This topic describes how to migrate data between ApsaraDB for Redis instances by using Data Transmission Service (DTS).

Prerequisites

  • The source and destination ApsaraDB for Redis instances are created.
    Note
  • The available storage space of the destination ApsaraDB for Redis instance is larger than the total size of the data in the source ApsaraDB for Redis instance.

Limits

CategoryMigration description
Limits on the source database
  • Bandwidth requirements: The server to which the source database belongs must have sufficient outbound bandwidth. Otherwise, the data migration speed decreases.
  • You must enable the data logging and append-only file (AOF) logging features for the source database.
  • Limits on operations: If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data may be inconsistent between the source and destination databases. To ensure data consistency, we recommend that you select full data migration and incremental data migration as the migration types.
  • Limits on migrating data from a standalone Redis instance to a Redis cluster: Each command can be run only on a single slot in a Redis cluster. If you perform operations on multiple keys in the source database and the keys belong to different slots, the following error occurs:
    CROSSSLOT Keys in request don't hash to the same slot
    We recommend that you perform operations on only one key during data migration. This prevents the data migration task from being interrupted.
  • To ensure the stability of data migration, DTS inserts a key prefixed with DTS_REDIS_TIMESTAMP_HEARTBEAT into the source database to record when data is updated. If the source database is deployed in a cluster architecture, DTS inserts the key into each shard of the cluster. The key is filtered out during data migration. After the data migration task is complete, the key expires.
  • If the source database is a read-only database or the source database account that is used to run the data migration task does not have the permissions to run the SETEX command, the reported latency may be inaccurate.
Other limits
  • A storage-optimized instance of ApsaraDB for Redis Enhanced Edition (Tair) cannot be configured as the source or destination database.
  • If the destination database is out of memory, data eviction is triggered. By default, the data eviction policy (maxmemory-policy) of an ApsaraDB for Redis instance is set to volatile-lru. Therefore, data inconsistency may occur between the source and destination databases. In this case, the data migration task can run as expected.

    To prevent data inconsistency between the source and destination databases, we recommend that you set the data eviction policy of the destination database to noeviction. If the destination database is out of memory, DTS fails to write data and the data migration task fails. However, data loss does not occur in the destination database due to data eviction.

    Note For more information about data eviction policies, see How does ApsaraDB for Redis evict data by default?
  • If you run the EVAL or EVALSHA command to call Lua scripts, DTS cannot identify whether these Lua scripts are executed in the destination database. This is because the destination database does not explicitly return the execution results of Lua scripts during incremental data migration.
  • When you run the PSYNC or SYNC command to transfer data of the LIST type, DTS does not perform the FLUSH operation on the existing data in the destination database. As a result, the destination database may contain duplicate data records.
  • If an expiration policy is enabled for specific keys in the source database, these keys may not be deleted at the earliest opportunity after they expire. Therefore, the number of keys in the destination database may be less than that in the source database. You can run the INFO command to view the number of keys in the destination database.
    Note The number of keys that do not have the expiration policy enabled or have not expired is the same between the source and destination databases.
  • To ensure compatibility, the version of the destination Redis database must be the same as or later than that of the source Redis database.
    Note If the version of the destination database is earlier than that of the source database, database compatibility issues may occur.
  • Before you migrate data, evaluate the impact of data migration on the performance of the source and destination databases. We recommend that you migrate data during off-peak hours. During full data migration, DTS uses read and write resources of the source and destination databases. This may increase the loads of the database servers.
  • During full data migration, concurrent INSERT operations cause fragmentation in the tables of the destination cluster. After full data migration is complete, the size of used tablespace of the destination cluster is larger than that of the source instance.
  • DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination instance, stop or release the data migration task. You can also run the REVOKE command to revoke the write permissions from the accounts that are used by DTS to access the destination instance. Otherwise, the data in the source database overwrites the data in the destination database after the task is resumed.
  • If both the source and destination databases are of ApsaraDB for Redis Community Edition or Enhanced Edition, take note of the following limits: If the database configurations are changed, such as the specifications or port number, DTS cannot obtain continuous log data or correct connection information. As a result, the data migration task is interrupted. To ensure data consistency, we recommend that you delete the data in the destination database and reconfigure the data migration task after the database configurations are changed.
  • If the destination instance is deployed in a cluster architecture and the amount of memory used by a shard in the destination instance reaches the upper limit, or if the available storage space of the destination instance is insufficient, the data synchronization task fails due to out of memory (OOM).
  • If Transparent Data Encryption (TDE) is enabled for the source or destination instance, you cannot use DTS to migrate data.
Special casesIf the source database is a self-managed Redis database, take note of the following limits:
  • If you perform a primary/secondary switchover on the source database when the data migration task is running, the task fails.
  • DTS calculates migration latency based on the timestamp of the latest migrated data in the destination database and the current timestamp in the source database. If no DML operation is performed on the source database for an extended period of time, the migration latency may be inaccurate. If the latency of the data migration task is too high, you can perform a DML operation on the source database to update the latency.
    Note If you select an entire database as the object to be migrated, you can create a heartbeat table. The heartbeat table is updated or receives data every second.

Migration types

Migration typeDescription
Full data migration

DTS migrates the existing data of objects from the source database to the destination database.

Incremental data migration

After full data migration is complete, DTS migrates incremental data from the source database to the destination database. Incremental data migration allows data to be migrated smoothly without interrupting services of self-managed applications during data migration.

Commands that are supported by incremental data migration

  • APPEND
  • BITOP, BLPOP, BRPOP, and BRPOPLPUSH
  • DECR, DECRBY, and DEL
  • EVAL, EVALSHA, EXEC, EXPIRE, and EXPIREAT
  • FLUSHALL and FLUSHDB
  • GEOADD and GETSET
  • HDEL, HINCRBY, HINCRBYFLOAT, HMSET, HSET, and HSETNX
  • INCR, INCRBY, and INCRBYFLOAT
  • LINSERT, LPOP, LPUSH, LPUSHX, LREM, LSET, and LTRIM
  • MOVE, MSET, MSETNX, and MULTI
  • PERSIST, PEXPIRE, PEXPIREAT, PFADD, PFMERGE, PSETEX, and PUBLISH
  • RENAME, RENAMENX, RESTORE, RPOP, RPOPLPUSH, RPUSH, and RPUSHX
  • SADD, SDIFFSTORE, SELECT, SET, SETBIT, SETEX, SETNX, SETRANGE, SINTERSTORE, SMOVE, SPOP, SREM, and SUNIONSTORE
  • ZADD, ZINCRBY, ZINTERSTORE, ZREM, ZREMRANGEBYLEX, ZUNIONSTORE, ZREMRANGEBYRANK, and ZREMRANGEBYSCORE

Procedure

  1. Go to the Data Migration Tasks page.
    1. Log on to the Data Management (DMS) console.
    2. In the top navigation bar, click DTS.
    3. In the left-side navigation pane, choose DTS (DTS) > Data Migration.
    Note
  2. From the drop-down list next to Data Migration Tasks, select the region in which the data migration instance resides.
    Note If you use the new DTS console, you must select the region in which the data migration instance resides in the upper-left corner.
  3. Click Create Task. On the page that appears, configure the source and destination databases.
    SectionParameterDescription
    N/ATask Name

    The name of the task. DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to specify a unique task name.

    Source DatabaseSelect an existing DMS database instance
    The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements.
    • If you use an existing instance, DTS automatically applies the parameter settings of the source database.
    • If you do not use an existing instance, you must configure the parameters for the database.
    Database TypeSelect Tair/Redis.
    Access MethodSelect Alibaba Cloud Instance. In this example, the source and destination databases are ApsaraDB for Redis instances. You can also follow the procedure to configure data migration tasks for other types of databases.
    Instance RegionThe region where the source ApsaraDB for Redis instance resides.
    Replicate Data Across Alibaba Cloud AccountsSpecifies whether data is replicated across Alibaba Cloud accounts. In this example, No is selected.
    Instance IDThe ID of the source ApsaraDB for Redis instance.
    Database PasswordThe database password of the source ApsaraDB for Redis instance. The database account that corresponds to the password must have the read permissions. If you forget the password, you can reset the password. For more information, see Change or reset the password.
    • If you use the default account whose username is the same as the instance ID, you can enter only the password.
    • If you create a custom account, you must specify the password in the <Custom account>:<Password> format. Example: testaccount:Test1234.
    Note This parameter is optional and can be left empty if no database password is set.
    Destination DatabaseSelect an existing DMS database instance
    The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements.
    • If you use an existing instance, DTS automatically applies the parameter settings of the instance.
    • If you do not use an existing instance, you must configure the parameters for the database.
    Database TypeSelect Tair/Redis.
    Access MethodSelect Alibaba Cloud Instance.
    Instance RegionThe region where the destination ApsaraDB for Redis instance resides.
    Instance IDThe ID of the destination ApsaraDB for Redis instance.
    Database PasswordThe database password of the destination ApsaraDB for Redis instance. The database account that corresponds to the password must have the read permissions. If you forget the password, you can reset the password. For more information, see Change or reset the password.
    • If you use the default account whose username is the same as the instance ID, you can enter only the password.
    • If you create a custom account, you must specify the password in the <Custom account>:<Password> format. Example: testaccount:Test1234.
  4. In the lower part of the page, click Test Connectivity and Proceed.
    Warning If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of a database or an instance, or to the security group rules of an ECS instance, security risks may arise. Therefore, before you use DTS to migrate data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhance the security of your username and password, limit the ports that are exposed, authenticate API calls, regularly check the whitelist or ECS security group rules and forbid unauthorized CIDR blocks, or connect the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.
  5. Configure objects to migrate and advanced settings.
    • Basic Settings
      ParameterDescription
      Migration TypeSelect Full Data Migration and Incremental Data Migration.

      For more information, see Migration types.

      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 identical table names, the precheck is passed. Otherwise, an error is returned during the precheck and the data migration task cannot be started.

      • 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, DTS does not migrate data records that have the same primary keys as data records in the destination database.
        • If the source and destination databases have different schemas, only some columns are migrated or the data migration task fails. Proceed with caution.
      Source Objects

      Select one or more objects from the Source Objects section and click the Rightwards arrow icon to add the objects to the Selected Objects section.

      Note You can select databases as the objects to be migrated. Keys cannot be selected as objects to be migrated.
      Selected Objects
      • The objects that you select to be migrated. In this scenario, you cannot rename the objects.
      • In the Selected Objects section, right-click an object. In the dialog box that appears, select the commands that you want to migrate. For more information, see Commands that are supported by incremental data migration.
    • Advanced Settings
      ParameterDescription
      Set Alerts
      Specifies whether to set alerts for the data migration task. If the task fails or the migration latency exceeds the threshold, the alert contacts will receive notifications. Valid values:
      Retry Time for Failed Connections
      The retry time range for failed connections. If the data migration task fails, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the 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 migration task. Otherwise, the data migration task fails.
      Note
      • If an instance serves as the source database of multiple data migration tasks, the less value of this parameter that is set for the instance 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.
      Extend Expiration Time of Destination Database Key
      The extended time period for keys migrated from the source database to the destination instance to remain valid. Unit: seconds. If specific commands are used, such as the following commands, we recommend that you set the parameter to 600 to ensure data consistency: 
      expire key seconds
pexpire key milliseconds
expireat key timestamp
pexpireat key timestampMs
      Configure ETL
      Specifies whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. Valid values:
  6. In the lower part of the page, click Next: Save Task Settings and Precheck.
    Note
    • Before you can start the data migration task, DTS performs a precheck. You can start the data migration task only after the task passes the precheck.
    • If the task fails to pass the precheck, click View Details next to each failed item. After you troubleshoot the issues based on the causes, run a precheck again.
    • 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 issues. 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.
  7. Wait until the Success Rate value becomes 100%. Then, click Next: Purchase Instance.
  8. On the Purchase Instance page, specify the Instance Class parameter for the data migration instance. The following table describes the parameter.
    SectionParameterDescription
    ParametersInstance Class

    DTS provides several instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Specifications of data migration instances.

  9. Read and select the check box to agree to Data Transmission Service (Pay-as-you-go) Service Terms.
  10. Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.