In the realm of cloud-based application development, AWS Cognito serves as a powerful user authentication and authorization service, specifically designed to facilitate sign-in across multiple platforms. A common challenge encountered involves managing users who sign in using multiple Identity Providers (IdPs) like Google and Facebook, but share the same email address. Failing to handle such scenarios appropriately can result in fragmented user identities, leading to a poor user experience. This article explores best practices to manage such situations in AWS Cognito, employing technical examples for clarity.

Overview of AWS Cognito

AWS Cognito enables developers to add user sign-up, sign-in, and access control to their web and mobile apps swiftly. Cognito supports federated identities that allow users to authenticate using third-party IdPs, such as Google, Facebook, and others, alongside native Cognito user pools.

Key Components

• User Pools: This feature helps manage sign-up and sign-in functionality for app users.
• Identity Pools (Federated Identities): Allows linking user identities from various platforms.

The Challenge of Multiple IdPs with the Same Email

Users often prefer the flexibility of signing in with different IdPs without creating separate accounts. This can lead to potential conflicts within AWS Cognito if the same email address is used across different IdPs. This section deals with merging multiple identities into a single user profile.

Problems Encountered

• User Fragmentation: The same real-world user may end up having multiple separate accounts.
• Data Inconsistencies: Each account can have different user attributes, leading to inconsistencies.
• Access Control Issues: Managing permissions and roles across fragmented identities becomes cumbersome.

Best Practices for Managing Multiple IdPs

The objective is to unify the user experience across different IdPs, enabling seamless access through a single user profile.

1. Enable Email-based Linking

Linking users with the same email is crucial. With AWS Cognito, you can use Lambda triggers to orchestrate the linking:

• `custom:last_id_provider`
• `custom:linked_accounts`
• Step 1: Prompt the user to select a preferred IdP.
• Step 2: Authenticate the user with the IdP.
• Step 3: Use Lambda triggers to check for existing users with the same email and merge identities accordingly.