Lambda connect to VPC - Elasticache

When AWS documentation refers to a Lambda function being "attached to" a VPC, it means the Lambda function is configured to run within that VPC. The terms "attached to" and "inside" are used interchangeably in this context.

By default, Lambda functions run in a Lambda-managed VPC that has internet access. When you configure a Lambda function to connect to your VPC (where your ElastiCache is located), the function will use elastic network interfaces (ENIs) in your VPC's subnets to access resources privately within that VPC.

To connect your Lambda function to your ElastiCache instance, you need to:

1. Configure the Lambda function to use the same VPC as your ElastiCache
2. Select private subnets in that VPC
3. Assign appropriate security groups that allow communication with ElastiCache

When you connect your Lambda to your VPC, the function will lose direct internet access unless you configure it properly. This means your Lambda might lose access to AWS services like Parameter Store, Cognito, and API Gateway. To maintain this access while connected to your VPC, you have two options:

1. Set up a NAT Gateway in a public subnet and configure your private subnet's route table to route internet-bound traffic through the NAT Gateway.

2. Use VPC Endpoints (AWS PrivateLink) to privately connect to AWS services without going through the public internet.

There is no way to connect a Lambda function to ElastiCache while keeping the Lambda completely outside of your VPC. ElastiCache is designed to be accessed from within a VPC for security reasons.

VPC peering would not be applicable in this case because you don't have access to or control over the Lambda service's managed VPC. VPC peering is for connecting VPCs that you create and manage.
Sources
Giving Lambda functions access to resources in an Amazon VPC - AWS Lambda
Dedicated subnets are preferred for lambda function association with VPCs? | AWS re:Post
Enable internet access for VPC-connected Lambda functions - AWS Lambda
Serverless deployment and management - AWS Serverless Multi-Tier Architectures with Amazon API Gateway and AWS Lambda
Community | Build GenAI Applications Using Amazon Bedrock With AWS PrivateLink To Protect Your Data Privacy