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rds-read-replicas.md

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RDS Read Replicas for Read Scalability

  • Read replicas help us to scale our read operations
  • We can create up to 15 read replicas within the same AZ, cross AZ or cross region
  • Replication is async between the RDS and replicas => reads are eventually consistent
  • Replicas can be promoted as standalone databases

Read Replica Use Cases

  • Read replicas are used for reading => only SELECT kind of statements should be executed on them
  • Example of use case: analytics on top of the main database which can cause performance degradation of the main DB

Read Replica Network Cost

  • In AWS there is a network cost when data goes from one AZ to another
  • Exception for managed services, since RDS read replicas are considered to be managed services, within the same region we don't pay a free for inter-AZ data transfer

RDS Multi AZ (Disaster Recovery)

  • It is a synchronous replication to another stand-by database in another AZ
  • We have only one DNS name for the database, the failover in case of a disaster happens automatically => will increaser availability
  • Failover happens automatically in case of an AZ outage, loss of networking connectivity or instance storage failure
  • No manual intervention required
  • Not used for scaling!
  • The Read Replicas can also have Multi AZ synchronous replication enabled

From Single-AZ to Multi-AZ

  • In case we go for a database from single-AZ setup to multi-AZ:
    • There is no downtime (no need to stop the DB)
    • Only thing to do is to click on modify for the database
  • The following will happen internally:
    • A snapshot is taken automatically
    • A new DB is restored automatically from the snapshot in a new AZ
    • Synchronization is established between the two databases

Aurora Replicas - Auto Scaling

Aurora Auto Scaling

  • Clients must use the writer endpoint for accessing the writer; they must connect to the reader endpoint to access the read replicas

Global Aurora

  • Aurora Cross Region Read Replicas:
    • Useful for disaster recovery
    • Simple to set up
  • Aurora Global Database (recommended)
    • 1 region is defined as the primary region (can be used for reads and writes)
    • Up to 5 secondary regions, can be used for reads only. Replication lag is less than 1 second
    • We can have up to 16 read replicas per secondary region
    • In case of a disaster, the RTO for promoting another region is less than 1 minute
    • Typical cross-region replication takes less then 1 second