This topic introduces SQL Server read-only instances. If your application initiates a small number of write requests but a large number of read requests, a single instance may not be able to resist the read pressure. As a result, services may be affected. To achieve the elastic expansion of the read ability and share the pressure of the database, you can create one or more read-only instances in a region. The read-only instances can handle massive read requests and increase the application throughput.

Overview

A read-only instance is a read-only copy of the master instance. Changes to the master instance are also automatically synchronized to all relevant read-only instances.
Note
  • For RDS SQL Server, only the SQL Server 2017 Cluster (AlwaysOn) Edition supports read-only instances.
  • Each read-only instance adopts a single-node architecture (without slave nodes).
The following topology shows the positioning of the read-only instance.

Network diagram

Pricing

The billing method of read-only instances is Pay-As-You-Go. The following table lists the prices of common instances.

Hourly prices of specifications and storage

Region

rds.mssql.s2.large

2-core 4 GB

rds.mssql.s2.xlarge

2-core 8 GB

rds.mssql.s3.large

4-core 8 GB

rds.mssql.m1.medium

4-core 16 GB

rds.mssql.c1.large

8-core 16 GB

rds.mssql.c1.xlarge

8-core 32 GB

rds.mssql.c2.xlarge

16-core 64 GB

Storage
China mainland's regions $0.225 $0.447 $0.459 $0.851 $0.888 $1.732 $3.389 $0.0003/GB
China (Hong Kong) $0.264 $0.522 $0.537 $0.993 $1.035 $2.02 $3.954 $0.0004/GB
US (Virginia) $0.273 $0.542 $0.556 $1.028 $1.072 $2.093 $4.096 $0.0003/GB
US (Sillicon Vally) $0.292 $0.579 $0.595 $1.099 $1.146 $2.237 $4.378 $0.0003/GB
Singapore $0.311 $0.616 $0.632 $1.170 $1.220 $2.381 $4.661 $0.0004/GB
Australia $0.315 $0.622 $0.646 $1.209 $1.259 $2.415 $4.829 $0.0005/GB
Malaysia $0.296 $0.586 $0.601 $1.112 $1.159 $2.262 $4.428 $0.0004/GB
Indonesia $0.311 $0.616 $0.632 $1.170 $1.220 $2.381 $4.661 $0.0004/GB
Japan $0.311 $0.615 $0.632 $1.171 $1.221 $2.381 $4.660 $0.0005/GB
Germany (Frankfurt) $0.311 $0.615 $0.632 $1.171 $1.221 $2.381 $4.660 $0.0005/GB
UK (London) $0.311 $0.615 $0.632 $1.171 $1.221 $2.381 $4.660 $0.0005/GB
UAE (Dubai) $0.327 $0.646 $0.665 $1.230 $1.283 $2.500 $4.895 $0.0007/GB
India (Mumbai) $0.296 $0.586 $0.601 $1.112 $1.159 $2.262 $4.428 $0.0004/GB

Features

Read-only instances offer the following features:
  • Account and database management: No account or database maintenance is required for a read-only instance. Both the account and database are synchronized through the master instance.
  • Billing: Read-only instances support billing measured per hour, which is user-friendly and cost-efficient.
  • Specifications: The specifications of a read-only instance can differ from those of the master instance, and can be changed at any time. We recommend that the specifications of the read-only instance be equal to or higher than those of the master instance; otherwise the read-only instance may have high latency or workloads.
  • Network type: can differ from that of the master instance.
  • Whitelist: When a read-only instance is created, it automatically copies the whitelist of the master instance. However, the whitelist of the read-only instance is independent from that of the master instance. You can modify the whitelist of the read-only instance by referring to Configure a whitelist for an RDS for SQL Server instance.
  • Monitoring and alarms: Up to 20 system performance monitoring views can be used, which includes disk capacity, IOPS, connections, CPU utilization, and network traffic. Users can view the load of instances at ease.

Limits

  • Quantity of read-only instances:
    Database Quantity
    SQL Server Up to 7 read-only instances can be created for each master instance.
  • Read-only instances do not support backup settings or manual backup.
  • Instance recovery:
    • Read-only instances do not support the creation of temporary instances through backup files or a point in time. Read-only instances do not support the overwriting of instances using backup sets.
    • After creating a read-only instance, the master instance does not support data recovery through the direct overwriting of instances using backup sets.
  • You cannot migrate data to read-only instances.
  • You cannot create or delete databases for read-only instances.
  • You cannot create or delete accounts for read-only instances.
  • You cannot authorize accounts or modify account passwords for read-only instances.

FAQ

Can the accounts on the master instance be used on the read-only instances?

Accounts on the master instance are synchronized to the read-only instances. You can use the accounts to read data from the read-only instances but cannot write data into the read-only instances.