The Impact of Misusing 500 Errors on Server Stability

Introduction

HTTP status codes in the 500 range, often called 5xx errors, indicate that a server knows it has encountered an error or is otherwise incapable of performing the request. While these error codes are necessary for server-client communication, their misuse can lead to significant issues, including server instability and unnecessary resource utilization.

Unnecessary Server Restarts

Let’s consider a real-world example, that have happend with our client, to illustrate the impact of misusing 500 errors. In this case, a production server was experiencing unexpected restarts. The application restarted several times a day, sometimes up to 10 restarts per day. All metrics were normal, including CPU utilization and memory consumption. The number of users was within normal limits as was the system load itself. However, the AWS events log showed numerous environmental health transitions from OK to Severe or Degraded and back to OK. These transitions were triggered by 500 errors in the server’s responses.

It was discovered that the server’s exception handler was returning a 500 error in response to a DataIntegrityViolationException. This exception is typically thrown when there is a violation of an integrity constraint in a relational database system. In this case, the exception was being caused by the API consumer, not a server error. However, because the server was returning a 500 errors, AWS interpreted this as a server-side issue and restarted the server unnecessarily.

To resolve this issue, the exception handler was modified to return a 409 error (Conflict) instead of a 500 error. This change accurately reflected the nature of the error and prevented AWS from misinterpreting it as a server health issue. As a result, the unnecessary server restarts were eliminated, and the server’s stability was improved.

This is an example of a fix:

@ExceptionHandler(DataIntegrityViolationException.class)
	public ResponseEntity<Object> handleDataIntegrityViolationException(DataIntegrityViolationException ex, HttpServletRequest request, HttpServletResponse response) {
		log.error("Data Integrity Violation occurred: {}", ex.getMessage(), ex);
		return ResponseEntity.status(HttpStatus.CONFLICT).build();
	}        

Tests:

@WebMvcTest(PaymentTransactionController.class)
@AutoConfigureMockMvc(addFilters = false)
class PaymentTransactionControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private TransactionComponent transactionComponent;

    @MockBean
    private ProductRepository productRepository;

    @MockBean
    private StripeComponent stripeComponent;

    private final ObjectMapper objectMapper = new ObjectMapper();

    @Test
    void saveTransaction_ExistingTransactionProvided_ShouldReturnConflict() throws Exception {
        PaymentTransaction pTrans = new PaymentTransaction();
        PaymentTransactionDTO pTransDTO = new PaymentTransactionDTO();
        String jsonStringRequest = objectMapper.writeValueAsString(pTransDTO);

        when(transactionComponent.saveTransaction(pTrans)).thenThrow(DataIntegrityViolationException.class);

        mockMvc.perform(post("/transactions").content(jsonStringRequest).contentType(MediaType.APPLICATION_JSON))
               .andExpect(status().isConflict());
    }

}        

Lessons Learned

This case study highlights the importance of using the correct HTTP status codes. Misusing 500 errors can lead to false positives in server health checks, unnecessary server restarts, and other issues. By correctly handling exceptions and returning appropriate status codes, you can maintain server stability and ensure a smooth user experience.

Conclusion

While 500 errors are a necessary part of server-client communication, their misuse can lead to server instability and other issues. By understanding the potential impacts of these errors and implementing proper error handling and mitigation strategies, you can maintain server stability and ensure a smooth user experience.

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