This topic provides an overview of read-only ApsaraDB RDS for SQL Server instances. If your database system receives a small number of write requests but a large number of read requests, the primary RDS instance may be overwhelmed by the read requests and your workloads may be interrupted. In this case, you can create one or more read-only RDS instances to offload read requests from the primary RDS instance. This increases the read capability of your database system and the throughput of your application.

Background information

When you create a read-only RDS instance, it replicates data from the secondary RDS instance to the read-only RDS instance. The data of the created read-only RDS instance is consistent with that of the primary RDS instance. Data updates on the primary RDS instance are synchronized to all the read-only RDS instances that are attached with the primary RDS instance.
Note
  • You can create read-only RDS instances only when the primary RDS instance runs SQL Server 2017 EE or 2019 EE.
  • Each read-only RDS instance runs in a single-node architecture. In this architecture, no secondary RDS instance is provided as a standby for a read-only RDS instance.
The following figure shows the topology of read-only RDS instances.

Billing

Read-only RDS instances are charged on a pay-as-you-go or subscription basis. For more information about pricing, see Read-only ApsaraDB RDS instance types.

Features

  • Read-only RDS instances support both the pay-as-you-go and subscription billing methods. The pay-as-you-go billing method is flexible, and allows you to release your read-only RDS instances when the instances are no longer needed. The subscription billing method is cost-effective for long-term commitments.
  • Read-only RDS instances reside in the same region as the primary RDS instance, but possibly in different zones.
  • The specifications of read-only RDS instances can differ from the specifications of the primary RDS instance. You can change the specifications of read-only RDS instances at any time. We recommend that the specifications of read-only RDS instances be higher than or equal to the specifications of the primary RDS instance. If the specifications of a read-only RDS instance are lower than the specifications of the primary RDS instance, the read-only RDS instance may encounter issues such as high latency or heavy load.
  • The network types of read-only RDS instances can differ from the network type of the primary RDS instance.
  • You do not need to manage accounts or databases on read-only RDS instances. The account and database data on read-only RDS instances is synchronized from the primary RDS instance.
  • When you create a read-only RDS instance, ApsaraDB RDS replicates the IP address whitelists of the primary RDS instance to the read-only RDS instance. However, the IP address whitelists on the read-only RDS instance are independent of the IP address whitelists on the primary RDS instance. For more information about how to modify the IP address whitelists of a read-only RDS instance, see Configure an IP address whitelist for an ApsaraDB RDS for SQL Server instance.
  • Read-only RDS instances support monitoring and alerting. You can monitor about 20 metrics, such as the disk usage, input/output operations per second (IOPS), number of connections, CPU utilization, and network traffic.

Limits

  • You can create up to seven read-only RDS instances for each primary RDS instance.
  • You cannot configure backup policies or manually create backups for read-only RDS instances. These are configured and created on the primary RDS instance.
  • You cannot create a temporary RDS instance from a data backup file or to a specific point in time. In addition, you cannot overwrite the data of a read-only RDS instance by using a data backup file.
  • After a read-only RDS instance is created, you cannot use a data backup file to overwrite the data on the primary RDS instance.
  • You cannot migrate data to read-only RDS instances.
  • You cannot create or delete databases on read-only RDS instances.
  • You cannot create or delete accounts, grant the permissions on specific databases to accounts, or change the passwords of accounts on read-only RDS instances.

FAQ

After I create accounts on my primary RDS instance, can I manage the accounts on the read-only RDS instances?

No, although the accounts created on the primary RDS instance are replicated to its read-only RDS instances, you cannot manage the accounts on the read-only RDS instances. The accounts have only the read permissions on the read-only RDS instances.