CODEWITHSUNDEEP
amazon-web-servicesamazon-rds
HUGO-DEV
HUGO-DEV

Reputation: 935

AWS RDS Read Replica act as Failover Standby

I am currently assessing whether to use RDS MySQL Multi-AZ or Single AZ with Read Replica.

Considerations are budget and performance, as Multi-AZ cost twice as much as Single AZ and have no ability to offload read operations, Single AZ with Read Replica seems to be a logical choice.

However, I saw a way to manually 'promote' the Read Replica to master in the event of master's failure, but is there a way to automate this?

Note: There was a similar question but it did not address my question: Read replicas in RDS AWS

Upvotes: 3

Views: 5168

Answers (1)

mipo256
mipo256

Reputation: 3214

I think the problem is that you are a bit confused with these features. Let me help - you can launch AWS RDS in Multi-AZ deployment mode. In this case, AWS will do the following:

  1. It will allocate a DNS record for you. This DNS record represents a single entry point to your master database, which is, lets assume, currently active and able to serve connections.

  2. In the case of master failure for any reason, AWS will simply address hidden by the DNS record (quite fast, within 1-2 minutes) to be pointed to your stand by, which is located on another AZ.

  3. When the master will become available again, then your stand by, which have served writes also needs now to synchronize everything with the master. You do not need to take care about it - AWS will manage it for you

In case of read replica:

  1. AWS will allocate you 2 different DNS records - one for master, another for read replica. Read replica can be on the same AZ as a master, or even in an another Region

  2. You can, and must in you application choose what DNS name to use in different scenarios. I mean, you, most probably, will have 2 different connection pools - one for master, another for read replica. Replication itself will be asynchronous

  3. In the case of read replica, AWS solves the problem of replication by its own - you do not need to worry about it. But since the replica is read only AWS does not solve, by nature, the synchronization problem between read replica and master, because the replica is aimed to be read only, it should not accept any write traffic

Addressing your question directly:

Technically, you can try to make you read replica serve as a failover, but in this case you will have to implement a custom solution for synchronization with the master, because during the time the master was down, your read replica certainly received N amount of writes. AWS does not solve this synchronization problem in this case

In redards to Mutli-AZ - you cannot use your Multi-AZ standby as read replica, since it is not supported in AWS. I highly recommend to check out this documentation. I think it will help you sort the things out, have a nice day!)

Upvotes: 4

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