When to create new RDS Instance vs new database?

Question

I have two AWS RDS Postgres Instances. Sometimes I create new instances for applications that are (very) vaguely related to other applications. Which always leads me to the question; should I just create a new database in an existing instance or keep things separate and create a new instance instead?

Answer 1

I would recommend that you use the same database server (Amazon RDS instance).

You can logically separate the data via either:

• CREATE DATABASE: Full logical separation. You login to one database and never see the other one. OR
• CREATE SCHEMA: Data is kept separate, but can be referenced from the other. Quite common for staging areas, such as doing ETL in a Staging Schema, then publishing to a Production Schema.

From your description, I'd say that CREATE DATABASE would be appropriate.

The benefit is that you only need to manage one database and there is little impact on cost unless you need to increase the size of the database instance to handle the higher load (but it would still be cheaper than running two separate databases).

Just keep an eye on the CloudWatch metrics to be sure that the database is handling the increased load correctly.

Normally, the biggest reason for using a different server is because they are owned/managed by different teams. However, in your situation the same team seems to 'own' both data stores, so that wouldn't be an issue.