Overview of Read Replicas

What Is a Read Replica?

In read-intensive scenarios, a single DB instance may be unable to handle the read pressure and service performance may be affected. To offload read pressure on the primary DB instance, you can create one or more read replicas in the same region as the primary instance. These read replicas can process a large number of read requests and increase application throughput. You need to separately configure connection addresses of the primary DB instance and each read replica on your applications so that all read requests can be sent to read replicas and write requests to the primary DB instance.

Read Replica Architecture

A read replica uses a single-node architecture (with no standby node). RDS synchronizes changes from the primary instance to all read replicas using the native PostgreSQL replication. If network latency on the primary instance is high, data synchronization between the primary instance and the read replicas will be affected. Read replicas and the primary instance must be in the same region but can be in different AZs.

Figure 1 Topology

Billing

Read replicas are billed separately. They support yearly/monthly and pay-per-use billing modes. Yearly/monthly billing provides greater discounts than pay-per-use billing and is recommended for long-term users.

For details, see the read replica description on RDS Pricing Details.

Functions

• Reducing load: Read replicas help offload read-intensive workloads from the primary instance.
• Region and AZ: Read replicas and the primary instance are in the same region but can be in different AZs.
• Storage type: Read replicas have the same storage type as the primary instance.
• Storage capacity: The storage capacity of a read replica is the same as that of the primary instance.
• Specifications: Read replica specifications can differ from those of the primary instance but must be greater than or equal to the primary instance's specifications. Otherwise, high latency or heavy load may occur on the read replica.
• Network: The VPC and security group of a read replica are the same as those of the primary instance. EIPs can be bound to or unbound from read replicas.
• Administrator account and password: The administrator account of a read replica is root and its password is the same as that of the primary instance. When you change the administrator password of the primary instance, the password of its read replica will also be changed.
• Databases and accounts: You do not need to maintain databases or accounts on read replicas. They are synchronized from the primary instance.
• Monitoring and alarms: The system monitors the performance of read replicas.

• Anomaly diagnosis: The system can diagnose anomalies such as out-of-memory risks, frequent slow SQL queries, connection bottleneck risks, and insufficient capacity, and provide handling suggestions. For details, see Instance Summary.

Constraints

• Primary DB instances and read replicas billed on the yearly/monthly basis can be unsubscribed only through the RDS console.
• If both the primary instance and its read replicas use yearly/monthly billing, unsubscribe the read replicas first. Otherwise, refund processing for the read replicas may fail.
• Expired primary instances that use yearly/monthly billing are moved to the recycle bin and deleted after the retention period. If a primary instance has read replicas, the read replicas cannot be refunded once the primary instance is deleted.
• Read replicas cannot be purchased separately. You can purchase them only for your created instance.
• A maximum of five read replicas can be created for a DB instance.
• You cannot stop a read replica separately. Stopping a primary instance will automatically stop its read replicas.
• Backup settings: Read replicas do not support automated backups or manual backups. No backups are generated for read replicas.
• Instance restoration: Read replicas cannot be created from backups or point-in-time recovery, and backup sets cannot be used to overwrite a read replica.
• Data migration: Migrating data to a read replica is not supported.
• Database management: Databases cannot be created or deleted on read replicas.
• Account management: Read replicas do not provide permissions to create accounts. Create accounts on the primary instance. For details, see Creating a Database Account.
• Recycle bin: Read replicas are not moved to the recycle bin after being deleted.