Azure Functions Best Practices I've Learned the Hard Way

After building several production Azure Functions applications, including a large-scale offer management platform at Dynata, I’ve learned some valuable lessons. Here are my best practices for building reliable Azure Functions.

1. Handle Poison Messages Gracefully

One of the first things I learned is that Azure Functions can create infinite retry loops with bad messages:

[Function("ProcessOrder")]
public async Task Run(
    [ServiceBusTrigger("orders", Connection = "ServiceBusConnection")] 
    ServiceBusReceivedMessage message,
    ServiceBusMessageActions messageActions)
{
    try
    {
        var order = JsonSerializer.Deserialize<Order>(message.Body);
        await _orderProcessor.ProcessAsync(order);
        await messageActions.CompleteMessageAsync(message);
    }
    catch (ValidationException ex)
    {
        // Dead letter messages that will never succeed
        await messageActions.DeadLetterMessageAsync(message, 
            deadLetterReason: "ValidationFailed",
            deadLetterErrorDescription: ex.Message);
    }
    catch (Exception ex)
    {
        // Let transient errors retry
        _logger.LogError(ex, "Failed to process order");
        throw;
    }
}

2. Use Durable Functions for Long-Running Workflows

For complex workflows that span multiple services, Durable Functions are a game-changer:

[Function("OrderOrchestrator")]
public static async Task<OrderResult> RunOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var orderId = context.GetInput<Guid>();
    
    // Each activity can be retried independently
    var inventory = await context.CallActivityAsync<bool>(
        "CheckInventory", orderId);
    
    if (!inventory)
        return new OrderResult { Success = false, Reason = "Out of stock" };
    
    var payment = await context.CallActivityAsync<PaymentResult>(
        "ProcessPayment", orderId);
    
    await context.CallActivityAsync("SendConfirmation", orderId);
    
    return new OrderResult { Success = true };
}

3. Implement Proper Logging with Application Insights

Structured logging is essential for troubleshooting:

public class OrderProcessor
{
    private readonly ILogger<OrderProcessor> _logger;

    public async Task ProcessAsync(Order order)
    {
        using var scope = _logger.BeginScope(new Dictionary<string, object>
        {
            ["OrderId"] = order.Id,
            ["CustomerId"] = order.CustomerId
        });

        _logger.LogInformation("Processing order started");
        
        // Process order...
        
        _logger.LogInformation("Processing order completed in {Duration}ms", 
            stopwatch.ElapsedMilliseconds);
    }
}

4. Configure Scaling Appropriately

Don’t let your functions scale infinitely and overwhelm downstream services:

{
  "extensions": {
    "serviceBus": {
      "maxConcurrentCalls": 16,
      "maxConcurrentSessions": 8,
      "prefetchCount": 100
    }
  }
}

5. Use Managed Identities

Stop putting connection strings in your config:

[Function("ProcessBlob")]
public async Task Run(
    [BlobTrigger("uploads/{name}", Connection = "StorageConnection")] 
    Stream blob,
    string name)
{
    // Connection uses Managed Identity - no secrets needed!
    await _blobProcessor.ProcessAsync(blob, name);
}

Key Takeaways

• Always plan for failure and poison messages
• Use Durable Functions for complex workflows
• Invest in proper logging and monitoring
• Be mindful of scaling and downstream dependencies
• Embrace Managed Identities for security

These patterns have saved me countless hours of debugging and helped build more reliable systems. What Azure Functions patterns have worked well for you?