# Configuring and managing a Multi-AZ deployment

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## Multi-AZ DB instance deployments

* **Note**\
  The high availability option isn't a scaling solution for read-only scenarios. You can't use a standby replica to serve read traffic. To serve read-only traffic, use a Multi-AZ DB cluster or a read replica instead. For more information about Multi-AZ DB clusters, see [Multi-AZ DB cluster deployments](https://github.com/kevinslin/aws-reference-notes/blob/main/services/amazon_rds/multi-az-db-clusters-concepts.md). For more information about read replicas, see [Working with DB instance read replicas](https://github.com/kevinslin/aws-reference-notes/blob/main/services/amazon_rds/USER_ReadRepl.md).
* **Important**\
  Using a snapshot to create the standby instance avoids downtime when you convert from Single-AZ to Multi-AZ, but you can experience a performance impact during and after converting to Multi-AZ. This impact can be significant for workloads that are sensitive to write latency.\
  While this capability lets large volumes be restored from snapshots quickly, it can cause a significant increase in the latency of I/O operations because of the synchronous replication. This latency can impact your database performance. We highly recommend as a best practice not to perform Multi-AZ conversion on a production DB instance.\
  To avoid the performance impact on the DB instance currently serving the sensitive workload, create a read replica and enable backups on the read replica. Convert the read replica to Multi-AZ, and run queries that load the data into the read replica's volumes (on both AZs). Then promote the read replica to be the primary DB instance. For more information, see [Working with DB instance read replicas](https://github.com/kevinslin/aws-reference-notes/blob/main/services/amazon_rds/USER_ReadRepl.md).
* **Note**\
  You can force a failover manually when you reboot a DB instance. For more information, see [Rebooting a DB instance](https://github.com/kevinslin/aws-reference-notes/blob/main/services/amazon_rds/USER_RebootInstance.md).
* **Note**\
  The default TTL can vary according to the version of your JVM and whether a security manager is installed. Many JVMs provide a default TTL less than 60 seconds. If you're using such a JVM and not using a security manager, you can ignore the rest of this topic. For more information on security managers in Oracle, see [The security manager](https://docs.oracle.com/javase/tutorial/essential/environment/security.html) in the Oracle documentation.

## Multi-AZ DB cluster deployments

* **Important**\
  Multi-AZ DB clusters aren't the same as Aurora DB clusters. For information about Aurora DB clusters, see the [Amazon Aurora User Guide](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/CHAP_AuroraOverview.html).
* **Important**\
  To prevent replication errors in RDS for MySQL Multi-AZ DB clusters, we strongly recommend that all tables have a primary key.
* **Note**\
  The delay is shown in microseconds.
* **Note**\
  The default TTL can vary according to the version of your JVM and whether a security manager is installed. Many JVMs provide a default TTL less than 60 seconds. If you're using such a JVM and not using a security manager, you can ignore the rest of this topic. For more information on security managers in Oracle, see [The security manager](https://docs.oracle.com/javase/tutorial/essential/environment/security.html) in the Oracle documentation.


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