Which solution will improve the reliability of the application?

Explanations:

Using Amazon RDS Proxy allows connection pooling, which efficiently manages database connections, especially for applications with many short-lived connections. It can help mitigate the “too many connections” error by reusing existing connections and provides a single endpoint for the Lambda function, improving reliability during high traffic.

Increasing the max_connections setting may temporarily alleviate the connection limit issue but does not address the root cause of the connection spikes. This could lead to higher resource consumption and potential performance degradation, and it requires a reboot of all instances, which can lead to downtime. Additionally, it does not optimize the management of connections like RDS Proxy does.

Configuring instance scaling based on the DatabaseConnections metric does not provide an immediate solution for high connection spikes and can lead to unpredictable behavior during peak times. It would still require handling of connections directly by the Lambda function, which does not efficiently manage short-lived connections. The Aurora reader endpoint also does not pool connections like RDS Proxy does.

Although Amazon RDS Proxy can manage connections efficiently, the reference to the Aurora Data API in this context is incorrect since the Aurora Data API is primarily for serverless Aurora and does not apply to standard RDS Proxy usage for a PostgreSQL DB cluster. Thus, this option does not provide a valid solution for managing connections.