# RDS Read Replicas

* Multiple database engines have **built-in replication functionality to create a special type of database instance called a read replica from a source database instance**.<br>

* The **source DB instance becomes the primary database instance**.<br>

* **Updates made to the primary database instance are asynchronously copied to the read replica**.<br>

* You can **reduce the load on your primary database instance by routing read queries from your applications to the read replica**.

* You **can promote a read replica into a standalone database instance**.
  * When you promote a read replica, the **database instance is rebooted before it becomes available**.<br>

* The following are where read-replicas are useful:<br>

  * Scaling beyond the compute or I/O capacity of a single DB instance for **read-heavy database workloads**.
    * You can **direct this excess read traffic to one or more read replicas**.<br>
  * Serving read traffic while the **source database instance is unavailable**.
    * In these cases, you can **direct read traffic to your read replicas**.
      * For this use case, keep in mind that the data on the **read replica might be "stale" because the source database instance is unavailable**.<br>
  * **Business reporting** or data warehousing scenarios.
    * You **might want business reporting queries to run against a read replica, rather than your production database instance**.<br>
  * Implementing **disaster recovery**.
    * You **can promote a read replica to a standalone instance as a disaster recovery solution if the primary database instance fails**.

* **Data goes from 1 AZ to another = Data Charges**.

* **Data stays in the same AZ = Free**.


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