization for dependent objects.

Supported SQL operations

Operation type	SQL operation
DML	`INSERT`, `UPDATE`, `DELETE`
DDL	Only data synchronization tasks created after October 1, 2020 (UTC+8) support DDL operation synchronization. Important For data synchronization tasks created before September 9, 2022 (UTC+8), you must create triggers and functions in the source database to capture DDL information before you configure the task. For more information, see Use triggers and functions to implement incremental DDL migration for PostgreSQL. Data of the BIT type cannot be synchronized during incremental data synchronization. DTS can synchronize the following DDL statements if the database account for the source database is a privileged account, the major version of the ApsaraDB RDS for PostgreSQL instance is 9.5 or later, and the minor version is 20210228 or later. For more information about how to update the minor version, see Update the minor engine version. `CREATE TABLE`, `DROP TABLE` `ALTER TABLE` (including `RENAME TABLE`, `ADD COLUMN`, `ADD COLUMN DEFAULT`, `ALTER COLUMN TYPE`, `DROP COLUMN`, `ADD CONSTRAINT`, `ADD CONSTRAINT CHECK`, `ALTER COLUMN DROP DEFAULT`) `TRUNCATE TABLE` (The source PostgreSQL database must be PostgreSQL 11 or later.) `CREATE INDEX ON TABLE` Important Additional information in DDL statements, such as `CASCADE` or `RESTRICT`, cannot be synchronized. DDL statements from a session that executes the `SET session_replication_role = replica` command cannot be synchronized. DDL statements that are executed by calling a `FUNCTION` are not supported. If multiple SQL statements submitted by the source database at the same time contain both DML and DDL statements, the DDL statements are not synchronized. If multiple SQL statements submitted by the source database at the same time contain DDL statements for objects that are not synchronized, the DDL statements are not synchronized. DDL statements executed directly within a plug-in through the Server Programming Interface (SPI) are not supported.

Important

DDL statements are synchronized only in forward tasks (source to destination). Reverse tasks automatically filter out DDL statements.
When you execute DDL to modify a table schema during bidirectional synchronization, ensure DML writes on both databases are consistent with the new schema. For example, if you delete a column from the source, the destination may still write data based on the old schema that includes the deleted column, causing failures.

Procedure

Purchase a two-way data synchronization instance. For more information, see Purchase a synchronization instance.
Important
On the buy page, set both the source and destination database type to PostgreSQL, and set the synchronization topology to Two-way Synchronization.
Go to the Data Synchronization page of the new DTS console.
Note
Alternatively, log on to the DMS console. In the top menu bar, choose Data + AI. Then, in the navigation pane on the left, choose Data Transmission (DTS) > Data Synchronization.
In the upper-left corner of the page, select the region of the destination instance.
Find the synchronization instance that you purchased and click Configure Task in the Actions column of the first synchronization task.

Configure the source and destination databases.

Warning

After you select the source and destination instances, review the Limits at the top of the page. Otherwise, the task may fail or data inconsistency may occur.

Category	Parameter	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Database Type	Select PostgreSQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	The source instance region that you selected at purchase. This value cannot be changed.
	Instance ID	Select the source ApsaraDB RDS for PostgreSQL instance ID.
	Database Name	Enter the name of the database to which the objects to be synchronized belong on the source ApsaraDB RDS for PostgreSQL instance.
	Database Account	Enter an account with elevated privileges on the source ApsaraDB RDS for PostgreSQL instance. The account must be the owner of the specified database. For more information, see Create an account and Create a database.
	Database Password	Enter the password for the specified database account.
	Encryption	Specifies whether to encrypt the connection to the source database. You can configure this parameter based on your business requirements. In this example, Non-encrypted is selected. If you want to establish an SSL-encrypted connection to the source database, perform the following steps: Select SSL-encrypted, upload CA Certificate, Client Certificate, and Private Key of Client Certificate as needed, and then specify Private Key Password of Client Certificate. Note If you set Encryption to SSL-encrypted for a self-managed PostgreSQL database, you must upload CA Certificate. If you want to use the client certificate, you must upload Client Certificate and Private Key of Client Certificate and specify Private Key Password of Client Certificate. For information about how to configure SSL encryption for an ApsaraDB RDS for PostgreSQL instance, see SSL encryption.
Destination Database	Database Type	Select PostgreSQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	The destination instance region that you selected at purchase. This value cannot be changed.
	Instance ID	Select the destination ApsaraDB RDS for PostgreSQL instance ID.
	Database Name	Enter the name of the database to which the objects to be synchronized belong on the destination ApsaraDB RDS for PostgreSQL instance.
	Database Account	Enter an account with elevated privileges on the destination ApsaraDB RDS for PostgreSQL instance. The account must be the owner of the specified database. For more information, see Create an account and Create a database.
	Database Password	Enter the password for the specified database account.
	Encryption	Specifies whether to encrypt the connection to the source database. You can configure this parameter based on your business requirements. In this example, Non-encrypted is selected. If you want to establish an SSL-encrypted connection to the source database, perform the following steps: Select SSL-encrypted, upload CA Certificate, Client Certificate, and Private Key of Client Certificate as needed, and then specify Private Key Password of Client Certificate. Note If you set Encryption to SSL-encrypted for a self-managed PostgreSQL database, you must upload CA Certificate. If you want to use the client certificate, you must upload Client Certificate and Private Key of Client Certificate and specify Private Key Password of Client Certificate. For information about how to configure SSL encryption for an ApsaraDB RDS for PostgreSQL instance, see SSL encryption.

After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
- Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.
- If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the task objects.

On the Configure Objects page, specify the objects to synchronize.

Parameter	Description
Synchronization Types	DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization. Note If Schema Synchronization is selected, DTS synchronizes the table schemas (including foreign keys) from the source database to the destination database.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful. Note If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Object name mapping. Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.
Synchronization Topology	Select Two-way Synchronization.
Exclude DDL Operations	Yes: DDL operations are not synchronized. No: DDL operations are synchronized. Important To ensure stability of the two-way synchronization, only the forward task (source to destination) lets you choose whether to synchronize DDL operations. The reverse task (destination to source) automatically filters out DDL operations.
Conflict Resolution Policy	If you encounter any of the conflict types described above, select a conflict resolution policy based on your business requirements. TaskFailed (Fail and report an error upon conflict) The task will error out and stop if a conflict occurs. The task enters a failed state and requires manual intervention for recovery. Ignore (Keep the existing record in the destination instance) The system skips the conflicting statement and continues the synchronization. The existing record in the destination instance is retained. Overwrite (Overwrite the conflicting record in the destination instance) If a conflict occurs, the system overwrites the conflicting record in the destination instance with the data from the source. Note If the synchronization task is paused or restarted and has latency, these policies are temporarily ineffective. During the latency period, destination data is overwritten by default.
Capitalization of Object Names in Destination Instance	Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note You can select schemas or tables as synchronization objects. If you select tables, other objects, such as views, triggers, and stored procedures, are not synchronized to the destination database. If a table to be synchronized contains columns of the SERIAL type and you select Schema Synchronization as a synchronization type, we recommend that you also select the sequences or synchronize the entire schema.
Selected Objects	To rename a single object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map a single object name. To rename multiple objects in bulk, click Batch Edit in the upper-right corner of the Selected Objects box. For more information, see Map multiple object names in bulk. Note To select specific SQL operations to synchronize at the schema or table level, right-click the target object in the Selected Objects section, and then select the SQL operations in the dialog box. To filter data with WHERE conditions, right-click a table in the Selected Objects section, and then set the filter conditions in the dialog box. For more information, see Filter data by using SQL conditions. Using the object name mapping feature may cause dependent objects to fail to synchronize.

Click Next: Advanced Settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.
Retry Time for Failed Connections	If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails. Note If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances. DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.
Retry Time for Other Issues	If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Note This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
Enable Throttling for Incremental Data Synchronization	You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
Environment Tag	Optional. Select an environment tag to label the instance. This is not required in this example.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert contact. For more information, see Configure monitoring and alerting during task configuration.

Click Data Verification to configure a data verification task.
To use the data verification feature, see Configure data verification.

Save the task and perform a precheck.
- To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.
- If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.
- If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.
- If the precheck generates warnings:
  For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.
  For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.
After Success Rate reaches 100%, click Back.
Configure the reverse synchronization task.
1. Wait for the forward synchronization task to complete initial synchronization. The Status must show Running.
2. Locate the reverse synchronization task and click Configure Task.
3. Follow Step 5 through Step 9 to configure the reverse synchronization task.
  Important
  When you configure the reverse task, swap the source and destination instances: the forward task's destination becomes the reverse task's source, and vice versa. Verify that all instance details (database name, account, and password) are correct.
  The Instance Region fields for the reverse task cannot be changed. The reverse task requires fewer parameters than the forward task. Configure only the parameters shown in the console.
  The Processing Mode of Conflicting Tables check for the reverse task does not detect tables that were synchronized by the forward task.
  The reverse task cannot synchronize objects from the Selected Objects list of the forward task.
  Do not use the object name mapping feature for the reverse task. Doing so may cause data inconsistencies.
After configuring the reverse task, wait for the Status of both synchronization tasks to show Running. The two-way synchronization is now fully configured.