(Almost) No one likes writing code meant to store data to a repository, queues, blobs. Let alone triggering your code when some event occurs in one of those areas. Luckily for us the Azure Functions team has decided to use bindings for this.
By leveraging the power of bindings, you don’t have to write your own logic to store or retrieve data. Azure Functions provides all of this functionality out of the box!

Bindings give you the possibility to retrieve data (strong-typed if you want) from HTTP calls, blob storage events, queues, CosmosDB events, etc. Not only does this work for input, but also for output. Say you want to store some object to a queue or repository, you can just use an output binding in your Azure Function to make this happen. Awesome, right?

Most of the documentation and blogposts out there state you should define your bindings in a file called function.json. An example of these bindings is shown in the block below.

{
  "bindings": [
    {
      "name": "order",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue-items",
      "connection": "MY_STORAGE_ACCT_APP_SETTING"
    },
    {
      "name": "$return",
      "type": "table",
      "direction": "out",
      "tableName": "outTable",
      "connection": "MY_TABLE_STORAGE_ACCT_APP_SETTING"
    }
  ]
}

The above sample specifies an input binding for a Queue and an output binding for a some Table Storage. While this works perfectly, it’s not the way you want to implement this when using C# (or F# for that matter), especially if you are using Visual Studio!

How to use bindings with Visual Studio

To set up a function binding in via Visual Studio you just have to specify some attributes for the input parameters of your code. These attributes will make sure the function.json file is created when the code is being compiled.

After creating your first Azure Function via Visual Studio you will get a function with these attributes immediately. For my URL Minifier solution I’ve used the following HttpTrigger.

[FunctionName("Get")]
public static async Task<HttpResponseMessage> Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "{slug}")] HttpRequestMessage req, string slug,
    TraceWriter log)

Visual Studio (well, actually the Azure Function tooling) will make sure this will get translated to a binding block which looks like this.

"bindings": [
  {
    "type": "httpTrigger",
    "route": "{slug}",
    "methods": [
      "get"
    ],
    "authLevel": "anonymous",
    "name": "req"
  }
],

You can do this for every type of trigger which is available at the moment.

Sadly, this type of development hasn’t been described a lot in the various blogposts and documentation, but with a bit of effort you can find out how to implement most bindings by yourself.

I haven’t worked with all of the different type of bindings yet.

One which I found quite hard to implement is the output binding for a Cosmos DB repository. Though, in hindsight it was rather easy to do once you know what to look for. What worked for me, is creating an Azure Function via the portal first and see which type of binding it uses. This way I found out for a Cosmos DB output binding you need to use the DocumentDBAttribute. This attribute needs a couple of variables, like the database name, collection name and of course the actual connection string. After providing all of the necessary information your Cosmos DB output binding should look something like the one below.

[FunctionName("Create")]
public static HttpResponseMessage Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = "create")]HttpRequestMessage req,
    [DocumentDB("TablesDB", "minified-urls", ConnectionStringSetting = "Minified_ConnectionString", CreateIfNotExists = true)] out MinifiedUrl minifiedUrl,
    TraceWriter log)

Notice I had to remove the async keyword? That’s because you can’t use async if there is an out-parameter.

The thing I had the most trouble with is finding out which value should be in the ConnectionStringSetting. If you head down to the Connection String tab of your Cosmos DB in the Azure portal you will find a connection string in the following format.

DefaultEndpointsProtocol=https;AccountName=[myCosmosDb];AccountKey=[myAccountKey];TableEndpoint=https://[myCosmosDb].documents.azure.com

If you use this setting, you’ll be prompted with a NullReferenceException for a ServiceEndpoint. After having quite a bit of time on troubleshooting this issue I decided the problem probably had to use some other value in the ConnectionStringSetting.

Having tired a couple of things I finally discovered you have to specify the setting as follows:

AccountEndpoint=https://[myCosmosDb].documents.azure.com:443/;AccountKey=[myAccountKey];

Running the function will work like a charm now.

I’m pretty sure this will not be the only ‘quirk’ you will come across when using the bindings, but as long as we can all share the information it will become easier in the future!

Where will I store the secrets?

When using attributes you can’t rely much on retrieving your secrets via application settings or the like. Well, the team has you covered!

You can just use your regular application settings, as long as you hold to a naming convention where the values have to be uppercase and use underscores for separation. So instead of hardcoding the values “TablesDB” and “minified-urls” inside my earlier code snippet, one can also use the following.

[FunctionName("Create")]
public static HttpResponseMessage Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = "create")]HttpRequestMessage req,
    [DocumentDB("MY-DATABASE", MY-COLLECTION", ConnectionStringSetting = Minified_ConnectionString", CreateIfNotExists = true)] out MinifiedUrl minifiedUrl,
    TraceWriter log)

By convention, the actual values will now be retrieved via the application settings.

Awesome!

Yeah, but Application Settings aren’t very secure

True!

I’ve already written about this in an earlier post. While using the Application Settings are fine to store some configuration data, you don’t want to specify secrets over there. Secrets should be stored inside Azure Key Vault.

Of course, you can’t use Azure Key Vault in these attributes.

Lucky for us, the Azure Functions team still got us covered with an awesome feature called Imperative bindings! The sample code is enough to get us cracking on creating a binding where the connection secrets are still stored inside Azure Key Vault (or somewhere else for that matter).

Because I’m using a Cosmos DB connection, I need to specify the DocumentDBAttribute inside the Binder. Something else you should note is when you want to use an output binding, you can’t just use create binding to a MinifiedUrl object. If you only specify the object type, the Binder will assume it’s an input binding.

If you want an output binding, you need to specify the binding as an IAsyncCollector. Check out the code below to see what you need to do in order to use the DocumentDBAttribute in combination with imperative bindings.

// Retrieving the secret from Azure Key Vault via a helper class
var connectionString = await secret.Get("CosmosConnectionStringSecret");
// Setting the AppSetting run-time with the secret value, because the Binder needs it
ConfigurationManager.AppSettings["CosmosConnectionString"] = connectionString;

// Creating an output binding
var output = await binder.BindAsync<IAsyncCollector<MinifiedUrl>>(new DocumentDBAttribute("TablesDB", "minified-urls")
{
    CreateIfNotExists = true,
    // Specify the AppSetting key which contains the actual connection string information
    ConnectionStringSetting = "CosmosConnectionString",
});

// Create the MinifiedUrl object
var create = new CreateUrlHandler();
var minifiedUrl = create.Execute(data);

// Adding the newly created object to Cosmos DB
await output.AddAsync(minifiedUrl);

As you can see, there’s a lot of extra code in the body of the function. We have to give up some of the simplicity in order to make the code and configuration a bit more secure, but that’s worth it in my opinion.

If you want to check out the complete codebase of this solution, please check out the GitHub repository, it contains all of the code.

I need more bindings!

Well, a couple of days ago there was some amazing announcement. You can now create your own bindings! Donna Malayeri has some sample code available on GitHub on how to create a Slack binding. There is also a documentation page in the making on how to create these type of bindings.

At this time this feature is still in preview, but if you need some binding which isn’t available at the moment, be sure to check this out. I can imagine this will become quite popular once it has been released. Just imagine creating bindings to existing CRM systems, databases, your own SMTP services, etc.

Awesome stuff in the making!