Android SDK中联合身份登录后，向Cognito用户池添加用户的最优方式？

1. Optimal Way to Add Users to Cognito User Pool After Federated Login #

The best approach is to lean into Cognito's native federation tools with automatic user pool sync instead of building custom logic from scratch. Here's how to implement it:

• Configure your Cognito Identity Pool to integrate with your User Pool, then enable the "Auto-create users" setting in the Identity Pool's authenticated identities configuration. When a user logs in via a third-party provider (like Google or Facebook), Cognito will automatically create a matching user in your User Pool and link the federated identity to it—no manual API calls required from your app.
• If you need to add custom attributes or run post-creation logic, use a Post Authentication Lambda trigger on your User Pool. This trigger fires right after the user is created, letting you populate extra attributes or execute custom workflows without touching your client code.

2. Handling Third-Party Logins in Android SDK (Auto-Add to User Pool + Custom Flow) #

First, a quick clarification: By default, Cognito Identity Pool only issues temporary AWS credentials for federated users—it doesn’t auto-create a User Pool user. Here’s the optimal path, plus feedback on your proposed solution:

Optimal Native Solution (Minimal Custom Code) #

• Set Up Identity Pool + User Pool Integration: Just like the first question, configure your Identity Pool to trust your User Pool as an identity provider and enable "Auto-create users". When your Android app uses the SDK to authenticate via Google/Facebook, Cognito handles creating the User Pool user and linking the federated identity in the background. This is the most secure and low-maintenance option since it leverages Cognito’s built-in capabilities.

Custom Flow (If You Need User Input Before Creating the User) #

Your proposed solution—logging in, checking for existing users via email, then collecting additional info—is completely feasible, but you need to handle it securely:

• Never call admin APIs directly from Android: Avoid using AdminCreateUser or ListUsers in your app code. These require IAM admin permissions, and exposing those credentials client-side is a critical security risk. Instead, use a backend service (like AWS Lambda or your own API) to handle these operations.
• Step-by-step secure flow:
  1. After the user logs in via a third-party provider in your Android app, pull their basic profile info (email, name, etc.) from the IdP.
  2. Send the user’s email to your backend, which calls ListUsers on the User Pool to check if a matching user already exists.
  3. If the user exists: Have your backend call AdminLinkProviderForUser to link the federated identity to the existing User Pool user. Then fetch the user’s current info and display a form for them to fill in any missing attributes.
  4. If the user doesn’t exist: Show your info collection form in the Android app. Once the user submits the form, send the details (email + custom attributes) to your backend, which calls SignUp to create the User Pool user, then links the federated identity with AdminLinkProviderForUser.
• Alternative: Cognito Hosted UI: If you don’t want to build the info form yourself, use Cognito Hosted UI. It can be configured to prompt users for additional attributes on their first third-party login, and automatically creates the User Pool user for you.

Should You Register Users Directly in the Android App? #

Only use client-side registration (like CognitoUserPool.signUp()) if you’re collecting user info before they log in via a third-party provider. If the user already authenticated with a federated IdP, it’s safer to handle User Pool creation/linking via your backend or Cognito’s auto-creation feature to avoid exposing sensitive permissions.

内容的提问来源于stack exchange，提问作者Mehmet Kemal Bayer