Azure Blob Storage | AcquireLeaseAsync, synchronously wait until lock is released

Question

Right now I\'m making a HTTP serverless function with Azure Functions and with that I need to make my incoming requests synchronous when accessing a single Blob Storage Item

Answer 1

The error (probably a code=409) what you are received is correct when you acquired a lease blob. This is a perfect behavior for serverless distributed model, where more functions (jobs, workers, etc.) want to have an exclusive access on the lease blob.

Only one of them can be a winner and the others must periodically asking again for their acquire lease. It is recommended during this acquiring period use a random waiting time.

The following code snippet shows an example of this "loop logic" in the azure function:

    // Acquireing a Lease blob
    public async static Task LockingEntity(dynamic entity, int leaseTimeInSec = 60, string leaseId = null)
    {
       while (true)
       {
          try
          {
             string lid = await entity.AcquireLeaseAsync(TimeSpan.FromSeconds(leaseTimeInSec), leaseId);
             if (string.IsNullOrEmpty(lid))
                await Task.Delay(TimeSpan.FromMilliseconds(new Random(Guid.NewGuid().GetHashCode()).Next(250, 1000)));
             else
                return lid;
          }
          catch (StorageException ex)
          {
             if (ex.RequestInformation.HttpStatusCode != 409)
                throw;
          }
      }
   }

the usage of the above method in the azure function:

    string leaseId = await LockingEntity(blockBlob);
    if (string.IsNullOrEmpty(leaseId) == false)
    {
       try
       {
          string esp = await blockBlob.DownloadTextAsync(Encoding.UTF8, AccessCondition.GenerateLeaseCondition(leaseId), null, null);
          state = JsonConvert.DeserializeObject(esp);
          // …
       }
       finally
       {
          await blockBlob.ReleaseLeaseAsync(AccessCondition.GenerateLeaseCondition(leaseId));
       }
    }

Also, have a look at my article Using Azure Lease Blob for more patterns, etc. in the serverless event driven distributed architecture.