Data Transmission Service (DTS) supports two-way data synchronization between two PostgreSQL databases, such as databases in ApsaraDB RDS for PostgreSQL instances and self-managed PostgreSQL databases. This feature applies to scenarios such as active geo-redundancy (unit-based) and geo-disaster recovery. This topic describes how to configure two-way data synchronization between ApsaraDB RDS for PostgreSQL instances.
Prerequisites
The source and destination ApsaraDB RDS for PostgreSQL instances are created. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
For more information about the supported versions of the source and destination databases, see Overview of data synchronization scenarios.
The wal_level parameter is set to logical for the source and destination ApsaraDB RDS for PostgreSQL instances. For more information, see Modify the parameters of an ApsaraDB RDS for PostgreSQL instance.
Limits
Category | Description |
Category | Description |
Limits on the source and destination databases |
DTS cannot synchronize temporary tables in source database, internal triggers, or some internal procedures and functions written in the C programming language. DTS can synchronize custom parameters of the COMPOSITE, ENUM, and RANGE types. The tables to be synchronized must have the PRIMARY KEY, FOREIGN KEY, UNIQUE, or CHECK constraints. |
Other limits |
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Special cases |
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Billing
Synchronization type | Task configuration fee |
Synchronization type | Task configuration fee |
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 data synchronization only between two PostgreSQL databases. DTS does not support two-way data synchronization between multiple PostgreSQL databases.
Conflict detection
To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes. If data records are updated on both nodes, DTS responds to conflicts based on the conflict resolution policy that you specify for the data synchronization task.
DTS checks and fixes conflicts to maximize the stability of two-way synchronization instances. DTS can detect the following types of conflicts:
Uniqueness conflicts caused by INSERT operations
INSERT operations that do not comply with the uniqueness constraint cannot be synchronized. For example, if a record with the same primary key value is inserted into the two synchronization nodes at almost the same time, one of the inserted records fails to be synchronized. The synchronization fails because a record with the same primary key value already exists on the other node.
Inconsistent records caused by 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 primary keys or unique keys of the records to insert may conflict with those of existing records in the destination instance.
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.
During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS does not ensure that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes.
DTS provides conflict resolution policies to prevent conflicts that may occur during data synchronization. You can select a conflict resolution policy when you configure two-way data synchronization.
Supported objects to be synchronized
SCHEMA and TABLE.
Including PRIMARY KEY, UNIQUE KEY, FOREIGN KEY, DATATYPE (built-in data type), and DEFAULT CONSTRAINT.
VIEW, PROCEDURE (PostgreSQL V11 or later.), FUNCTION, RULE, SEQUENCE, EXTENSION, TRIGGER, AGGREGATE, INDEX, OPERATOR, and DOMAIN.
SQL operations that can be synchronized
Operation type | SQL statement |
Operation type | SQL statement |
DML | INSERT, UPDATE, and DELETE |
DDL |
|
DDL operations can be synchronized only in the forward direction from the source database to the destination database. DDL operations cannot be synchronized from the destination database to the source database because DDL operations are ignored in the reverse direction.
Procedure
Purchase an instance for two-way data synchronization. For more information, see Purchase a DTS instance.
On the buy page, set both the Source Instance and Destination Instance parameters to PostgreSQL and set the Synchronization Topology parameter to Two-way Synchronization.
Go to the Data Synchronization page of the new DTS console.
You can also log on to the Data Management (DMS) console. In the top navigation bar, move the pointer over Data Development and choose .
In the upper-left corner of the page, select the region in which the purchased data synchronization instance resides.
Find the data synchronization instance and click Configure Task in the Actions column of the first data synchronization task.
Configure the source and destination databases. The following table describes the parameters.
After you configure the source and destination databases, we recommend that you read the Limits that are displayed on the page. Otherwise, the task may fail or data inconsistency may occur.
Section
Parameter
Description
Section
Parameter
Description
N/A
Task Name
The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database
Database Type
The type of the source database. Select PostgreSQL.
Access Method
The access method of the source database. Select Alibaba Cloud Instance.
Instance Region
The source region that you selected on the buy page. You cannot modify this parameter.
Instance ID
The ID of the source ApsaraDB RDS for PostgreSQL instance.
Database Name
The name of the source database in the source ApsaraDB RDS for PostgreSQL instance.
Database Account
A privileged account of the source ApsaraDB RDS for PostgreSQL instance. The account must be the owner of the source database. For more information about how to create an account for an ApsaraDB RDS for PostgreSQL instance and grant permissions to this account, see Create an account and Create a database.
Database Password
The password that is used to access the database.
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.
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
The type of the destination database. Select PostgreSQL.
Access Method
The access method of the destination database. Select Alibaba Cloud Instance.
Instance Region
The destination region that you selected on the buy page. You cannot modify this parameter.
Instance ID
The ID of the destination ApsaraDB RDS for PostgreSQL instance.
Database Name
The name of the destination database in the destination ApsaraDB RDS for PostgreSQL instance.
Database Account
A privileged account of the destination ApsaraDB RDS for PostgreSQL instance. The account must be the owner of the destination database. For more information about how to create an account for an ApsaraDB RDS for PostgreSQL instance and grant permissions to this account, see Create an account and Create a database.
Database Password
The password that is used to access the database.
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.
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.
Click Test Connectivity and Proceed in the lower part of the page.
Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.
If the source or destination database is a self-managed database and its Access Method is not set to Alibaba Cloud Instance, click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.
Configure the objects to be synchronized.
In the Configure Objects step, configure the objects that you want to synchronize.
Parameter
Description
Parameter
Description
Synchronization Types
The synchronization types. By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization.
If you select Schema Synchronization, DTS synchronizes the schemas of the tables to be synchronized from the source database to the destination database. The schemas include foreign keys.
Processing Mode of Conflicting Tables
Precheck and Report Errors: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the precheck, and the data synchronization task cannot be started.
If the source and destination databases contain tables with identical names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables that are synchronized to the destination database. For more information, see Map object names.
Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases.
If you select Ignore Errors and Proceed, data inconsistency may occur and your business may be exposed to potential risks.
If the source and destination databases have the same schema and a data record in the destination database has the same primary key value or unique key value as a data record in the source database:
During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained.
During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten.
If the source and destination databases have different schemas, data may fail to be initialized. In this case, only some columns are synchronized, or the data synchronization task fails. Proceed with caution.
Synchronization Topology
The synchronization topology of the data synchronization task. Select Two-way Synchronization.
Enable Exactly-Once Write
Specifies whether to enable the Exactly-Once write feature. If the objects to be synchronized include tables that do not have primary keys and UNIQUE constraints, select Yes. For more information, see Synchronize tables without primary keys or UNIQUE constraints.
Exclude DDL Operations
Yes: excludes DDL operations.
No: synchronizes DDL operations.
DDL operations can be synchronized only in the forward direction from the source database to the destination database. DDL operations are ignored in the reverse direction from the destination database to the source database. Therefore, this parameter is displayed only when you configure the task in the forward direction.
Conflict Resolution Policy
If you encounter the conflicts described in the Conflict detection section of this topic, select a conflict resolution policy based on your business requirements.
TaskFailed
If a conflict occurs during data synchronization, the data synchronization task reports an error and exits the process. The task enters a failed state, and you must manually resolve the conflict.
Ignore
If a conflict occurs during data synchronization, the data synchronization task ignores the current statement and continues the process. The conflicting records in the destination database are used.
Overwrite
If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten.
If a latency occurs when you pause or restart a data synchronization task, the selected conflict resolution policy does not take effect during the latency. By default, data in the destination database is overwritten by the data that is synchronized during the latency.
Capitalization of Object Names in Destination Instance
The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to ensure that the capitalization of object names is consistent with that in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Source Objects
Select one or more objects from the Source Objects section and click the
icon to add the objects to the Selected Objects section.
You can select columns, tables, or schemas as objects to be synchronized.
Selected Objects
To rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see the "Map the name of a single object" section of the Map object names topic.
To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see the "Map multiple object names at a time" section of the Map object names topic.
To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize.
To specify WHERE conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Use SQL conditions to filter data.
If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be synchronized.
Click Next: Advanced Settings to configure advanced settings.
Parameter
Description
Parameter
Description
Dedicated Cluster for Task Scheduling
By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Retry Time for Failed Connections
The retry time range for failed connections. If the source or destination database fails to be connected after the data synchronization task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set this parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails.
If you specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range takes precedence.
When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
Retry Time for Other Issues
The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set this parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails.
The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration
During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server.
This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter.
Enable Throttling for Incremental Data Synchronization
Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
Environment Tag
The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment tag is selected.
Monitoring and Alerting
Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values:
No: does not enable alerting.
Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see the "Configure monitoring and alerting when you create a DTS task" section of the Configure monitoring and alerting topic.
Click Next Step: Data Verification to configure data verification.
For more information about how to use the data verification feature, see Configure a data verification task.
Save the task settings and run a precheck.
To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization task only after the task passes the precheck.
If the data synchronization task fails the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, rerun the precheck.
If an alert is triggered for an item during the precheck:
If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issue. Then, run a precheck again.
If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.
Wait until the success rate becomes 100%. Then, click Back.
Configure a data synchronization task in the reverse direction.
Wait until initial synchronization is complete and the data synchronization task in the forward direction is in the Running state.
Find the data synchronization task in the reverse direction and click Configure Task.
Configure the data synchronization task in the reverse direction by performing the operations described from Step 5 to Step 9.
When you configure the data synchronization task in the reverse direction, you must select the correct source and destination instances. The source instance in the reverse direction is the destination instance in the forward direction. The destination instance in the reverse direction is the source instance in the forward direction. You must also make sure that the parameter settings such as the database name, account, and password are consistent.
When you configure the source and destination databases of the data synchronization task in the reverse direction, the Instance Region parameter cannot be modified. The number of parameters that you need to configure for a data synchronization task in the reverse direction is less than that for a data synchronization task in the forward direction. Configure the parameters that are displayed in the console.
When DTS checks for conflicting tables in the reverse direction, the tables that have been synchronized to the destination instance in the forward direction are ignored in the Processing Mode of Conflicting Tables.
You cannot select the Selected Objects of the data synchronization task in the forward direction for the data synchronization task in the reverse direction.
We recommend that you do not use the object name mapping feature when you configure the data synchronization task in the reverse direction. Otherwise, data inconsistency may occur.
After the data synchronization task in the reverse direction is configured, wait until both tasks enter the Running state. Two-way data synchronization is configured.