When we talk about data transfer there are a number of different data paths where you might be charged for transferring data IN to and OUT of your RDS database depending on the source and destination, for example:

• Data transferred IN to your RDS database from the internet
• Data transferred OUT from your RDS database to the internet
• Data transferred OUT to Amazon CloudFront
• Data transferred OUT to AWS Regions
• Data transferred OUT to EC2 instances in the same availability zone
• Data transferred between availability zones for multi-az replication
• Data transferred between an EC2 instance and an RDS instance in different availability zones of the same region
• Data transferred when a snapshot copy is transferred to a different region

Across the different DB engines, there is consistency when it comes to data transfer costs, so let’s take a look at each of the price points individually.  In the following examples, I will be taking the pricing from the MySQL DB engine.

Firstly, let me show you under which circumstances you will NOT be charged for data transfer and so it completely free, this includes:

• Any data that is transferred IN to your RDS database from the internet
• Any Data that is transferred OUT to Amazon CloudFront
• Any data that is transferred OUT to EC2 instances in the same availability zone
• Data transferred between availability zones for multi-az replication - For those who might be new to Multi-AZ, it simply means Multi-Availability Zone, which is a feature that is used to help with resiliency and business continuity. When Multi-AZ is configured, a secondary RDS instance, known as a replica, is deployed within a different availability zone within the same region as the primary instance. That's its single and only purpose, to provide a failover option for a primary RDS instance.  As a result, there is a requirement for data replication between the primary and the secondary.

Data transferred OUT from your RDS database to the internet: This element of data transfer is very dependent on how much data you transfer per month and is charged per GB.  As a result, the resulting pricing is reflected as a tiered structure as you can see.

The first GB each month is free, and then as the amount of data transferred increases in Terabytes, the amount charged in GB decreases.  The more you transfer out to the internet, the cheaper per GB your data transfer becomes.

If data is transferred from RDS out to another region, then you will incur a small charge per GB, and as it stands at the time of writing this course, this is a flat fee of $0.02 per GB for ANY region that the data is transferred to.  

In this instance, charges will apply for Amazon EC2 regional data transfer both sides of the transfer and is charged at $0.01/GB in each direction.

As explained earlier, Snapshots in RDS are your backups of your database tables and instances, and these snapshots can be both exported out of Amazon RDS and copied to another region.  These Cross-region snapshots are a great way to help you implement a disaster recovery strategy across your RDS database infrastructure.

When copying your snapshots across regions you are just charged for the amount of data transferred based on the size of the snapshot, based on $0.02 per GB.