Amazon Cognito

Amazon Cognito lets you add user sign-up, sign-in, and access control to your web and mobile apps quickly and easily. It can scale to millions of users and supports sign-in with social identity providers, such as Facebook, Google, and Amazon, and enterprise identity providers via SAML 2.0.

In this case, I am using Cognito as it is very easy to start with and integrate, plus I am using a bunch of features that can integrate with Lambda where I can put my custom logic.

For more details check out the documentation.

Pricing

If you are using Amazon Cognito Identity to create a User Pool, you pay based on your monthly active users (MAUs) only. A user is counted as a MAU if, within a calendar month, there is an identity operation related to that user, such as sign-up, sign-in, token refresh, or password change. You are not charged for subsequent sessions or for inactive users within that calendar month.

Pricing Tier (MAUs)	Price per MAU
First 50,000	Free
Next 50,000	$0.00550
Next 900,000	$0.00460
Next 9,000,000	$0.00325