4 reasons why your lambda function cannot communicate with RedShift
Tags: aws, lambda, dataengineering, redshift
Published: 2023-07-25 14:54:13

**AWS Lambda functions** are widely used in data engineering for **ETL** (Extract, Transform, Load) processes. They allow you to read data from multiple sources and to apply cleansing and filtering before loading the data into the final destination. One common issue **data engineers** encounter when working with lambda functions is **difficulties** in establishing a connection to **RedShift**’s database that’s in a **private Subnet**.


![AWS Lambda not able to access RedShift](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/zlwqntn2ogobh7lzi0js.png)

In this article, we’ll discover why a **Lambda Function** fails to access **RedShift**’s database.

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<u>**Reason 1: Lambda’s VPC configuration**</u>

As we know, the **Virtual Private Cloud** (VPC) lets you launch AWS resources in a logically isolated virtual network that you define. In our case, the **RedShift** cluster is launched within a **private-facing** subnet with **no internet** access. That is to say, all attempts to access the database from the **public internet** will eventually fail, including lambda function requests.

**> Solution:**

Place your lambda function and RedShift cluster within the same VPC’s private subnet.

_Note: Ensure this IAM Managed policy **AWSLambdaVPCAccessExecutionRole** is attached to your lambda’s IAM role._

<u>**Reason 2: Misconfiguration of Security Groups**</u>

Security group acts as a **virtual firewall** that controls **inbound** and **outbound** traffic for EC2 instances, Lambda functions, and other AWS resources. It is essential to **configure** security groups correctly to allow communication between the **Lambda** function and **Redshift**.

**> Solution:**

- Configure the security group associated with your Lambda function to allow **outbound** traffic to the cluster’s Security group.
- Configure the security group associated with your Redshift cluster to allow **inbound** traffic from the Lambda function.

<u>**Reason 3: Lambda’s role lacks IAM Permissions**</u>

AWS **IAM **(Identity and Access Management) **Roles **are a fundamental part of the **AWS security** model. They define a set of permissions that determine what actions and resources an AWS service or user can access. They are commonly used for **granting permissions** to AWS services, allowing them to access other AWS resources **securely** without exposing any credentials. This means the **IAM role** that is assigned to your **Lambda function** should have sufficient **permissions** to access the database.

**> Solution:**

Make sure that your lambda’s execution role has the following policy: **AmazonRedshiftDataFullAccess**

<u>**Reason 4: Inaccessible Database Connection Credentials**</u>

**AWS Secrets Manager** which provides a **secure **and **scalable** solution for **storing** and managing **secrets** such as database credentials, API keys, and other sensitive information. In our case, **Lambda** and **RedShift** are both placed in a private subnet, so we need to find a way to allow access to credentials stored in **AWS Secret Manager**. The most cost-effective option would be to use an AWS Secrets Manager **VPC endpoint**.

**> Solution:**

For a **Lambda function** to access an AW**S secret** from within a **VPC** you should check the following steps:

- Ensure lambda’s execution role has the following policy: **SecretsManagerReadWrite**.

- Make sure that the AWS Secrets Manager and the Lambda function are in **the same AWS region**. Note: Secrets are region-specific and cannot be accessed from a different region.

- Configure the **outbound** rules of the Lambda function’s **security group** to allow access to the Secrets Manager service **endpoint**.

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To sum up, a **lambda function** might fail to connect to **RedShift** for multiple reasons but we could group them into these two categories: **network** (VPC, Security Group, Endpoints) and **security** (IAM permissions). By following the troubleshooting steps outlined in this article, you would be able to identify and address the root cause of the issue.

_References:_

- **AWS Managed policy to access VPC:**
https://docs.aws.amazon.com/aws-managed-policy/latest/reference/AWSLambdaVPCAccessExecutionRole.html
- **AWS Managed policy to access RedShift:**
https://docs.aws.amazon.com/aws-managed-policy/latest/reference/AmazonRedshiftDataFullAccess.html
- **Lambda inside a custom VPC:**
https://docs.aws.amazon.com/lambda/latest/dg/configuration-vpc.html#vpc-internet
- **Connecting to Amazon Redshift using an interface VPC endpoint:**
https://docs.aws.amazon.com/redshift/latest/mgmt/security-private-link.html