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 initial 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 initial full data synchronization. This may increase the loads on the database servers. If you synchronize a large amount of data or if the server specifications do not meet your requirements, database services may become unavailable. 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.
• 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 How does ApsaraDB for Redis evict data by default?
• 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 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 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.
• If the source or destination instance resides in a region outside the Chinese mainland, two-way data synchronization is supported only between instances located within the same region. For example, if the source instance resides in the Japan (Tokyo) region, data can be synchronized only within the Japan (Tokyo) region and cannot be synchronized to the Germany (Frankfurt) region.
• Limits on synchronizing 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 synchronization. This prevents the data synchronization task from being 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.

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 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.
   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 lower-right corner 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 Data 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 descriptive name that makes it easy to identify the task. You do not need to use a unique task name.
      Source Instance Details	Instance Type	The type of the source database. Select Redis Instance.
      	Instance Region	The source region that you selected on the buy page. You cannot change the value of this parameter.
      	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 that are 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 source instance is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
      Destination Instance Details	Instance Type	The type of the destination database. Select Redis Instance.
      	Instance Region	The destination region that you selected on the buy page. You cannot change the value of this parameter.
      	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 that are 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 source 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 for ApsaraDB instances, such as ApsaraDB RDS for MySQL and ApsaraDB for MongoDB, 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 (Conflicting records in the destination instance are 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.
      	Select the processing mode of conflicting tables	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 instances. 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 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. You cannot select keys as the objects to be synchronized.
      Rename Databases and Tables	N/A	The objects to be synchronized. In this scenario, you cannot rename the objects.
      Replicate Temporary Tables When DMS Performs DDL Operations	N/A	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	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 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, click the icon next to each failed item to view details.
        • After you troubleshoot the issues based on the causes, run a precheck again.
        • If you do not need to troubleshoot the issues, ignore failed items and run a precheck again.
   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 substeps ii to viii that are described in Step 5.