Memcache rejecting all connections

Question

I am trying to fetch data saved inside a rds mysql database using a lambda function and cache it using memcache but the lambda keeps fetching only from the database all the time without caching. When I try to just test the memcache connection, it gives "error104: connection reset by peer". The database is in a public subnet wile lambda and memcache are in private subnets. They all are in the same vpc and security groups are set up properly along with NACL. When I try to connect the cache within a ec2 instance, it says connected but when a command is run, it says "Connection closed by foreign host.". So, I can't connect to my memcache at all. I also, tried rebooting, delete and recreating, everything. I am really lost. I'm on a free tire so anything that is not within free limit is not preferred. Since VPC endpoints are not free, does it really require an endpoint for lambda to access memcache inside the same private subnets?

re:Post Agent · Answer

It sounds like you're experiencing connectivity issues with your ElastiCache Memcached cluster. There are several potential reasons for this and steps you can take to troubleshoot:

1. Network Configuration: Even though you mentioned that the security groups and NACLs are set up properly, it's worth double-checking. Ensure that the security group for your Memcached cluster allows inbound traffic on port 11211 (default Memcached port) from the security groups associated with your Lambda function and EC2 instance.

2. VPC Endpoints: Since your Lambda function is in a private subnet, it needs a way to communicate with ElastiCache. Make sure you have set up a VPC endpoint for ElastiCache in your VPC. This allows resources in your private subnets to communicate with ElastiCache without requiring an internet gateway, NAT device, VPN connection, or AWS Direct Connect connection.

3. Lambda Execution Role: Verify that the IAM role associated with your Lambda function has the necessary permissions to interact with ElastiCache.

4. ElastiCache Subnet Group: Ensure that the subnet group for your ElastiCache cluster includes the private subnets where your Lambda function resides.

5. Connection Timeout: The "connection reset by peer" error often indicates a timeout issue. You might need to adjust the connection timeout settings in your Lambda function or EC2 instance.

6. TLS Configuration: If you're using TLS for secure connections to ElastiCache, make sure your client is properly configured to use TLS. This includes setting up the correct SSL context and potentially importing the necessary certificates.

7. Client Library: Ensure you're using a compatible and up-to-date Memcached client library in your Lambda function and EC2 instance.

8. ElastiCache Cluster Health: Check the status of your ElastiCache cluster in the AWS Management Console to ensure it's in the "available" state and there are no ongoing issues.

If you've verified all these points and are still unable to connect, you may want to consider creating a new ElastiCache cluster to see if the issue persists. If it does, it might be worth reaching out to AWS support for further assistance, as there could be an underlying issue with the ElastiCache service in your region.

Remember to implement proper error handling and retry logic in your Lambda function to gracefully handle temporary connection issues with ElastiCache.
      
**Sources**  
[Read and write data to the cache - Amazon ElastiCache](https://docs.aws.amazon.com/AmazonElastiCache/latest/dg/read-write-cache-mem.html)  
[SNS Publish Timeout when using Lambda | AWS re:Post](https://repost.aws/questions/QUZezG8AdMSdq3vqZD9PAJXg/sns-publish-timeout-when-using-lambda)  
[I can't connect my RDS MYSQL local database to my node application or at can't test at MYSQL workbench. | AWS re:Post](https://repost.aws/questions/QUzjNhI_rbRrS3uWPr2jDSww/i-can-t-connect-my-rds-mysql-local-database-to-my-node-application-or-at-can-t-test-at-mysql-workbench)

Memcache rejecting all connections

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