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This is a good question, while RDS in free-tier is limited to single-AZ instances it is possible to have a read replica in the same AZ. As the read-replica is just another DB instance then as long as it is also (e.g.) a db.t2.micro then it sounds like it should still be in-scope of free tier. And as the read-replica is in the same AZ then there will be no cross-AZ data charges (not that there should be any charge for that anyway, see the horizontal green line in Figure 3. Workload components across Availability Zones of https://aws.amazon.com/blogs/architecture/overview-of-data-transfer-costs-for-common-architectures/ ).
The other limiting factors are 750 hours and 20GB of storage per month. So doing the maths (and assuming that a read replica in the same AZ does fall within the scope of free tier, and I'm not saying that it does) you would need to limit it to 375 hours per month, with 10GB on your primary and 10GB on the read replica.
I agree that there isn't anything in the AWS documentation that is explicitly clear on this one way or the other.
Hello.
The free tier of RDS is for single-AZ configurations.
Therefore, if you create a read replica, you will be charged a fee.
The 750 hours is the amount of RDS usage time within your account, not for a single RDS instance.
In other words, if you launch two RDS instances, you will be using them for 1,488 hours a month.
https://aws.amazon.com/rds/free/?nc1=h_ls
750 hours of Amazon RDS Single-AZ db.t2.micro, db.t3.micro, and db.t4g.micro Instances usage running MySQL, MariaDB, PostgreSQL databases each month. If running more than one instance, usage is aggregated across instance classes.
I think it's easier to understand by looking at the fee estimate when RDS starts up.
The image below is a price estimate for setting up with t3.micro.
If you set up Multi-AZ, you will see that the price is for two devices.
I don't wanna pay any charge, so I must keep paying attention to the RDS. Thanks a lot
Please read the documentation below.
If you add a read replica, it will exceed 750 hours per month, so it will not be covered by the free tier and you will be charged a fee.
https://aws.amazon.com/rds/faqs/?nc1=h_ls
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Thank you for your kind answer. It helps a lot!!
No worries. The worst that could happen is you create a 10GB read replica and run for 375 hours and end up getting charged for it, and that would cost USD7.35 in us-east-1 https://calculator.aws/#/estimate?id=f09e8d1dea44de34af8a612eea2900b102d8eedd
You may want to create the single-AZ read replica, run it for a day and then terminate it (i.e. go back to single instance) and then at the end of the month see whether you were charged for it or not (would be about 25c). if it turns out it's free then come back on here and let us all know :-)