Why is My Go HTTP Server Failing ApacheBench Tests with 'Connection Refused' or 'Operation Timed Out' Errors?

Troubleshooting Connectivity Issues with Go HTTP Server under ApacheBench

Your Go HTTP server is encountering connectivity issues during ApacheBench testing. ApacheBench returns errors like "Connection refused" or "Operation timed out," indicating a problem with the server's ability to handle incoming requests.

Identifying the Root Cause: Port Exhaustion

Upon investigation, it becomes evident that the server is running out of ephemeral ports. macOS has a default ephemeral port range of 49152-65535, which is insufficient to handle a high volume of concurrent requests without keepalive.

Mechanism of Port Exhaustion

Each HTTP/1.0 request without keepalive consumes a unique ephemeral port on the server. When this port range is depleted, the operating system queues requests for unused ports.

Impact on Performance

Once the port queue fills up, the operating system throttles further incoming connections. As a result, clients may encounter connection failures or timeouts.

Mitigating the Issue

To mitigate the issue, consider the following options:

• Use HTTP/1.1 or Keepalive (with ApacheBench): ApacheBench provides a -k option to enable HTTP/1.1 keepalive. This allows connections to be reused based on the concurrency settings of the tool, reducing port consumption.
• Employ a Load Generator with Keepalive: Other load generators, such as wrk, support HTTP/1.1 and keepalive capabilities out of the box. Using these tools eliminates the need for manual keepalive configuration.
• Optimize Server Performance (Optional): While your server code performs minimal work, optimizing it can improve overall performance and reduce the load on the load generator and operating system. Consider incorporating meaningful work into the server code.

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