We next turn our attention to RDS Instances. So an RDS Instance is a database service running in a virtual computing environment. So essentially it's a virtual machine that is running a database engine for you. A user can decide the database engine, CPU, memory, storage, and network capabilities of the RDS Instance. RDS provides a wide selection of instance types that are optimized to fit different use cases. Instance types comprise varying combinations of CPU, memory, storage, and network capacity.

So how do you choose an RDS Instance type? So essentially, we divide our instances into three series, and this is independent of the actual instance hardware. This has to do with how many active database nodes are running. Essentially we divide our instances up into different types or series based on the level of HA, or high-availability service that they provide. So the Basic Edition has decoupled storage and compute, with a single computing node. So if that node fails, the database fails. This is not high availability, there's only a single node. But the Basic Edition offers very high cost effectiveness, because there's only a single compute node, so your costs are low.

The High-Availability Edition is a classic HA architecture with one master node and one slave node, with SSD storage providing better performance. The HA edition is what we recommend for most production applications. And then there's the Enterprise Edition. This has one master node and two slave nodes, it's a three node system. This offers our highest reliability and availability guarantee, and it offers strong data consistency by synchronizing logs between these three database instances. This is something you would use for financial level or super critical production applications. And then we also have the special edition for Microsoft SQL Server 2017, called Cluster Edition, which uses the always on SQL Server functionality to provide HA. We won't talk about that edition more, because it only applies to SQL Server 2017.

So let's look at the critical difference between Basic and HA. So, Basic Edition contains only a single node with no slave node for fault recovery. As a result, it's half the price of High-Availability Edition. So it's a great choice for low cost deployments, or for testing and development deployments. The High-Availability Edition has two instances in the same or different zones, with automatic data synchronization, and automatic failover. In the HA and Enterprise Editions, if one of the nodes fails, if the master node fails, one of the slaves can immediately take over and become the new master. I should point out that in Basic Edition, you can still recover from a failure, it's just not automatic. You would have to create a new temporary database instance, and restore from a previous backup. Whereas with HA, if the master node fails, the system can fail over to one of the slave nodes in near real time. 

So just to recap on the editions here, Basic Edition has a single node. This is a good choice for small websites or development and testing environments. High-Availability Edition has two nodes. This is a good choice for production databases at medium or large scale enterprises. And then Enterprise Edition uses a three node architecture. This is what you want to use for core databases with extremely high data security requirements. So this would be something in financial securities, insurance, banking, et cetera. This is for scenarios where you really cannot tolerate a failure at all. Instance types, there are essentially two.

There's what we call the Common Instance, this is our most cost-effective instance. A great advantage of this type is that storage capacity is not linked in any way with the rest of the hardware configuration. So you have a lot of flexibility, but you are sharing hardware with other users. So there's the potential for another user's database throughput or high CPU usage, to have an impact on you, although this is not common. Then there's the Dedicated Instance or Dedicated Host. In this case, you have fully dedicated CPU cores with threads that are assigned to you, an IO capacity that's assigned to you, so that even if another user is using a lot of resources on the machine, your ability to use the resources allocated to you, is guaranteed. This also has what we call reserved storage space for higher stability.

So typically, we recommend the Dedicated Instance because of the dedicated CPU, memory and storage that you're guaranteed. And I should point out, both of these are shared OS scenarios. You may not be the only tenant from a hardware, the difference is that with the Dedicated Instance, you have guaranteed performance. In the next section, we'll take a look at some of RDS's features.