Synchronize data from a MongoDB database to an AnalyticDB for PostgreSQL instance - Data Transmission Service

Data Transmission Service (DTS) allows you to synchronize data from a MongoDB database to an AnalyticDB for PostgreSQL instance. This topic describes how to synchronize data from an ApsaraDB for MongoDB replica set instance to an AnalyticDB for PostgreSQL instance by using DTS.

Prerequisites

The destination AnalyticDB for PostgreSQL instance is created. The available storage space of this instance is larger than the total size of data in the source ApsaraDB for MongoDB instance. For more information about how to create an AnalyticDB for PostgreSQL instance, see Create an instance.
Note
The available storage space of the destination database is 10% larger than the total size of data in the source database. This is a recommended prerequisite.
A database, a schema, and a table with a primary key column are created in the destination AnalyticDB for PostgreSQL instance to receive data. For more information, see SQL syntax.
Important
- Make sure that the data type of data in the destination table is compatible with the data in the source ApsaraDB for MongoDB instance. For example, if the _id field of the ApsaraDB for MongoDB instance is of the ObjectId type, the data type of the AnalyticDB for PostgreSQL instance must be varchar.
- Do not name the columns of the destination table in the AnalyticDB for PostgreSQL instance as _id or _value.
The endpoints of all shard nodes are obtained and the usernames and passwords of the accounts used to log on to the shard nodes are consistent if the source database is an ApsaraDB for MongoDB sharded cluster instance. For more information, see Apply for an endpoint for a shard or ConfigServer component.

Usage notes

Category	Description
Limits on the source database	Bandwidth requirements: The server on which the source database is deployed must have sufficient outbound bandwidth. Otherwise, the data synchronization speed is affected. If you want to modify collections in the destination database, such as configuring name mapping for collections, you can synchronize up to 1,000 collections in a single data synchronization task. If you run a task to synchronize more than 1,000 collections, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the collections or configure a task to synchronize the entire database. If the source database is an ApsaraDB for MongoDB sharded cluster instance, the _id field in the collection to be synchronized must be unique. Otherwise, data inconsistency may occur. If the source database is an ApsaraDB for MongoDB sharded cluster instance, the number of Mongos nodes in the instance cannot exceed 10. You must also make sure that the source ApsaraDB for MongoDB sharded cluster instance does not contain orphaned documents. Otherwise, data inconsistency may occur and the task may fail. For more information, see the MongoDB documentation and the How do I delete orphaned documents of a MongoDB database deployed in the sharded cluster architecture? section of the FAQ topic. You cannot use the standalone ApsaraDB for MongoDB instance, Azure Cosmos DB for MongoDB cluster or an Amazon DocumentDB elastic cluster as the source database. The oplog feature must be enabled for the source database and must retain log data for at least seven days. Alternatively, change streams must be enabled to ensure that DTS can subscribe to data changes in the source database within the last seven days. Otherwise, DTS may fail to obtain data changes in the source database and data synchronization fails. In some circumstances, data inconsistency or data loss may occur. Issues that arise in such circumstances are not covered by the service level agreement (SLA) of DTS. Important We recommend that you use the oplog to record data changes in the source database. Only MongoDB 4.0 and later allow you to use change streams to obtain data changes in the source database. Two-way synchronization is not supported when you use change streams to obtain data changes in the source database. If the source database is a non-elastic Amazon DocumentDB cluster, you must enable change streams and set the Migration Method parameter to ChangeStream and the Architecture parameter to Sharded Cluster. Limits on operations to be performed on the source database: During full data synchronization, do not change the schemas of databases or collections or data of the ARRAY type. Otherwise, the data synchronization task fails, or data inconsistency occurs between the source and destination databases. If you perform only full data synchronization, do not write data to the source database during data synchronization. Otherwise, data inconsistency occurs between the source and destination databases. If the source MongoDB instance uses the sharded cluster architecture, do not run commands that change the data distribution of the objects to be synchronized during the synchronization task. These commands include shardCollection, reshardCollection, unshardCollection, moveCollection, and movePrimary. Otherwise, data inconsistency may occur. If the source database is a MongoDB instance that uses the sharded cluster architecture and the balancer of the source database balances data, latency may occur in the instance. DTS cannot connect MongoDB database over a SRV endpoint.
Other limits	You can select only collections as the objects to be synchronized. The table to receive data in the destination AnalyticDB for PostgreSQL instance must have a unique primary key column rather than a composite primary key, and the value `bson_value("_id")` must be assigned to the primary key column in the Selected Objects section. The table to receive data in the destination AnalyticDB for PostgreSQL instance cannot contain fields that are named _id or _value. Otherwise, data synchronization fails. If the data type of the destination AnalyticDB for PostgreSQL instance is incompatible with the data in the source ApsaraDB for MongoDB instance, the data synchronization task fails. DTS cannot synchronize data from the admin or local database. The destination table cannot be an append-optimized (AO) table. Transaction information is not retained. When transactions are synchronized to the destination database, the transactions are converted into single records. Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers. During full data synchronization, concurrent INSERT operations cause fragmentation in the collections of the destination database. After full data synchronization is complete, the storage usage of collections in the destination database is larger than that of collections in the source database. Make sure that the precision settings for columns of the FLOAT or DOUBLE data type meet your business requirements. DTS uses the `ROUND(COLUMN,PRECISION)` function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for columns of the FLOAT data type to 38 digits and the precision for columns of the DOUBLE data type to 308 digits. DTS attempts to resume data synchronization tasks that failed within the last seven days. Before you switch your workloads to the destination database, you must stop or release the failed tasks. You can also execute the `REVOKE` statement to revoke the write permissions from the accounts that are used by DTS to access the destination database. Otherwise, the data in the source database overwrites the data in the destination instance after the task is automatically resumed. DTS calculates the latency of incremental data synchronization based on the timestamp of the latest synchronized data in the destination database and the current timestamp in the source database. If no update operation is performed on the source database for an extended period of time, the synchronization latency may be inaccurate. If the latency of the data synchronization task is excessively high, you can perform an update operation on the source database to update the latency. If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Billing

Synchronization type	Task configuration fee
Full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

Synchronization types

Synchronization type	Description
Full data synchronization	DTS synchronizes the historical data of the selected objects from the source ApsaraDB for MongoDB instance to the destination AnalyticDB for PostgreSQL instance.
Incremental data synchronization	After full data synchronization is complete, DTS synchronizes incremental data from the source ApsaraDB for MongoDB instance to the destination AnalyticDB for PostgreSQL instance. Note Only the operations that are performed to insert, update, or delete documents in a collection can be synchronized during incremental data synchronization When a DTS task synchronizes incremental data of a file, Only `$set` command can be synchronously run.

Permissions required for database accounts

Database	Required permissions	References
Source MongoDB instance	Read permissions on the source, admin, and local databases.	Manage the permissions of MongoDB database users
Destination AnalyticDB for PostgreSQL instance	Read and write permissions on the destination database	Create and manage a database account and Manage users and permissions Note You can use the initial account or an account that has the RDS_SUPERUSER permission.

Procedure

Use one of the following methods to go to the Data Synchronization page and select the region in which the data synchronization instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Synchronization.
3. In the upper-left corner of the page, select the region in which the data synchronization instance resides.
DMS console
Note
The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI and choose DTS (DTS) > Data Synchronization.
3. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.

Configure the source and destination databases. The following table describes the parameters.

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	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	The type of the source database. Select MongoDB.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Architecture	The architecture in which the source instance is deployed. In this example, Replica Set is selected. Note If the source ApsaraDB for MongoDB instance is deployed in the Sharded Cluster architecture, you must configure the Shard account and Shard password parameters.
	Migration Method	The method used to synchronize incremental data from the source database. Select a method based on your business requirements. Valid values: Oplog (recommended): This option is available if the oplog feature is enabled for the source database. Note By default, the oplog feature is enabled for both self-managed MongoDB databases and ApsaraDB for MongoDB instances. This feature allows you to synchronize incremental data at a low latency because of a fast log pulling speed. Therefore, we recommend that you select Oplog for the Migration Method parameter. ChangeStream: This option is available if change streams are enabled for the source database. For more information, see Change Streams. Note If the source database is an inelastic Amazon DocumentDB cluster, you can set the Migration Method parameter only to ChangeStream. If you select Sharded Cluster for the Architecture parameter, you do not need to configure the Shard account and Shard password parameters.
	Instance ID	The ID of the source ApsaraDB for MongoDB instance.
	Authentication Database	The name of the authentication database that stores the database accounts and passwords of the source ApsaraDB for MongoDB instance. If you did not change the name before, the default value admin is used.
	Database Account	The database account of the source ApsaraDB for MongoDB instance. For more information about the permissions required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the source database. You can select Non-encrypted, SSL-encrypted, or Mongo Atlas SSL based on your business requirements. The options available for the Encryption parameter are determined by the values selected for the Access Method and Architecture parameters. The options displayed in the DTS console prevail. Note If the Architecture parameter is set to Sharded Cluster, and the Migration Method parameter is set to Oplog for the ApsaraDB for MongoDB database, the Encryption parameter SSL-encrypted is unavailable. If the source database is a self-managed MongoDB database that uses the Replica Set architecture, the Access Method parameter is not set to Alibaba Cloud Instance, and the Encryption parameter is set to SSL-encrypted, you can upload a certification authority (CA) certificate to verify the connection to the source database.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	The type of the destination database. Select AnalyticDB for PostgreSQL.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination AnalyticDB for PostgreSQL instance resides.
	Instance ID	The ID of the destination AnalyticDB for PostgreSQL instance.
	Database Name	The name of the database that is used to receive the synchronized objects in the destination AnalyticDB for PostgreSQL instance.
	Database Account	The database account of the destination AnalyticDB for PostgreSQL instance. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password that is used to access the database.

Click Test Connectivity and Proceed in the lower part of the page.
Note
- 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
Synchronization Types	By default, Incremental Data Synchronization is selected. You can select only Full Data Synchronization. You cannot select Schema Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination database. The historical data is the basis for subsequent incremental synchronization.
DDL and DML Operations to Be Synchronized	Select the DDL and DML operations that you want to synchronize at the instance level during incremental data synchronization. Note To synchronize DDL and DML operations at the collection level during incremental data synchronization, right-click a collection in the Selected Objects section. In the dialog box that appears, select the operations that you want to synchronize.
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. Note 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. Warning 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.
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. Note You can select collections as the objects to be synchronized.
Selected Objects	Modify the database name. In the Selected Objects section, right-click the database to which the collections to be synchronized belong. Change the value of Database Name to the name of the schema that is used to receive data in the destination AnalyticDB for PostgreSQL instance. Optional. In the Select DDL and DML Operations to Be Synchronized section, select the operations that you want to synchronize during incremental data synchronization. Click OK. Modify table names. In the Selected Objects section, right-click a collection. Change the value of Table Name to the name of the table that is used to receive data in the destination AnalyticDB for PostgreSQL instance. Optional. You can specify conditions to filter data. For more information, see Specify filter conditions. Optional. In the Select DDL and DML Operations to Be Synchronized section, select the operations that you want to synchronize during incremental data synchronization. Specify the fields to be synchronized from the source ApsaraDB for MongoDB instance. By default, DTS maps the data of a collection to be synchronized and configures an expression in the Assign Value column. You must check whether the expression meets your requirements and specify the Column Name, Type, Length, and Precision parameters. In the `bson_value()` expression of the Assign Value column, view the field name of the row of data in the ApsaraDB for MongoDB instance. The field in `""` is the field name in the ApsaraDB for MongoDB instance. For example, if the expression is `bson_value("age")`, `age` is the field name of the row of data in the ApsaraDB for MongoDB instance. Optional: You can delete fields that do not need to be synchronized. Note Click the icon after the row of data to delete fields that do not need to be synchronized. Specify the fields to be synchronized. Perform one of the following procedures based on whether the `bson_value()` expression meets your requirements. The expression meets field requirements Configure the Column Name parameter. Note Enter the name of the column in the table that is used to receive data in the destination AnalyticDB for PostgreSQL instance. Select a data Type for each column. Important Make sure that the data type of the destination table is compatible with the data in the source ApsaraDB for MongoDB instance. Optional: Specify a data length and a precision for each column. Repeat the preceding steps to map the columns of the source table onto the columns of the destination table. The expression does not meet field requirements Note For example, fields with hierarchical relationships such as parent-child structure. In the Actions column, click the icon after the row of the data. Click + Add Column. Configure the Column Name, Type, Length, and Precision parameters. Enter the `bson_value()` expression in the Assign Value field. For more information, see the Example of value assignment section of this topic. Important You must assign `bson_value("_id")` to the primary key column of the destination table. You must specify the field and subfield of each column in the corresponding `bson_value()`expression based on the hierarchical relationship. Otherwise, data loss may occur or the task may fail. Repeat the preceding steps to map the columns of the source table onto the columns of the destination table. Click OK.

Click Next: Advanced Settings to configure advanced settings.

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. Note 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. Important 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 Synchronization	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. Note You can configure this parameter only if Full Data Synchronization is selected for the Synchronization Types parameter.
Only one data type for primary key _id in a table of the data to be synchronized	Whether the data type for primary key `_id` in a collection of the data to be synchronized is unique. Valid value: Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter. Yes: The data type is unique. During full data synchronization, DTS does not scan the data type for primary key `_id` of the data to be synchronized from the source database. No: The data type is not unique. During full data synchronization, DTS scans the data type for primary key `_id` of the data to be synchronized from the source database.
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, you do not need to configure this parameter.
Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.
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.

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.
Note
- 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.

Purchase an instance.

Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the buy page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for a subscription when you create a data synchronization instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go data synchronization instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the data synchronization instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides instance classes that vary in synchronization speed. You can select an instance class based on your business requirements. For more information, see Instance classes of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, specify the subscription duration and the number of data synchronization instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is available only if you select the Subscription billing method.

Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the dialog box that appears, click OK.
You can view the progress of the task in the task list.

Example of value assignment

Data structure of the source ApsaraDB for MongoDB instance

{
  "_id":"62cd344c85c1ea6a2a9f****",
  "person":{
    "name":"neo",
    "age":26,
    "sex":"male"
  }
}

Table schema of the destination AnalyticDB for PostgreSQL instance

Column name	Type
mongo_id	varchar Note The primary key column.
person_name	varchar
person_age	decimal

Configuration of additional columns

Important

You must specify the field and subfield of each column in the corresponding bson_value() expression based on the hierarchical relationship. Otherwise, data loss may occur or the task may fail. For example, if you specify only the person field of the source column by using the bson_value("person") expression, DTS cannot write the incremental data in the subfields of the person field, such as name, age, and sex, to the destination column.

Column name	Type	Value
mongo_id	STRING	bson_value("_id")
person_name	STRING	bson_value("person","name")
person_age	DECIMAL	bson_value("person","age")