Configure two-way synchronization between Tair instances - Data Transmission Service

This topic describes how to configure two-way synchronization between Tair instances, including ApsaraDB for Redis Enhanced Edition (Tair) instances and Tair instances that use cloud disks, by using Data Transmission Service (DTS). The data synchronization feature is applicable to scenarios such as active geo-redundancy and geo-disaster recovery.

Prerequisites

The source and destination Tair instances or Tair instances that use cloud disks and run Redis 5.0 are created. For more information, see Step 1: Create an ApsaraDB for Redis instance and Step 1: Create a Tair instance.
The source and destination instances have the same storage capacity.

Limits

Category	Description
Limits on the source database	A two-way data synchronization task supports only Tair instances that use local disks, Tair dynamic random access memory (DRAM)-based instances that use cloud disks, and Tair persistent memory-optimized instances that use cloud disks. To ensure the synchronization quality, Data Transmission Service (DTS) adds a key prefixed with DTS_REDIS_TIMESTAMP_HEARTBEAT to the source database. This key is used to record the time when data is synchronized to the destination database. If the source database is deployed in a cluster architecture, DTS adds this key to each shard. The key is filtered out during data synchronization. After the data synchronization 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 synchronization task does not have the permissions to run the SETEX command, the reported latency may be inaccurate. To ensure the stability of data synchronization, we recommend that you increase the value of the `repl-backlog-size` parameter in the redis.conf file. We recommend that you do not run the `FLUSHDB` or `FLUSHALL` command in the source database. If you run one of the commands, data inconsistency may occur between the source and destination databases. You must enable the append-only file (AOF) logging feature for the source database. 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.
Other limits	During data synchronization, if the number of shards in the source Tair instance is increased or decreased, or if you change the database specifications, 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 destination Tair instance before you reconfigure the data synchronization task. During data synchronization, if the endpoint of the source Tair instance is changed, you must reconfigure the data synchronization task. To ensure compatibility, the version of the destination database must be the same as or later than that of the source database. If the version of the destination database is earlier than that of the source database, database compatibility issues may occur. During initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers. 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. If the source or destination Tair instance resides in a region outside the Chinese mainland, two-way synchronization is supported only between instances within the same region. For example, if a Tair instance resides in the Japan (Tokyo) region, data can be synchronized only within the Japan (Tokyo) region and cannot be synchronized to or from the Germany (Frankfurt) region in two-way synchronization scenarios. During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases. A two-way 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. 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. 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 data eviction is triggered due to insufficient memory of the destination database, data inconsistency may occur between the source and destination databases. This is because the default data eviction policy (maxmemory-policy) of the destination database is set to volatile-lru. However, this does not affect the data synchronization task. In this case, we recommend that you set the data eviction policy of the destination database to noeviction. If the memory of the destination database is insufficient, DTS fails to write data and the data synchronization 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 the transparent data encryption (TDE) feature is enabled for the source or destination Tair instance, you cannot use DTS to synchronize data. During data synchronization, if resumable upload fails due to transient connections that occur on the source Redis database, full data may be re-synchronized to the destination database. This may cause data inconsistency between the source and destination databases.

Supported two-way synchronization topologies

DTS supports two-way synchronization only between two Tair instances. DTS does not support two-way synchronization among three or more Tair instances.

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, 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
XADD, XCLAIM, XDEL, XAUTOCLAIM, XGROUP CREATECONSUMER, and XTRIM

Note

The PUBLISH command cannot be synchronized.
If you run the EVAL or EVALSHA command to call Lua scripts, DTS cannot identify whether these Lua scripts are executed on the destination database. This is because the destination database does not explicitly return the execution results of Lua scripts during incremental data synchronization.
When you run the SYNC or PSYNC command to transfer data of the LIST type, DTS does not clear the existing data in the destination database. As a result, the destination database may contain duplicate data records.

Permissions required for database accounts

Database	Permissions and authorization method
Source Tair instance	The database accounts of the source and destination Tair instances must have read and write permissions. For more information about how to grant permissions to an account, see Create and manage database accounts.
Destination Tair instance

Procedure

Go to the Data Synchronization 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 Synchronization.
Note
- Operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
- You can also go to the Data Synchronization Tasks page of the new DTS console.
On the right side of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Note
If you use the new DTS console, you must select the region in which the data synchronization instance resides in the top navigation bar.

Configure the source and destination databases. The following table describes the parameters.

Warning

After you configure the source and destination databases, we recommend that you read the limits that are displayed on the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	Description
N/A	Task Name	The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select a DMS database instance	Select whether to use an existing instance. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the source database. Set this parameter to ApsaraDB for Redis Enhanced Edition (Tair).
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source Tair instance resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
	Instance ID	The ID of the source Tair instance.
	Database Password	The password of the source database. For information about the permissions that are required for the database account, see the Permissions required for database accounts section of this topic. Note 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 Tair instance is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
Destination Database	Select a DMS database instance	Select whether to use an existing instance. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the destination database. Set this parameter to ApsaraDB for Redis Enhanced Edition (Tair).
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination Tair instance resides.
	Instance ID	The ID of the destination Tair instance.
	Database Password	The password of the destination database. For information about the permissions that are required for the database account, see the Permissions required for database accounts section of this topic. Note For example, if the username of the account that you use to log on to the destination Tair instance is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.

In the lower part of the page, click Test Connectivity and Proceed.
If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must make sure that the ECS instance can access the database. If the database is deployed on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the whitelist of the database to allow DTS to access the database. For more information, see Add the CIDR blocks of DTS servers.
Warning
If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database or instance, or to the ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

Configure the objects to be synchronized and advanced settings.

Parameter	Description
Synchronization Types	By default, Incremental Data Synchronization is selected. You must also select Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination database. The historical data is the basis for subsequent incremental synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether data exists in the destination database. If no data exists in the destination database, the precheck is passed. If data exists in the destination database, an error is returned during the precheck, and the data synchronization task cannot be started. Ignore Errors and Proceed: skips the Check the existence of objects in the destination database. check item. Warning If you select Ignore Errors and Proceed, data loss may occur in the destination database because data records in the source database overwrite the data records that have the same keys in the destination database. Proceed with caution.
Synchronization Topology	The synchronization topology of the data synchronization task. Select Two-way Synchronization.
Source Objects	Select one or more objects from the Source Objects section and click the icon to move the objects to the Selected Objects section. Note You can select only databases as the objects to be synchronized. Keys cannot be selected as the objects to be synchronized.
Selected Objects	If you want to select a database to which data is synchronized from DB 0 to DB 255 or filter the data to be synchronized by prefix, you can use the object name mapping feature or the filtering feature. In the Selected Objects section, right-click the database that you want to synchronize. In the Edit Schema dialog box, configure the parameters. For more information, see Map object names and Set filter conditions. Note You cannot map multiple object names at a time.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Set Alerts	Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values: No: does not configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert contacts. For more information, see Configure monitoring and alerting.
Retry Time for Failed Connections	The retry time range for failed connections. If the source or destination database fails to be connected after the data synchronization task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set this 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 synchronization task. Otherwise, the data synchronization task fails. Note If you specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range 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 synchronized from the source database to the destination database to remain valid. Unit: seconds. If specific commands such as the following commands are used, we recommend that you set this parameter to 600 to ensure data consistency. `EXPIRE key seconds PEXPIRE key milliseconds EXPIREAT key timestamp PEXPIREAT key timestampMs`
Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.

Save the task settings and run a precheck.
- To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
- If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
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 View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, 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 issue. 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.
Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for your subscription when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. We recommend that you select the pay-as-you-go billing method for short-term use. If you no longer require a pay-as-you-go instance, you can release the instance to reduce costs.
	Resource Group	The resource group to which the instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides various synchronization specifications that provide different performance. The synchronization speed varies based on the synchronization specifications that you select. You can select a synchronization specification based on your business requirements. For more information, see Specifications of data synchronization instances.
	Duration	If you select the subscription billing method, set the subscription duration and the number of instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is displayed only if you select the subscription billing method.Subscription

Read and select the Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.
Wait until initial synchronization is complete and the data synchronization task in the forward direction is in the Running state.
Note
If both Full Data Synchronization and Incremental Data Synchronization are selected in the Configure Objects and Advanced Settings step, Incremental Data Synchronization is displayed in the Data Synchronization Tasks section of the Data Synchronization page.
Find the reverse synchronization task below the forward synchronization task and click Configure Task.
Configure the reverse synchronization task by performing Steps 3 to 7.
Important
- The number of parameters that you need to configure for a reverse synchronization task is less than that for a forward synchronization task. Configure the parameters by following the on-screen instructions.
- The source Tair instance in the reverse synchronization task is the destination Tair instance in the forward synchronization task, and the destination Tair instance in the reverse synchronization task is the source Tair instance in the forward synchronization task. You need to verify the consistency of the instance information, such as the names, accounts, and passwords of databases.
Wait until the success rate becomes 100%. Then, click Back.
Wait until both the forward synchronization task and the reverse synchronization task are in the Running state. Two-way synchronization is configured.