Azure Functions Documentation

Handling Errors in Azure Functions

Effective error handling is crucial for building robust and reliable serverless applications. Azure Functions provides mechanisms to detect, log, and respond to errors that occur during function execution.

Common Error Scenarios

Errors can arise from various sources:

• Runtime Errors: Exceptions thrown by your function code (e.g., null reference exceptions, type errors).
• Configuration Errors: Issues with function.json, host.json, or application settings.
• Binding Errors: Problems with input or output bindings, such as incorrect connection strings or data formats.
• Service Errors: Issues with dependent Azure services (e.g., storage, Cosmos DB, Service Bus).
• Throttling and Limits: Exceeding execution time limits or other service constraints.

Logging and Diagnostics

Azure Functions integrates seamlessly with Application Insights for comprehensive monitoring and diagnostics.

• Logging within your function: Use standard logging libraries (e.g., console.log in Node.js, ILogger in C#) to record important information.
• Structured Logging: Log events with context (e.g., correlation IDs, input parameters) to aid debugging.
• Application Insights: Automatically collects logs, exceptions, and telemetry. You can query this data to pinpoint the root cause of errors.

Example: Logging in Node.js

module.exports = async function (context, req) {
    context.log('JavaScript HTTP trigger function processed a request.');
    const name = (req.query.name || (req.body && req.body.name));
    const responseMessage = name
        ? 'Hello, ' + name + '. This HTTP triggered function executed successfully.'
        : "This HTTP triggered function executed successfully. Pass a name in the query string or in the request body for a personalized response.";

    if (!name) {
        context.log.warn('No name provided. Returning default message.');
    } else {
        try {
            // Simulate a potential error condition
            if (name.toLowerCase() === 'error') {
                throw new Error('Simulated error for user input "error"');
            }
            context.log(`Processing request for name: ${name}`);
        } catch (error) {
            context.log.error(`An error occurred: ${error.message}`, { error: error });
            context.res = {
                status: 500,
                body: "An internal error occurred while processing your request."
            };
            return; // Stop further execution
        }
    }

    context.res = {
        status: 200,
        body: responseMessage
    };
};
                

Handling Exceptions

Wrap your function logic in try...catch blocks to gracefully handle expected exceptions. This allows you to log the error, perform cleanup, and return an appropriate response.

Example: Exception Handling in C#

using System;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;

namespace MyNamespace
{
    public static class HttpTriggerWithErrorHandling
    {
        [FunctionName("HttpTriggerWithErrorHandling")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
            ILogger log)
        {
            log.LogInformation("C# HTTP trigger function processed a request.");

            string name = req.Query["name"];
            string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
            dynamic data = JsonConvert.DeserializeObject(requestBody);
            name = name ?? data?.name;

            string responseMessage = string.IsNullOrEmpty(name)
                ? "This HTTP triggered function executed successfully. Pass a name in the query string or in the request body for a personalized response."
                : $"Hello, {name}. This HTTP triggered function executed successfully.";

            try
            {
                if (string.Equals(name, "exception", StringComparison.OrdinalIgnoreCase))
                {
                    log.LogWarning("Simulating an exception as requested.");
                    throw new InvalidOperationException("This is a simulated exception.");
                }

                log.LogInformation($"Processing request for name: {name ?? "anonymous"}");

                return new OkObjectResult(responseMessage);
            }
            catch (Exception ex)
            {
                log.LogError(ex, "An error occurred during function execution.");
                return new StatusCodeResult(StatusCodes.Status500InternalServerError);
            }
        }
    }
}
                

Error Responses

When an error occurs, it's important to return a meaningful response to the caller. For HTTP-triggered functions, this typically means:

• Setting an appropriate HTTP status code (e.g., 500 Internal Server Error, 400 Bad Request).
• Providing a descriptive error message in the response body.

Dead-Letter Queues

For event-driven functions (e.g., triggered by Service Bus or Event Hubs), configure dead-letter queues. When a function fails to process a message after a certain number of retries, the message can be moved to a dead-letter queue for later inspection and reprocessing, preventing data loss.

Best Practices for Error Handling