Updated on 2023-04-26 GMT+08:00

From MongoDB to DDS

Supported Source and Destination Databases

Table 1 Supported databases

Source DB

Destination DB

  • On-premises MongoDB databases
  • MongoDB databases on an ECS
  • MongoDB database on other clouds
  • DDS instances
  • DDS instances

Prerequisites

  • You have logged in to the DRS console.
  • Your account balance is greater than or equal to $0 USD.
  • For details about the DB types and versions supported by real-time synchronization, see Supported Databases.
  • If a subaccount is used to create a DRS task, ensure that an agency has been added. To create an agency, see Agency Management.

Suggestions

  • The success of database synchronization depends on environment and manual operations. To ensure a smooth synchronization, perform a synchronization trial before you start the synchronization to help you detect and resolve problems in advance.
  • During the synchronization, ensure that no data is written to the destination database to keep data consistency before and after the synchronization.
  • Start your synchronization task during off-peak hours. A less active database is easier to synchronize successfully. If the data is fairly static, there is less likely to be any severe performance impacts during the synchronization.
    • If network bandwidth is not limited, the query rate of the source database increases by about 20 MB/s during full synchronization, and two to four CPUs are occupied.
  • Data-Level Comparison

    To obtain accurate comparison results, compare data at a specified time point during off-peak hours. If it is needed, select Start at a specified time for Comparison Time. Due to slight time difference and continuous operations on data, data inconsistency may occur, reducing the reliability and validity of the comparison results.

Precautions

Before creating a synchronization task, read the restrictions listed in the following table.

  • You are advised to create an independent database account for DRS task connection to prevent task failures caused by database account password modification.
  • After changing the account passwords for the source or destination databases, modify the connection information in the DRS task as soon as possible to prevent automatic retry after a task failure. Automatic retry will lock the database accounts.
Table 2 Precautions

Type

Restrictions

Database permissions

Source database (minimum permissions):

  • Permission requirements for full plus incremental synchronization:
    • Replica set: The source database user must have the readAnyDatabase permission for the admin database and the read permission for the local database.

Minimum permission requirements: The destination database user must have the dbAdminAnyDatabase permission for the admin database and the readWrite permission for the destination database.

Synchronization object constraints

  • Replica set: Only collections (including validator and capped and non-capped collections), indexes, and views can be synchronized.
  • The system database (such as local, admin, and config) and system collection cannot be synchronized. If service data is in the system database, run renameCollection to move the service data to the user database.
  • The statement for creating a view cannot contain a regular expression.
  • Collections that contain the _id field without indexes are not supported.
  • The first parameter of BinData() cannot be 2.
  • The mapped synchronization object name can contain 1 to 63 characters. The following characters are not allowed /\."$<>

Source database

  • During the incremental synchronization, the Oplog of the source database must be enabled.
  • If the storage space is sufficient, store the source database Oplog for as long as possible. The recommended retention period is three days.
  • The source database name cannot contain /\.$ or spaces. The collection name or view name cannot start with system. or contain the dollar sign ($).
  • The source cannot be a GaussDB(for Mongo) instance.

Destination database

  • The destination DB instance is running properly.
  • The destination DB instance must have sufficient storage space.
  • Data cannot be synchronized from a newer version database to an older version database.

Precautions

  • Objects that have dependencies must be synchronized at the same time to avoid synchronization failure. Common dependencies: collections referenced by views, and views referenced by views
  • Replica set: The MongoDB replica set instance must be available and have primary nodes.
  • If the source is a replica set instance, the following operations and commands are supported during incremental synchronization:
    • Adding, deleting, and updating documents
    • Creating and deleting collections
    • Creating and deleting indexes
    • Creating and deleting views
    • The convertToCapped, collMod, and renameCollection commands are supported.
  • If a Time-to-Live (TTL) index already exists in the collection of the source database or is created during an incremental synchronization, data consistency cannot be ensured when source and destination databases are in different time zone.
  • The value of block_compressor is determined by stats().wiredTiger.creationString.block_compressor of the collection in the source database. If the destination database contains corresponding empty collections, the compression parameters will not be synchronized. If the compression parameters in the source database are not supported by the destination database, configure the compression parameters based on net.compression.compressors of the destination database. If the destination database version is DDS 4.2, DRS does not synchronize compression parameters because the destination database does not support compression parameter settings.
  • If the MongoDB service of the source database is deployed with other services on the same server, set the value of the cacheSizeGB parameter to the half of the minimum idle cache for the WiredTiger engine of the source database.
  • During an incremental synchronization of collections, renaming collections is not recommended.
  • Specify the information about all primary and secondary nodes of the replica set to prevent the primary/standby switchover from affecting the synchronization task. If you enter information about multiple primary and secondary nodes, ensure that all nodes belong to the same replica set instance.
  • To ensure data consistency, do not perform operations (including but not limited to DDL and DML operations) on the destination database during the synchronization.
  • During synchronization, do not modify or delete the usernames, passwords, permissions, or ports of the source and destination databases.
  • During task startup or full migration, you are not advised to perform DDL operations on the source database, such as deleting databases, collections, indexes, documents, or views. Otherwise, the synchronization may fail. During task startup or full migration, you are not advised to perform DDL operations on the source database, such as deleting databases, collections, indexes, documents, or views. Otherwise, the synchronization may fail.
  • During the synchronization, data rollback caused by a primary/standby switchover of the source database is not supported.
  • Documents larger than 16 MB in the source database cannot be inserted or updated during full or incremental synchronization.
  • In the incremental synchronization phase, concurrent replay is performed at the collection level to maintain the synchronization performance. In the following scenarios, only single-thread write is supported and concurrent replay is not supported:
    • The collection index contains a unique key.
    • The value of capped of the collection attribute is true.

    In either of the preceding scenarios, the task delay may increase.

  • To accelerate the synchronization, delete unnecessary indexes from the source database and retain only necessary indexes before the synchronization. You are advised not to create indexes for the source database during the synchronization. If indexes must be created, create them in the background.
  • To prevent loopback, do not start tasks that migrate the same database to and out of the cloud at the same time.
    NOTE:

    The source and destination databases in a synchronization task can be the same database. To avoid replication loop, you must rename object names.

Procedure

This section uses DDS replica set as an example to describe how to configure a MongoDB to DDS replica set synchronization task over a VPC network.

  1. On the Data Synchronization Management page, click Create Synchronization Task.
  2. On the Create Synchronization Instance page, select a region and project, specify the task name, description, and the synchronization instance details, and click Create Now.

    • Task information description
      Figure 1 Synchronization task information
      Table 3 Task and recipient description

      Parameter

      Description

      Region

      The region where the synchronization instance is deployed. You can change the region.

      Project

      The project corresponds to the current region and can be changed.

      Task Name

      The task name must start with a letter and consist of 4 to 50 characters. It can contain only letters, digits, hyphens (-), and underscores (_).

      Description

      The description consists of a maximum of 256 characters and cannot contain special characters !=<>'&"\

    • Synchronization instance information
      Figure 2 Synchronization instance information
      Table 4 Synchronization instance settings

      Parameter

      Description

      Data Flow

      Select To the cloud.

      The destination database must be a database on the current cloud.

      Source DB Engine

      Select MongoDB.

      Destination DB Engine

      Select DDS.

      Network Type

      Available options: VPC, VPN or Direct Connect, and Public network. By default, the value is Public network.

      Destination DB Instance

      Select the DB instance you have created.

      Synchronization Instance Subnet

      Select the subnet where the synchronization instance is located. You can also click View Subnet to go to the network console to view the subnet where the instance resides.

      By default, the DRS instance and the destination DB instance are in the same subnet. You need to select the subnet where the DRS instance resides and ensure that there are available IP addresses. To ensure that the synchronization instance is successfully created, only subnets with DHCP enabled are displayed.

      Synchronization Mode

      Full+Incremental: This synchronization type allows you to synchronize data without interrupting services. After a full synchronization initializes the destination database, an incremental synchronization initiates and parses logs to ensure data consistency between the source and destination databases.
      NOTE:

      If you select Full+Incremental, data generated during the full synchronization will be continuously synchronized to the destination database, and the source remains accessible.

      Source DB Instance Type

      If Synchronization Mode is set to Full+Incremental, set this parameter based on the source database. Currently, the source database supports only replica set, which is a non-cluster database.

    • Enterprise Projects and Tags
      Figure 3 Enterprise projects and tags
      Table 5 Enterprise Projects and Tags

      Parameter

      Description

      Enterprise Project

      An enterprise project you would like to use to centrally manage your Global Accelerator resources. Select an enterprise project from the drop-down list. The default project is default. For more information about enterprise project, see Enterprise Management User Guide.

      To customize an enterprise project, click Enterprise in the upper right corner of the console. The Enterprise Management page is displayed. For details, see Creating an Enterprise Project in Enterprise Management User Guide.

      Tags

      • This setting is optional. Adding tags helps you better identify and manage your tasks. Each task can have up to 10 tags.
      • After a task is created, you can view its tag details on the Tags tab. For details, see Tag Management.

    If a task fails to be created, DRS retains the task for three days by default. After three days, the task automatically ends.

  3. On the Configure Source and Destination Databases page, wait until the synchronization instance is created. Then, specify source and destination database information and click Test Connection for both the source and destination databases to check whether they have been connected to the synchronization instance. After the connection tests are successful, select the check box before the agreement and click Next.

    • Source database information
      Figure 4 Source database information
      Table 6 Source database settings

      Parameter

      Description

      Database Types

      The source database type can be Self-built on ECS or a DDS DB Instance. In this example, DDS DB Instance is selected.

      VPC

      This option is available if the source database type is Self-built on ECS. VPC is used to isolate networks for different services.

      Subnet

      This option is available if the source database type is Self-built on ECS. Subnet provides dedicated and isolated networks for different services and controls access. The subnet must be in the AZ where the source database resides. You need to enable DHCP for creating the source database subnet.

      IP Address or Domain Name

      The IP address or domain name of the source database. This option is available if the source database type is Self-built on ECS.

      DB Instance Name

      The DDS DB instance as the source. This option is available if the source database type is DDS DB Instance.

      Authentication Database

      The name of the authentication database. For example: The default authentication database of DDS instance is admin.

      Database Username

      The username for accessing the source database.

      Database Password

      The password for the database username.

      SSL Connection

      Whether to encrypt the database connections. This option is available if the source database type is Self-built on ECS. If SSL is enabled, upload the SSL CA root certificate.

      NOTE:
      • The maximum size of a single certificate file that can be uploaded is 500 KB.
      • If SSL is not enabled, your data may be at risk.

      The IP address, domain name, username, and password of the source database are encrypted and stored in the system until the task is deleted.

    • Destination database configuration
      Figure 5 Destination database information
      Table 7 Destination database settings

      Parameter

      Description

      DB Instance Name

      The default value is the DB instance selected when the synchronization task is created and cannot be changed.

      Database Username

      The username for accessing the destination database.

      Database Password

      The password for the database username.

      The username and password of the destination database are encrypted and stored in the database and the synchronization instance during the synchronization. After the task is deleted, the username and password are permanently deleted.

  4. On the Set Synchronization Task page, select the objects to be synchronized, and then click Next.

    Figure 6 Synchronization objects
    Table 8 Synchronization object

    Parameter

    Description

    Flow Control

    You can choose whether to control the flow.

    • Yes

      You can customize the maximum migration speed.

      In addition, you can set the time range based on your service requirements. The traffic rate setting usually includes setting of a rate limiting time period and a traffic rate value. Flow can be controlled all day or during specific time ranges. The default value is All day. A maximum of three time ranges can be set, and they cannot overlap.

      The flow rate must be set based on the service scenario and cannot exceed 9,999 MB/s.

      Figure 7 Flow control
    • No
      The synchronization speed is not limited and the outbound bandwidth of the source database is maximally used, which will increase the read burden on the source database. For example, if the outbound bandwidth of the source database is 100 MB/s and 80% bandwidth is used, the I/O consumption on the source database is 80 MB/s.
      NOTE:
      • The flow control mode takes effect only in the full synchronization phase.
      • You can also change the flow control mode after creating a task. For details, see Modifying the Flow Control Mode.

    Synchronization Object

    The left pane displays the source database objects, and the right pane displays the selected objects. DRS supports table- and database-level synchronization. You can select data for synchronization based on your service requirements.

    If the synchronization objects in source and destination databases have different names, you can map the source object name to the destination one. For details, see Mapping Object Names. The mapped name can contain 1 to 63 characters. The following characters are not allowed /\."$<>

    If there are changes made to the source databases or objects, click in the upper right corner to update the objects to be synchronized.

    NOTE:
    • To quickly select the desired database objects, you can use the search function.
    • If there are changes made to the source databases or objects, click in the upper right corner to update the objects to be synchronized.
    • If the object name contains spaces, the spaces before and after the object name are not displayed. If there are multiple spaces between the object name and the object name, only one space is displayed.
    • The name of the selected synchronization object cannot contain spaces.

  5. On the Check Task page, check the synchronization task.

    • If any check fails, review the cause and rectify the fault. After the fault is rectified, click Check Again.

      For details about how to handle check failures, see Checking Whether the Source Database Is Connected in Data Replication Service User Guide.

    • If the check is complete and the check success rate is 100%, click Next.

      You can proceed to the next step only when all checks are successful. If there are any items that require confirmation, view and confirm the details first before proceeding to the next step.

      Figure 8 Pre-check

  6. On the displayed page, specify Start Time, Send Notification, SMN Topic, Synchronization Delay Threshold, and Stop Abnormal Tasks After, confirm that the configured information is correct, select the check box before the agreement, and click Submit to submit the task.

    Figure 9 Task startup settings
    Table 9 Task startup settings

    Parameter

    Description

    Started Time

    Set Start Time to Start upon task creation or Start at a specified time based on site requirements.

    NOTE:

    After a synchronization task is started, the performance of the source and destination databases may be affected. You are advised to start a synchronization task during off-peak hours.

    Send Notifications

    SMN topic. This parameter is optional. If an exception occurs during synchronization, the system will send a notification to the specified recipients.

    SMN Topic

    This parameter is available only after you enable Send Notifications and create a topic on the SMN console and add a subscriber.

    For details, see Simple Message Notification User Guide.

    Synchronization Delay Threshold

    During an incremental synchronization, a synchronization delay indicates a time difference (in seconds) of synchronization between the source and destination database.

    If the synchronization delay exceeds the threshold you specify, DRS will send alarms to the specified recipients. The value ranges from 0 to 3,600. To avoid repeated alarms caused by the fluctuation of delay, an alarm is sent only after the delay has exceeded the threshold for six minutes.

    NOTE:
    • If the delay threshold is set to 0, no notifications will be sent to the recipient.
    • In the early stages of an incremental synchronization, the synchronization delay is long because a large quantity of data is awaiting synchronization. In this case, no notifications will be sent.
    • Before setting the delay threshold, enable Send Notification.

    Stop Abnormal Tasks After

    Number of days after which an abnormal task is automatically stopped. The value must range from 14 to 100. The default value is 14.

    NOTE:

    Tasks in the abnormal state are still charged. If tasks remain in the abnormal state for a long time, they cannot be resumed. Abnormal tasks run longer than the period you set (unit: day) will automatically stop to avoid unnecessary fees.

  7. After the task is submitted, you can view and manage it on the Data Synchronization Management page.

    • You can view the task status. For more information about task status, see Task Statuses.
    • You can click in the upper-right corner to view the latest task status.
    • By default, DRS retains a task in the Configuration state for three days. After three days, DRS automatically deletes background resources, and the task status remains unchanged. When you restart the task configuration, DRS applies for resources again.