Would this work to distribute traffic to my RDS Read Replicas?

Question

I am using Amazon RDS for my database services and want to use the read replica feature to distributed the traffic amongst the my read replica volumes. I currently store the connection information for my database in a single config file. So my idea is that I could create a function that randomly picked from a list of my read-replica endpoints/addresses in my config file any time my application performed a read.

Is there a problem with this idea as long as I don't perform it on a write?

Answer 1

My guess is that if you have a service that has enough traffic to where you have multiple rds read replicas that you want to balance load across, then you also have multiple application servers in front of it operating behind a load balancer.

As such, you are probably better off having certain clusters of app server instances each pointing at a specific read replica. Perhaps you do this by availability zone.

The thought here is that your load balancer will then serve as the mechanism for properly distributing the incoming requests that ultimately lead to database reads. If you had the DB reads randomized across different replicas you could have unexpected spikes where too much traffic happens to be directed to one DB replica causing resulting latency spikes on your service.

Answer 2

The biggest challenge is that there is no guarantee that the read replicas will be up-to-date with the master or with each other when updates are made. If you pick a different read-replica each time you do a read you could see some strangeness if one of the read replicas is behind: one out of N reads would get stale data, giving an inconsistent view of the system.

Choosing a random read replica per transaction or session might be easier to deal with from the consistency perspective.