What causes the API response differences between my local setup and EC2, despite using the same database URL?

The difference in API response between your local setup and EC2, despite using the same database URL, could be caused by several factors:

1. Caching: Your EC2 instance or the services it uses (like API Gateway) might be caching responses. This can cause a delay in reflecting recent changes made to the database.

2. Network latency: When querying from EC2, there might be additional network hops or cross-Availability Zone (AZ) communication, which can introduce latency. This is especially true if your EC2 instance and the database are in different AZs.

3. Connection pooling: The EC2 instance might be using connection pooling, which can sometimes result in stale data if not configured properly.

4. DNS resolution: The database URL might be resolving to different IP addresses in your local environment versus the EC2 environment. In the EC2 setup, it might be resolving to a private IP within the VPC, while locally it's resolving to a public IP.

5. Load balancing: If your database is part of a cluster or has read replicas, the EC2 instance might be connecting to a different node than your local setup, which could have slightly outdated data due to replication lag.

6. Firewall or security group settings: There might be network-level restrictions in place for the EC2 instance that are affecting how quickly it can access and retrieve data from the database.

To troubleshoot this issue:

1. Check for any caching mechanisms in your application or infrastructure.
2. Verify that your EC2 instance is in the same AZ as your database, if possible.
3. Review your connection pooling settings.
4. Check DNS resolution on both your local machine and the EC2 instance.
5. Ensure that security groups and network ACLs are not introducing unnecessary latency.
6. Monitor database replication lag if you're using a distributed database setup.

By systematically checking these potential causes, you should be able to identify and resolve the discrepancy between your local and EC2 environments.
Sources
Connection Internal Delay | AWS re:Post
High Latency When Querying Aurora MySQL from Cross-AZ EC2 Instance | AWS re:Post