I have a program that writes to an append blob and periodically updates its metadata. Here is the code: link.

When the program flushes the write buffer and sends the write request to Azure Blob Storage, it fails with the following exception:

Unhandled exception. Azure.RequestFailedException: The condition specified using HTTP conditional header(s) is not met.
RequestId:ebf5d0e6-8f35-4ed6-811a-7ff7e8db7478
Time:2025-04-02T21:21:35.616Z
Status: 412 (The condition specified using HTTP conditional header(s) is not met.)
ErrorCode: ConditionNotMet

Content:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Error>
  <Code>ConditionNotMet</Code>
  <Message>The condition specified using HTTP conditional header(s) is not met.
RequestId:ebf5d0e6-8f35-4ed6-811a-7ff7e8db7478
Time:2025-04-02T21:21:35.616Z</Message>
</Error>

I suspect this happens because the write requests include the If-Match header, and Azure Blob Storage verifies that the ETag remains unchanged between requests. However, updating metadata modifies the ETag, causing subsequent write operations to fail.

Questions:

1. Is there a way to update metadata while continuing to write to an append blob?
2. Why does the Azure Storage Blobs client library enforce this behavior, preventing metadata updates while writing?

Any insights or workarounds would be greatly appreciated!

I have a program that writes to an append blob and periodically updates its metadata. Here is the code: link.

When the program flushes the write buffer and sends the write request to Azure Blob Storage, it fails with the following exception:

Unhandled exception. Azure.RequestFailedException: The condition specified using HTTP conditional header(s) is not met.
RequestId:ebf5d0e6-8f35-4ed6-811a-7ff7e8db7478
Time:2025-04-02T21:21:35.616Z
Status: 412 (The condition specified using HTTP conditional header(s) is not met.)
ErrorCode: ConditionNotMet

Content:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Error>
  <Code>ConditionNotMet</Code>
  <Message>The condition specified using HTTP conditional header(s) is not met.
RequestId:ebf5d0e6-8f35-4ed6-811a-7ff7e8db7478
Time:2025-04-02T21:21:35.616Z</Message>
</Error>

I suspect this happens because the write requests include the If-Match header, and Azure Blob Storage verifies that the ETag remains unchanged between requests. However, updating metadata modifies the ETag, causing subsequent write operations to fail.

Questions:

1. Is there a way to update metadata while continuing to write to an append blob?
2. Why does the Azure Storage Blobs client library enforce this behavior, preventing metadata updates while writing?

Any insights or workarounds would be greatly appreciated!

• azure
• azure-blob-storage

• Close the append blob write stream before updating metadata, then reopen it to continue writing, this avoids the ETag mismatch (412) error caused when metadata updates change the blob’s version. – Venkatesan Commented yesterday

1 Answer 1

Updating metadata while writing to an append blob

When you write using OpenWriteAsync(), Azure links that stream to the blob’s current version (ETag).If you update metadata while the stream is open, the version changes so that writes fail with an error.

I had checked the stream was closed after each write so it will not get stuck with an old version. This prevents errors because Azure allow writes and metadata updates to match the blob version.

using System;
using System.Collections.Generic;
using System.IO;
using System.Threading.Tasks;
using Azure.Storage.Blobs;
using Azure.Storage.Blobs.Models;
using Azure.Storage.Blobs.Specialized;

namespace AppendBlobMetadataDemo
{
    internal static class Program
    {
        public static async Task Main(string[] args)
        {
            var connectionString = "<Your ConnectionString>"; 
            var containerName = "<Your ContainerName>";
            var blobName = "<Your BlobName>";
            var blobServiceClient = new BlobServiceClient(connectionString);
            var containerClient = blobServiceClient.GetBlobContainerClient(containerName);
            await containerClient.CreateIfNotExistsAsync(PublicAccessType.None);
            var appendBlobClient = containerClient.GetAppendBlobClient(blobName);
            await appendBlobClient.CreateIfNotExistsAsync();
            for (int i = 0; i < 5; i++)
            {
                Console.WriteLine($"[Iteration {i}] Opening stream to write...");
                await using (var stream = await appendBlobClient.OpenWriteAsync(overwrite: false))
                {
                    var data = new byte[] { 0, 1, 2, 3, 4, 5 };
                    await stream.WriteAsync(data);
                    await stream.FlushAsync();
                    Console.WriteLine($"[Iteration {i}] Wrote {data.Length} bytes to blob.");
                }
                var metadata = new Dictionary<string, string>
                {
                    { "lastwrite", DateTime.UtcNow.ToString("yyyyMMddHHmmss") }, 
                    { "iteration", i.ToString() } 
                };
                Console.WriteLine($"[Iteration {i}] Setting metadata...");
                await appendBlobClient.SetMetadataAsync(metadata);
                Console.WriteLine($"[Iteration {i}] Done.\n");
                await Task.Delay(1000);
            }
            Console.WriteLine("Completed writing and metadata updates without any errors.");
        }
    }
}

After updating metadata, I opened a new stream each time to make sure writing worked properly.

Output: