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When working with AWS Cognito in Terraform, one of the most common and often misunderstood tasks is retrieving the User Pool ARN by name. Whether you’re configuring permissions, integrating authentication with your backend, or setting up cross-service access, getting the ARN (Amazon Resource Name) of a Cognito User Pool is often essential. However, Terraform doesn’t directly allow querying resources purely by name. This makes retrieving the ARN a bit more involved but entirely achievable with the right approach.

Why You Might Need the ARN of a Cognito User Pool

If you’re building a scalable infrastructure on AWS using Infrastructure as Code (IaC) with Terraform, you may encounter situations where you need to reference the get user pool arn by name in terraform  in IAM policies, Lambda triggers, or even CloudWatch rules. The User Pool ARN uniquely identifies the Cognito User Pool resource and is required in many service integrations that enforce strict permission policies.

Unfortunately, there’s no direct data source in Terraform to get user pool details just by name. So, you must plan an alternate approach. Let's explore how to get user pool ARN by name in Terraform in a smart, repeatable, and human-readable way.

Method 1: Referencing a Declared Resource

If you are managing your Cognito User Pool within Terraform, the most straightforward way to retrieve the ARN is by referencing the output of the created resource:

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resource "aws_cognito_user_pool" "example" { name = "my_user_pool" } output "user_pool_arn" { value = aws_cognito_user_pool.example.arn }

In this example, we’re declaring the resource and simply outputting its ARN. This is the ideal method because it ensures that the Terraform state is in full control of the resource. But what if the user pool already exists, or you only know the name?

Method 2: Using aws_cognito_user_pools Data Source with Filtering

Here comes the trickier and more real-world scenario—when the user pool is not created within your Terraform code, but you know the name. Terraform doesn’t provide a direct way to fetch an individual user pool by name, but you can work around this limitation using a custom script or external data source.

Here’s a sample solution using an external data source with a script:

Step 1: Create a Script to Fetch ARN by Name

Create a shell or Python script that uses the AWS CLI to retrieve the ARN of the Cognito user pool by its name:

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#!/bin/bash POOL_NAME=$1 POOL_ID=$(aws cognito-idp list-user-pools --max-results 60 | jq -r --arg NAME "$POOL_NAME" '.UserPools[] | select(.Name==$NAME) | .Id') POOL_ARN="arn:aws:cognito-idp:us-east-1:123456789012:userpool/$POOL_ID" echo "{\"arn\": \"$POOL_ARN\"}"

Make sure the script has execution permission and your AWS CLI is configured correctly.

Step 2: Use the Script in Terraform

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data "external" "cognito_user_pool" { program = ["bash", "${path.module}/get_user_pool_arn.sh", "my_user_pool"] } output "cognito_user_pool_arn" { value = data.external.cognito_user_pool.result["arn"] }

This way, you successfully get user pool ARN by name in Terraform using an external lookup. This is especially helpful in large infrastructure stacks where resources are partially managed outside Terraform.

Method 3: Use AWS SDK in Custom Provider or Lambda

For advanced use cases or production-grade setups, you might consider wrapping this logic in a custom Terraform provider using the AWS SDK or calling a Lambda function via Terraform that returns the required ARN. This method offers more robust error handling, security, and version control but is more complex to implement.

Caution: Stability and Drift

Whenever you reference resources created outside Terraform, consider the implications of state drift. Terraform doesn’t track changes to external resources unless explicitly imported or managed. So when using the method to get the user pool ARN by name, ensure your external script or data source is always consistent with the actual AWS state.

Final Thoughts

To get user pool ARN by name in Terraform, you need to be resourceful. While Terraform lacks a native way to do this, external scripts and data sources offer a viable workaround get user pool arn by name in terraform incorporating external logic or ensuring your user pool is declared within Terraform, you not only retrieve the required ARN but also maintain infrastructure as code discipline. Whether you're automating IAM roles, setting up monitoring, or deploying authentication-aware services, getting this ARN efficiently is a key building block in your cloud architecture.