Introduction

Azure functions enable you to quickly build a piece of functionality that can be triggered by an external system such as Azure Blob Storage, Storage Queue, CosmoDB, Event Hub and the list goes on. In this post, I will explain how Azure Blob Trigger works.

Sample Function

If you created a new Azure Function using Visual Studio, you will end up with the following code:

 public static class Function1
    {
        [FunctionName("Function1")]
        public static void Run([BlobTrigger("samples-workitems/{name}", Connection = "")]Stream myBlob, string name, TraceWriter log)
        {
            log.Info($"C# Blob trigger function Processed blob\n Name:{name} \n Size: {myBlob.Length} Bytes");
        }
    }

The secret lies in the attribute [BlobTrigger]. It accepts the path to the storage container which Azure Web Jobs will monitor. As you know, Azure Functions is based on Azure Web Jobs, so the same triggers are used.

How Azure Web Job knows about a file that is being added, updated or removed to a container?

Run the sample app you just created in visual studio, it should work on the local storage emulator. Now, you should be able to invoke the function by creating a blob container called samples-workitems in your development storage account and you should end up with the following structure.

You can see the container “samples-workitems” which Azure Web Jobs will monitor and invoke the function whenever a file is changed there. But there is also another container named “azure-webjobs-hosts” which is the secret for how Azure monitor the files in that container.

If you opened that container, you will find a folder called “blobreceipts” which has a folder for the function name.

Inside the folder with the function name, there are some other folders with strange names like below

These strange names are the ETag of each blob file added, edited or removed. So, when you change a file in any container, Web Jobs will create a new folder here with the new version ETag for that file and inside this folder you will find the same structure of the file being edited. In our case, there should be a folder called samples-workitems and inside this folder the file that was modified.

When a new file is added, Azure will check if  its ETag exist in the azure-webjobs-hosts folder and if not, then it will call the Azure Function. This way it will prevent duplicate calls for the same file. This pattern is called blob receipt.

Note that this process depends on Azure Blob Storage Logging which can take up to 10 minutes to write to the container azure-webjobs-hosts to improve performance. If you need your function to trigger faster then consider using Storage Queue trigger instead.