How to log errors in an azure function that uses an output binding to a service bus?

Question

I have an azure function that uses an output binding to send messages to a service bus topic. I need to log custom data if the messages fail to be added to the service bus, but only after the built-in retries fail. The problem is, I'm not sure where to write such a log statement? Would the following be acceptable? My guess is that this will log errors before the retry policy fails, so if a transient error occurs, the logging statement below will execute for each retry.

[FunctionName("MessageLoader")]
[return: ServiceBus("%Topic_Name%", Connection = "Topic_Connection")]
public static async Task<TopicMessage> Run(
    [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = "domain/load-message")] HttpRequestMessage req)
{
    try
    {
        var stringRequestBody = await req.Content.ReadAsStringAsync();
        return JsonConvert.DeserializeObject<TopicMessage>(stringRequestBody);
    }
    catch
    {
        _log.LogError("Could not send message: " + customBusinessObject.custom_message"); }
    }
}

Answer 1

If you want to check your function when sending output to service bus topic, then the way of the output binding usage on your side is wrong.

In your way, if send message get something wrong, it will go to the logic in catch. But please notice, you still need to return something. So this way is not right.

You should not use output binding as return type, please do like this:

using System;
using System.IO;
using System.Threading.Tasks;
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 _20201012
{
    public static class Function1
    {
        [FunctionName("Function1")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
            [ServiceBus("bowman1012", Connection = "ServiceBusConnectionString1")] IAsyncCollector<dynamic> outputServiceBus,
            ILogger log)
        {
            try
            {
                await outputServiceBus.AddAsync("Item1");
            } catch {
                log.LogError("Could not send message: ...");
            }
            return new OkObjectResult(null);
        }
    }
}