Read replicas are asynchronous copies of the primary database that are used only for read-only purposes. You can create and have multiple read replicas for your primary database. When new data is written to the primary database, AWS copies that data for you asynchronously to the read replicas. As the data is written asynchronously, AWS provides you with a way to monitor the lag between primary and read replicas using a CloudWatch alarm. Read replicas can be created from other read replicas. This helps you to avoid a performance hit if you have a highly transactional primary database and there is a need for you to have multiple read replicas. Not all the database engines support read replicas. Currently, you can create read replicas for Aurora, MySQL, MariaDB, and PostgreSQL. You cannot create read replicas for SQL Server or the Oracle database engine.
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