How to catch an Exception throw by Azure Table in an async Azure Function HTTP Triggered function

Question

I have an Azure Function HTTP triggered function which writes to Azure Table that may end in duplicated entries. I noticed that even if I try/catch'd the whole function, there will still be an Exception "leaked" to the function runner thus returning HTTP 500. Is there any way to catch this kind of exception?

Here's a minified version of the code:

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 Microsoft.WindowsAzure.Storage;

namespace FunctionTest
{
    public class Entry
    {
        public string PartitionKey { get; set; }
        public string RowKey { get; set; }
    }
    public static class Debug
    {
        [FunctionName("Debug")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)]
            HttpRequest req,
            [Table("Debug")]
            IAsyncCollector<Entry> tableBinding,
            ILogger log)
        {
            try
            {
                await tableBinding.AddAsync(new Entry()
                {
                    PartitionKey = "1111",
                    RowKey = "1111",
                });
                await tableBinding.FlushAsync();
            }
            catch (StorageException)
            {
                // we expect an Exception "The specified entity already exists"
                return new OkObjectResult("This passes test");
            }

            return new OkObjectResult("This passes test too");
        }
    }
}

The code is written under Azure Function runtime 2.0 (the .NET Core one).

Trigger /api/debug twice or more and you will see:

HTTP 500
The catch{} code is entered, and still returns an HTTP 500(!)
In Application Insights, two table dependency call per request (shouldn't happen, the documentation says table do not have auto retry)

You aren't catching all the exception types so of course an exception can leak. Are you 100% sure that you aren't missing an exception by not having a generic `system.exception` catch block? — Charleh, Feb 23 '19 at 09:52
It is also possible, but he said, that the flow enters the `catch` block and still reports 500. — kamil-mrzyglod, Feb 23 '19 at 13:38
@Charleh I've already done experiments before I came here to ask, and catching Exception simply doesn't work. The code provided is a minimal version to reproduce the problem, it's not what I wrote in the actual project. — Jamesits, Feb 23 '19 at 13:44

score 2 · Accepted Answer · answered Feb 23 '19 at 13:36

I guess, that using IAsyncCollector<> breaks things here. If you want to avoid such problems, try to exchange the following binding:

[Table("Debug")] IAsyncCollector<Entry> tableBinding

to:

[Table("Debug")] CloudTable tableBinding

Then, instead of using tableBinding.AddAsync() use the following snippet:

var op = TableOperation.Insert(new Entry());
await tableBinding.ExecuteAsync(op);

With that approach, you should be able to catch the exception, without leaking it to the Functions runtime.

This works for me. Note: class Entry needs to implement TableEntity. — Jamesits, Feb 23 '19 at 14:15

score -1 · Answer 2 · answered Feb 23 '19 at 13:17

-1

Your try/catch block should look like following to catch all errors

try
{

}
catch (StorageException)
{
    return new OkObjectResult("This passes test");
}
catch (Exception ex)
{
    // return different error code
}

answered Feb 23 '19 at 13:17

Rahul Ruikar

1,076
6
9

The code provided is only a minimal version to reproduce the problem. In my own code I've tried multiple ways including catching the generic Exception, and I have extensive logging so I'm pretty sure StorageException is the only exception type. – Jamesits Feb 23 '19 at 13:42

How to catch an Exception throw by Azure Table in an async Azure Function HTTP Triggered function

2 Answers2

Linked