Data Transmission Service:Configure two-way synchronization between ApsaraDB for Redis Enhanced Edition (Tair) instances

Prerequisites

Precautions

• During two-way data synchronization, the data synchronization task in the forward direction performs full data synchronization and incremental data synchronization. The data synchronization task in the reverse direction performs only incremental data synchronization.

  Warning

  To ensure data consistency, do not modify or write data to the same key in the source and destination databases when the two-way data synchronization tasks are running.

• DTS uses the resources of the source and destination databases during full data synchronization. This may increase the loads of the database servers. If you synchronize large amounts of data or the server specifications cannot meet your requirements, the database services may become unavailable. Before you synchronize data, evaluate the impacts of data synchronization on the performance of the source and destination instances. We recommend that you synchronize data during off-peak hours.

• We recommend that you do not run the FLUSHDB or FLUSHALL command in the source instance during data synchronization. If you run one of the commands, data inconsistency may occur between the source and destination instances.

• By default, the maxmemory-policy parameter that specifies how data is evicted is set to volatile-lru for ApsaraDB for Redis instances. If the destination instance has insufficient memory, data inconsistency may occur between the source and destination instances due to data eviction. In this case, the data synchronization task does not stop running.

  To prevent data inconsistency, we recommend that you set maxmemory-policy to noeviction for the destination instance. This way, the data synchronization task fails if the destination instance has insufficient memory, but data loss can be prevented for the destination instance.

  Note

  For more information about data eviction policies, see What is the default eviction policy of ApsaraDB for Redis?

• 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.

• If the direct connection mode is disabled for the destination ApsaraDB for Redis instance, DTS uses the proxy mode to write data to the destination instance.

  Note

  For more information about how to enable the direct connection mode for an instance, see Enable the direct connection mode.

• During data synchronization, if the number of shards in the source or destination ApsaraDB for Redis instance is increased or decreased, or if you change the specifications of the source or destination instance, such as scaling up the memory capacity, you must reconfigure the data synchronization task. To ensure data consistency, we recommend that you clear the data that has been synchronized to the source and destination ApsaraDB for Redis instances before you reconfigure the data synchronization task.

• During data synchronization, the endpoint of the source or destination ApsaraDB for Redis instance may be changed. For example, the endpoint of the source or destination instance is changed if the zone of the instance is changed or the network type is changed from classic network to Virtual Private Cloud (VPC). In this case, you must reconfigure the data synchronization task.

• Limits on synchronizing data from a standalone ApsaraDB for Redis instance to an ApsaraDB for Redis cluster instance: Each command can be run only on a single slot in an ApsaraDB for Redis cluster instance. 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 synchronization. Otherwise, the data synchronization task is interrupted.

• 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 the transparent data encryption (TDE) feature is enabled for the source or destination instance, you cannot use DTS to synchronize data.

• A two-way data synchronization instance contains a forward synchronization task and a reverse synchronization task. If an object is to be synchronized in both the forward and reverse synchronization tasks when you configure or reset the instance, the following rules apply:

  • Only one of the tasks can synchronize both the full data and incremental data of the object. The other task synchronizes only the incremental data of the object.

  • The source data of the current task can be synchronized only to the destination of the task. The synchronized data is not used as the source data of the other task.

Billing

Commands that can be synchronized

• APPEND

• BITOP, BLPOP, BRPOP, and BRPOPLPUSH

• DECR, DECRBY, and DEL

• EVAL, EVALSHA, EXEC, EXPIRE, and EXPIREAT

• 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, and PSETEX

• RENAME, RENAMENX, RPOP, RPOPLPUSH, RPUSH, and RPUSHX

• SADD, SDIFFSTORE, SELECT, SET, SETBIT, SETEX, SETNX, SETRANGE, SINTERSTORE, SMOVE, SPOP, SREM, and SUNIONSTORE

• UNLINK, ZADD, ZINCRBY, ZINTERSTORE, ZREM, ZREMRANGEBYLEX, ZUNIONSTORE, ZREMRANGEBYRANK, and ZREMRANGEBYSCORE

• SWAPDB (This command cannot be synchronized if the source or destination instance is deployed in the cluster architecture.)

Permissions required for database accounts

Procedure

1. Purchase a data synchronization instance. For more information, see Purchase a DTS instance.

   Important

   On the buy page, set the Source Instance parameter to Redis, the Destination Instance parameter to Redis, and the Synchronization Topology parameter to Two-way Synchronization.

2. Log on to the DTS console.

   Note

   If you are redirected to the Data Management (DMS) console, you can click the icon in the to go to the previous version of the DTS console.

3. In the left-side navigation pane, click Data Synchronization.

4. In the upper part of the Synchronization Tasks page, select the region in which the destination instance resides.

5. Configure the data synchronization task in the forward direction.

   1. Find the data synchronization instance, and click Configure Task in the Actions column of the first data synchronization task.

      Important

      A two-way data synchronization instance contains two data synchronization tasks. You must set parameters for each task. When you configure the second data synchronization task, find the task and click Configure Task in the Actions column.

      

   2. Configure the source and destination instances.

      

      Section	Parameter	Description
      N/A	Synchronization Task Name	The task name that DTS automatically generates. We recommend that you specify a name that can help you identify the task. You do not need to specify a unique task name.
      Source Instance Details	Instance Type	The type of the source instance. Select Redis Instance.
      	Instance Region	The region of the source instance that you selected on the buy page. The value of this parameter cannot be changed.
      	Instance ID	The ID of the source ApsaraDB for Redis instance. Important When you configure the data synchronization task in the reverse direction, select the ID of the destination ApsaraDB for Redis instance.
      	Database Password	The password of the database account of the source ApsaraDB for Redis instance. For information about the permissions required for the database account, see Permissions required for database accounts. Important The database password is in the <user>:<password> format. For example, if the username of the account that you use to log on to the instance is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
      Destination Instance Details	Instance Type	The type of the destination instance. Select Redis Instance.
      	Instance Region	The region of the destination instance that you selected on the buy page. The value of this parameter cannot be changed.
      	Instance ID	The ID of the destination ApsaraDB for Redis instance. Important When you configure the data synchronization task in the reverse direction, select the ID of the source ApsaraDB for Redis instance.
      	Database Password	The password of the database account of the destination ApsaraDB for Redis instance. For information about the permissions required for the database account, see Permissions required for database accounts. Important The database password is in the <user>:<password> format. For example, if the username of the account that you use to log on to the instance is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.

   3. In the lower-right corner of the page, click Set Whitelist and Next.

      Note

      • You do not need to modify the security settings of ApsaraDB instances, such as ApsaraDB RDS for MySQL and ApsaraDB for MongoDB instances, or self-managed databases that are hosted on Elastic Compute Service (ECS) instances. DTS automatically adds the CIDR blocks of DTS servers to the whitelists of ApsaraDB instances or the security group rules of ECS instances. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.

      • After data synchronization is complete, you can remove the CIDR blocks of DTS servers from the whitelists or security groups.

   4. Select the synchronization policy and the objects to be synchronized.

      

      Setting	Parameter	Description
      Select the synchronization policy	Conflict Resolution Policy	Overwrite (When a conflict occurs, the conflicting record in the destination instance is overwritten) During data synchronization, if data records have the same key but different values, the data record with the latest key value overwrites the conflicting records.
      	Processing Mode In Existed Target Table	Pre-check and Intercept: checks whether the destination instance is empty. If the destination instance is empty, the precheck is passed. If the destination instance is not empty, an error is returned during the precheck and the data synchronization task cannot be started. Ignore: skips the check for empty destination databases. Warning If you select Ignore, the data records in the source database overwrite the data records with the same keys in the destination database. Proceed with caution.
      Select the objects to be synchronized	N/A	Select one or more databases from the Available section and click the icon to move the databases to the Selected section. You can select only databases as the objects to be synchronized. Keys cannot be selected as the objects to be synchronized.
      Rename databases and tables	N/A	In this scenario, you cannot rename the objects.
      Specify whether to copy temporary tables to the destination database when DMS performs online DDL operations on the source tables	N/A	If you use DMS to perform online DDL operations on the source database, you can specify whether to synchronize temporary tables generated by online DDL operations. Yes: DTS synchronizes the data of temporary tables generated by online DDL operations. Note If online DDL operations generate a large amount of data, the data synchronization task may be delayed. No: DTS does not synchronize the data of temporary tables generated by online DDL operations. Only the original DDL data of the source database is synchronized. Note If you select No, the tables in the destination database may be locked.
      Configure Retry Time for Failed Connections	N/A	By default, if DTS fails to connect to the source or destination database, DTS retries within the next 720 minutes (12 hours). You can specify the retry time based on your needs. If DTS reconnects to the source and destination databases within the specified time, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Note When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time based on your business needs. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.

   5. In the lower-right corner of the page, click Next.

   6. Select the initial synchronization types.

      

      The value is set to Include full data + incremental data and cannot be changed. DTS synchronizes historical data from the source ApsaraDB for Redis instance to the destination ApsaraDB for Redis instance. Then, DTS synchronizes incremental data.

      Important

      • If all the required objects have been synchronized from the source instance to the destination instance, the data synchronization task in the reverse direction synchronizes only incremental data.

      • If a version-related error message appears, you can upgrade the source ApsaraDB for Redis instance to a specified version. For more information, see Upgrade the major version and Update the minor version of an instance.

   7. In the lower-right corner of the page, click Precheck.

      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 task fails to pass the precheck, you can click the icon next to each failed item to view details.

        • After you troubleshoot the issues based on the details, initiate a new precheck.

        • If you do not need to troubleshoot the issues, ignore the failed items and initiate a new precheck.

   8. Close the Precheck dialog box after the following message is displayed: Precheck Passed. Then, the data synchronization task in the forward direction starts.

6. Wait until initial forward synchronization is completed and the data synchronization task is in the Synchronizing state.

   You can view the status of the data synchronization task on the Synchronization Tasks page.

7. Configure the data synchronization task in the reverse direction.

   1. Find the second data synchronization task and click Configure Synchronization Channel in the Actions column.

      

   2. Repeat the substeps in Step 5.