Database replication (for example, read replicas)

Database replication is a fundamental concept that finds an important place in the study material for the AWS Certified Solutions Architect – Associate exam. It is a technique used to copy and maintain database objects, such as data from one database to another, ensuring high availability and fault tolerance of database systems.

Within the context of AWS, one of the prominent database replication techniques is the use of read replicas. AWS offers read replicas as part of its Relational Database Service (RDS). Read replicas provide a way to scale out beyond the capacity constraints of a single DB instance for read-heavy database workloads.

What are Read Replicas?

Read replicas are essentially copies of your primary database instance that reflect changes made to the data in a near real-time fashion. They allow you to offload read traffic from your primary database instance, thus improving the performance of the primary instance for write-heavy operations. This is especially important for applications that need to serve high volumes of read requests, such as reporting queries or secondary read-only application engines.

Types of Replication Supported by AWS RDS

AWS RDS supports two types of replication:

• Asynchronous Replication: Changes to the primary DB instance are written to the read replica after the primary instance has been written to. This method can lead to a minor lag between the primary database and the replica.
• Synchronous Replication: Featured in the Amazon Aurora, this type of replication writes data simultaneously to the primary instance and replica with virtually no lag. Though not commonly referred to as a read replica, Amazon Aurora replicas function like enhanced read replicas.

Benefits of Using Read Replicas on AWS

• Scalability: Distribute traffic across multiple read endpoints, allowing more concurrent users and higher throughput.
• Availability: Provide additional failover targets in case the primary instance fails, minimizing downtime.
• Data locality: Place read replicas in different regions closer to your users, reducing latency.
• Offload reporting: Run reporting and analytics queries against read replicas without impacting the performance of the primary instance.

Creating Read Replicas in AWS RDS

To create a read replica within AWS, you use either the AWS Management Console, AWS CLI or RDS API. Here is a basic example of how to create a read replica using the AWS CLI:

aws rds create-db-instance-read-replica \
–db-instance-identifier my-read-replica \
–source-db-instance-identifier my-original-rds-instance \
–availability-zone us-west-2a

This command creates a read replica named my-read-replica of the my-original-rds-instance, placing the instance in the us-west-2a availability zone.

Monitoring and Performance

Monitoring is crucial to maintaining healthy read replicas. AWS provides several metrics to keep track of, such as:

• Replica Lag: The amount of time a read replica lags behind the primary DB instance.
• CPU Utilization: How much computational power the read replica is using.
• Read IOPS: Input/output operations per second on the read replica.

AWS CloudWatch can be used to monitor these metrics, and setting up alarms for thresholds can prevent performance degradation.

Limitations and Considerations

• Latency: While read replicas are generally close to real-time, there may be replication lag, which is important for applications requiring up-to-date read data.
• Cost: Each read replica incurs additional costs, including instance costs and potential cross-AZ or cross-region data transfer costs.
• Maintenance: Read replicas must be monitored and maintained similarly to your primary database instance.
• Consistency: Read replicas are eventually consistent, meaning that the most recent write might not be immediately visible.

Conclusion

In the context of the AWS Certified Solutions Architect – Associate exam, understanding the configuration, benefits, costs, and best practices of database replication, particularly read replicas, is vital. Knowledge of when and how to implement replication effectively plays a key part in designing reliable, scalable, and efficient systems in the AWS cloud environment.