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 Tair instance and the destination Tair instance 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 ApsaraDB for Redis Enhanced Edition (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. The collections to be synchronized must have PRIMARY KEY or UNIQUE constraints, and all fields must be unique. Otherwise, the destination database may contain duplicate data records. 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 on 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. A Tair enhanced SSD (ESSD)-based instance cannot be configured as the source 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. If a table is synchronized in both the forward and reverse synchronization and both the full data and incremental data of the table are synchronized in the forward synchronization, DTS synchronizes only the incremental data of the table in the reverse synchronization. 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 the destination database is out of memory, data eviction is triggered. By default, the data eviction policy (maxmemory-policy) of a Tair instance is set to volatile-lru. Therefore, data inconsistency may occur between the source and destination databases. In this case, the data synchronization 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 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.

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

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 in 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	Permission 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 DMS console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
Note You can also go to the Data Synchronization Tasks page of the new DTS console.
From the drop-down list to the right of Data Synchronization Tasks, select the region in which your data synchronization instance resides.
Note If you use the new DTS console, select the region in which your data synchronization instance resides in the top navigation bar.

Configure the source and destination databases.

Warning After you select the source and destination instances, we recommend that you read the limits displayed in the upper part of the page. This helps you create and run the data synchronization task.


Section	Parameter	Description
N/A	Task Name	DTS automatically 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 Database	Select Instance	Select whether to use an existing instance. If you select an existing instance, DTS automatically applies the parameter settings of the instance. You do not need to configure the corresponding parameters again. If you do not use an existing instance, you must configure parameters for the source database.
	Database Type	The type of the source database. Select 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 Permissions required for database accounts. 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 Instance	Select whether to use an existing instance. If you select an existing instance, DTS automatically applies the parameter settings of the instance. You do not need to configure the corresponding parameters again. If you do not use an existing instance, you must configure parameters for the source database.
	Database Type	The type of the destination database. Select 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 Permissions required for database accounts. 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.
Note
- 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. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. 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.
- After data synchronization is complete, we recommend that you remove the CIDR blocks of DTS servers from the allowlists or security groups. You must remove the IP address whitelist group whose name contains dts from the whitelist of the ApsaraDB instance or the security rules of the ECS instance. For more information about the CIDR blocks that you must remove from the whitelist of the self-managed databases that are deployed in data centers or databases that are hosted on third-party cloud services, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.

Select objects for the task and configure 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 the destination database is empty. If the destination database is empty, the precheck is passed. If the destination database is not empty, an error is returned during the precheck, and the data synchronization task cannot be started. Ignore Errors and Proceed: skips the check for empty destination databases. Warning If you select Ignore Errors and Proceed, data records in the source database overwrite the data records that have the same keys in the destination database. Proceed with caution.
Synchronization Topology	Select Two-way Synchronization.
Source Objects	Select one or more objects from the Source Objects section and click the icon to add 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 remove a selected object, select the object that you want to remove in the Selected Objects section and click the icon to move it to the Source Objects section. In the Selected Objects section, right-click an object. In the dialog box that appears, select the commands that you want to synchronize. For more information, see Commands that can be synchronized.

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. If you select Yes, you must also specify the alert threshold and alert contacts. For more information, see Configure monitoring and alerting when you create a DTS task.
Retry Time for Failed Connection	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 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 synchronization task. Otherwise, the data synchronization task fails. Note If you set different retry time ranges for multiple DTS tasks that have the same source or destination database, the shortest retry time range that is set 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 the 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 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: 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.

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 generated for an item during the precheck, perform the following operations based on the scenario:
  In scenarios where you cannot ignore the alert item, click View Details next to the failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
  In scenarios where you can ignore the alert item, click Confirm Alert Details next to the failed item. 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 the instance 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 charged on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go instance, you can release the pay-as-you-go instance to reduce costs.
	Instance Class	DTS provides several instance classes that have different performance in synchronization speed. You can select an instance class based on your business scenario. For more information, see Specifications of data synchronization instances.
	Subscription 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 or one to three years. Note This parameter is displayed only if you select the subscription billing method.

Read and select the check box for 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 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 one, and the destination Tair instance in the reverse synchronization task is the source Tair instance in the forward one. You need to verify the consistency of the instance information, such as the names, accounts, and passwords of databases.
Wait until the Success Rate value becomes 100%. Then, click Back.
After the reverse synchronization task is configured, wait until both tasks are in the Running state. Two-way synchronization is configured.