Configure two-way data 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 migrate a large volume of data or if the server specifications do not meet your requirements, 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. Otherwise, data may become inconsistent between the source and destination instances.
• If the data eviction policy (maxmemory-policy) of the destination instance is not set to noeviction, data may become inconsistent between the source and destination instances. For more information about data eviction policies, see How does ApsaraDB for Redis evict data by default?
• If an expiration policy is enabled for some keys in the source database, these keys may not be deleted in a timely manner after they expired. 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 an expiration policy or have not expired is the same in the source and destination databases.
• If direct connection is disabled for the destination ApsaraDB for Redis instance, DTS uses the proxy forwarding mode to write data to the destination instance.
  Note For more information about how to enable direct connection, 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 the database specifications are changed (for example, the memory capacity is scaled up), you must reconfigure the task. To ensure data consistency, we recommend that you clear the data that has been synchronized to the source and destination Redis databases before you reconfigure the task.
• During data synchronization, if the endpoint of the source or destination ApsaraDB for Redis instance is changed, you must submit a ticket to update the endpoint. Operations that may cause endpoint changes include instance zone changes and network changes from the classic network to Virtual Private Cloud (VPC). Otherwise, the append-only files (AOF) of the source or destination ApsaraDB for Redis instance may be reset. In this case, you must reconfigure the task.
• If the source or the destination instance is located in a region outside the Chinese mainland, two-way data synchronization is supported only between instances located with the same region. For example, two-way data synchronization is supported between instances within the Japan (Tokyo) region. Two-way data synchronization between an instance in the Japan (Tokyo) region and another instance in the Germany (Frankfurt) region is not supported.

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 operation is not supported if the source or destination ApsaraDB for Redis 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.
   Notice On the buy page, set Source Instance to Redis, set Destination Instance to Redis, and set Synchronization Topology to Two-way Synchronization.
2. Log on to the DTS console.
3. In the left-side navigation pane, click Data Synchronization.
4. At the top of the Synchronization Tasks page, select the region where the destination instance resides.
5. Configure the data synchronization task in the forward direction.
   1. Find the data synchronization instance, and click Configure Synchronization Channel in the Actions column of the first data synchronization task.
      Notice 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 Synchronization Channel in the Actions column.

      
   2. Configure the source and destination instances.
      

      Section	Parameter	Description
      None	Synchronization Task Name	DTS generates a task name. We recommend that you specify an informative name to identify the task. You do not need to use a unique task name.
      Source Instance Details	Instance Type	Select Redis Instance.
      	Instance Region	The source region that you selected on the buy page. The value of this parameter cannot be changed.
      	Instance ID	Select the ID of the source ApsaraDB for Redis instance. Notice When you configure the data synchronization task in the reverse direction, select the ID of the destination ApsaraDB for Redis instance.
      	Database Password	Enter the database password of the ApsaraDB for Redis instance. For information about the permissions that are required for the account, see Permissions required for database accounts. Notice The database password is in the <user>:<password> format. For example, if the username of a custom account is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
      Destination Instance Details	Instance Type	Select Redis Instance.
      	Instance Region	The region of the destination cluster. The region is the same as the destination region that you selected on the buy page. The value of this parameter cannot be changed.
      	Instance ID	Select the ID of the destination ApsaraDB for Redis instance. Notice When you configure the data synchronization task in the reverse direction, select the ID of the source ApsaraDB for Redis instance.
      	Database Password	Enter the database password of the ApsaraDB for Redis instance. For information about the permissions that are required for the account, see Permissions required for database accounts. Notice The database password is in the <user>:<password> format. For example, if the username of a custom account 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 for ApsaraDB instances (such as ApsaraDB RDS for MySQL and ApsaraDB for MongoDB) and ECS-hosted databases. DTS automatically adds the CIDR blocks of DTS servers to the whitelists of ApsaraDB instances or the security group rules of Elastic Compute Service (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.
      

      Section	Parameter	Description
      Select the synchronization policy	Conflict Resolution Policy	Overwrite 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.
      	Select the processing mode of conflicting tables	Pre-check and Intercept: checks whether the destination database is empty. If the destination database is empty, the precheck is passed. If the database 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	None	Select one or more databases from the Available section and click to move the databases to the Selected section. You can select only databases as the objects to be synchronized. You cannot select keys as the objects to be synchronized.
      Rename Databases and Tables	None	In this scenario, you cannot rename objects.
      Replicate Temporary Tables When DMS Performs DDL Operations	None	If you use Data Management (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.
      Retry Time for Failed Connections	None	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. Configure initial synchronization.
      

      The value is set to Include full data + incremental data. DTS synchronizes historical data from the source ApsaraDB for Redis instance to the destination ApsaraDB for Redis instance. Then, DTS synchronizes incremental data.

      Notice

      • 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.
   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.
        • You can troubleshoot the issues based on the causes and run a precheck again.
        • If you do not need to troubleshoot the issues, you can ignore failed items and run a precheck again.
   8. Close the Precheck dialog box after the following message is displayed: The precheck is 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 state 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 substeps ii to viii that are described in Step 5.