I’m storing my blog’s images in Azure storage and serve them via Azure CDN for a better performance. However, the images didn’t have a Cache-Control header so far, so they wouldn’t get cached as much / as long as I wanted to. And while setting the CacheControl property on my Azure Storage blobs manually is possible, I didn’t want to do that, but rather have it automated. Azure and automation? Functions to the rescue!

First of all, my Storage account contains a container named blog, in which I’ve got all relevant images stored:

To get started, I created a new Function with the BlobTrigger – C# template:

I changed the path from the provided samples-workitems to blog (my container), and selected the appropriate Storage account.

This is the corresponding function.json that was created for me:

	"bindings": [
			"name": "myBlob",
			"type": "blobTrigger",
			"direction": "in",
			"path": "blog/{name}",
			"connection": "myStorageAccount_STORAGE"
	"disabled": false

For my code, I had to make some small changes only. First of all, I added a reference to the Microsoft.WindowsAzure.Storage package. I also created a variable named newCacheSettings that defines the default value I want to use for my CacheControl property (I set it to 3 months here). Lastly, within my Run function, I simply check the value of the CacheControl property of the triggering blob, and if it differs from my default value, I update it.
Please note that I’m using Block Blobs, so Append Blobs and Page Blobs won’t get updated by the code below.

#r "Microsoft.WindowsAzure.Storage"
using Microsoft.WindowsAzure.Storage.Blob;

private static string newCacheSettings = "public, max-age=7776000"; // 3 months

public static void Run(CloudBlockBlob myBlob, string name, TraceWriter log)
	log.Info($"C# Blob trigger function Processed blob\n Cache:{myBlob.Properties.CacheControl}");
	// only set the cache control property if it differs (e.g., if it is empty)
	if(myBlob.Properties.CacheControl != newCacheSettings) {
		myBlob.Properties.CacheControl = newCacheSettings;
		log.Info($"New Cache:{myBlob.Properties.CacheControl}");

That’s it! Just a few lines of code, and once again Functions has simplified a manual task for me.

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