One-way data synchronization between Tair (Redis OSS-Compatible) instances - Data Transmission Service

Data Transmission Service (DTS) supports one-way synchronization between Redis databases. This feature is applicable to scenarios such as active geo-redundancy and geo-disaster recovery. This topic describes how to configure one-way synchronization between ApsaraDB for Tair (Redis Compatible) instances.

Prerequisites

The destination ApsaraDB for Tair (Redis compatible) instance is created. For more information, see Step 1: Create an instance.
The available storage space of the destination ApsaraDB for Tair (Redis compatible) instance must be larger than the used storage space of the source ApsaraDB for Tair (Redis compatible) instance.

Usage notes

Category	Description
Limits on source database	To ensure synchronization quality, DTS inserts a key with the prefix DTS_REDIS_TIMESTAMP_HEARTBEAT into the source database to record update timestamps. If the source database is a cluster architecture, DTS inserts this key into each shard. This key is filtered during synchronization and expires when the synchronization task ends. If the source database is a read-only instance or the DTS account does not have write (SETEX) permission, 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. Otherwise, data inconsistency between the source and destination databases may occur. If an expiration policy is enabled for specific 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. If the source is a Storage Medium of Persistent Memory Tair (Enterprise Edition) instance, ensure that the appendonly parameter value is set to `yes`. For more information, see procedure. If the source database is a self-managed Redis database: 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.
Other limits	The timeout period for data replication between the master and replica nodes in a Redis database is specified by the repl-timeout parameter. We recommend that you run the `config set repl-timeout 600` command to set the timeout period to 600 seconds for the source instance. If the source instance stores a large amount of data, you can increase the value of the repl-timeout parameter based on your business requirements. During data synchronization, if the number of shards in the source Redis 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 Redis database before you reconfigure the data synchronization task. If the source or destination instance is a self-managed Redis database and the connection address of the self-managed Redis database changes during the data synchronization, you need to 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 of 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. 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 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). By default, the maxmemory-policy parameter that specifies how data is evicted is set to volatile-lru for Tair (Redis OSS-compatible) 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 Tair? If the transparent data encryption (TDE) feature is enabled for the source or destination instance, you cannot use DTS to synchronize data. If any of the following situations occur during data synchronization, the full data may be resynchronized to the destination, resulting in data inconsistency. A transient connection occurs on the source or destination Redis, causing breakpoint resume to fail. A master-slave switch or failover occurs on the source or destination Redis. The connection address of the source or destination Redis changes. The synchronization object of the DTS instance is modified. If the ApsaraDB Tair (compatible with Redis) instance has enabled the TLS (Transport Layer Security) encryption feature, you must connect to DTS using SSL-encrypted (does not support TLSv1.3). Currently, it is not supported to connect an ApsaraDB Tair (compatible with Redis) instance with SSL enabled to DTS as an Alibaba Cloud Instance. If the synchronization instance contains both full and incremental tasks, restarting the synchronization instance may cause DTS to re-execute the full and incremental tasks. If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Billing

Synchronization type	Task configuration fee
Full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

Supported synchronization topologies

One-way one-to-one synchronization
One-way one-to-many synchronization
One-way cascade synchronization

For more information about synchronization topologies and usage notes, see Synchronization topologies.

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
SWAPDB and UNLINK (These two commands can be synchronized only if the engine version of the source Redis instance is 4.0)
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 DTS runs the SYNC or PSYNC command to transfer data of the LIST type, DTS does not clear the existing data. As a result, the destination database may contain duplicate data records.

Permissions required for database accounts

Database	Permissions and authorization method
Source ApsaraDB for Tair (Redis compatible) instance	The database account must have read permissions. For more information about how to grant permissions to an account, see Create and manage database accounts.
Destination ApsaraDB for Tair (Redis compatible) instance	The database account must have read and write permissions. For more information about how to grant permissions to an account, see Create and manage database accounts.

Procedure

Use one of the following methods to go to the Data Synchronization page and select the region in which the data synchronization instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Synchronization.
3. In the upper-left corner of the page, select the region in which the data synchronization instance resides.
DMS console
Note
The actual 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.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI and choose DTS (DTS) > Data Synchronization.
3. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.

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

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 Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select Tair/Redis.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region to which the source ApsaraDB for Tair (Redis compatible) instance belongs.
	Replicate Data Across Alibaba Cloud Accounts	In this example, data is synchronized between instances of the same Alibaba Cloud account. Select No.
	Instance ID	Select the ID of the source ApsaraDB for Tair (Redis compatible) instance.
	Authentication Method	Select an authentication method based on your business requirements. In this example, select Password Login. Note Only Redis databases of version 6.0 or later support Account + Password Login. If you select Secret-free login, make sure that you enable the password-free access feature in the Redis database. For information about how to enable password-free access for a Tair (Redis OSS-Compatible) instance, see Enable password-free access.
	Database Password	Enter the database account password for connecting to the source ApsaraDB for Tair (Redis compatible) instance. For information about the account permissions, see Permissions required for database accounts. Note The database password is in the <user>:<password> format. For example, if the username of the custom account that you use to log on to the Redis database is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
	Encryption	Specifies whether to encrypt the connection to the source database. Select Non-encrypted or SSL-encrypted based on your business requirements. Note If you set Access Method is not to Alibaba Cloud Instance and select SSL-encrypted for the self-managed Redis database, you must upload a CA Certificate and enter a CA Key.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select Tair/Redis.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region to which the destination ApsaraDB for Tair (Redis compatible) instance belongs.
	Replicate Data Across Alibaba Cloud Accounts	In this example, data is synchronized between instances of the same Alibaba Cloud account. Select No.
	Instance ID	Select the ID of the destination ApsaraDB for Tair (Redis compatible) instance.
	Authentication Method	Select an authentication method based on your business requirements. In this example, select Password Login. Note Only Redis databases of version 6.0 or later support Account + Password Login. If you select Secret-free login, make sure that you enable the password-free access feature in the Redis database. For information about how to enable password-free access for a Tair (Redis OSS-Compatible) instance, see Enable password-free access.
	Database Password	Enter the database password of the destination Tair (Redis OSS-Compatible) instance. For information about the permissions 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 custom account that you use to log on to the Redis database is admin and the password is Rp829dlwa, the database password is admin:Rp829dlwa.
	Encryption	Specifies whether to encrypt the connection to the source database. Select Non-encrypted or SSL-encrypted based on your business requirements. Note If you set Access Method is not to Alibaba Cloud Instance and select SSL-encrypted for the self-managed Redis database, you must upload a CA Certificate and enter a CA Key.

Click Test Connectivity and Proceed in the lower part of the page.
Note
- Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.
- If the source or destination database is a self-managed database and its Access Method is not set to Alibaba Cloud Instance, click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the objects to be synchronized.

In the Configure Objects step, configure the objects that you want to synchronize.

Parameter	Description
Synchronization Types	Select Incremental Data Synchronization or Full Data Synchronization + Incremental Data Synchronization. Note For DTS instances that have Incremental Data Synchronization selected, we recommend that you also select Full Data Synchronization.
Synchronization Topology	Select One-way Synchronization. Note This parameter is available only when both the source and destination instances are Tair (Enterprise Edition).
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.
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 objects to be synchronized. If you need to synchronize data for specific keys in a database, you can use the data filtering feature in the Selected Objects section.
Selected Objects	If you want to specify a database to receive data (DB 0 to DB 255) or filter data to be synchronized by prefix, you can use the mapping or filtering feature. Right-click the database to be synchronized in the Selected Objects section, and then configure the parameters in the Edit Schema dialog box that appears. For more information, see Map object names and Specify filter conditions.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
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.
Retry Time for Other Issues	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set this parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note This parameter is available only when Synchronization Types includes Full Data Synchronization.
Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, you do not need to configure this parameter.
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. If specific commands, such as the following commands, are used, we recommend that you set this parameter to ensure data consistency. Note In scenarios that involve distributed locks, the distributed locks may fail to be released in time. `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.
Monitoring and Alerting	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 enable alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see the "Configure monitoring and alerting when you create a DTS task" section of the Configure monitoring and alerting topic.

Click Next Step: Data Verification to configure data verification.
For more information about how to use the data verification feature, see Configure a data verification task.

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 data synchronization task fails the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, rerun the precheck.
- 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.

Purchase a data synchronization instance.

Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the buy 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 a subscription when you create a data synchronization 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. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go data synchronization instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the data synchronization instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides instance classes that vary in synchronization speed. You can select an instance class based on your business requirements. For more information, see Instance classes of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, specify the subscription duration and the number of data synchronization 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 available only if you select the Subscription billing method.

Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Purchase And Start, and in the OK dialog box that appears, click OK.
You can view the task progress on the data synchronization page.
Note
If your DTS instance includes both full and incremental tasks (Synchronization Types includes both Full Data Synchronization and Incremental Data Synchronization), it will be displayed as Incremental Data Synchronization on the synchronization task list page.