When migrating existing business services to Azure PaaS as part of an App Modernization project, you may find yourself seriously considering serverless computing using Azure Functions, especially if your target architecture includes MicroServices.
Azure Functions let you focus on what counts — your requirements, your time and your code — and less about boilerplate code, infrastructure and processes.

When creating new APIs in any technology, one thing is very important: Documenting those APIs so that others can use them. This is especially important in large enterprises or situations where you are exposing these APIs to the public.

This blog series guides you through creating a C# Function App, creating self-documenting APIs, ensuring the quality of that generated documentation, and separating documentation based on the audience.

The blog post assumes the following:

• You are familiar with C#
• You have knowledge of software development fundamentals
• You have completed Part One of this series

At AIS, we’ve determined that the one of the best approaches to documenting your APIs is to use OpenAPI (formerly Swagger) to have the APIs (nearly) document themselves. This saves time in the long run and even enables API clients to automatically generate client code to interact with your APIS. This helps with shelf life – if 6 months or a year down the road, we decide a better approach is best.

For these articles, I will walk you through the steps for creating well-documented Azure Functions for our fictitious shopping site called “Bmazon” and its modernization effort.

We Need Better Documentation

In the Previous Post, we got the Azure Functions application to start generating OpenAPI docs for our functions, but the results were somewhat underwhelming:

Here you can see that, even though the CreateOrder call takes an Order object in the body of the HTTP Post, there is no documentation describing this. This is because, unlike when writing traditional dotnet core APIs, the order is not a parameter to the function. Swashbuckle only has access to the function signature and anything that can be discovered through reflection.

This output is not very helpful to our clients. They need to know our inputs, potential HTTP Codes to expect (it just assumes that it will return a 200), and other pertinent information like what the method does and what the return data will look like.

For instance, if we add data validation to this method, we may wind up returning a 400 (Bad Request). We could also possibly return a 409 (Conflict) if the order already exists.

Since you’re reading this, you know there is a way to do this. Let’s get to it.

Give Swashbuckle More Info

In order for the OpenAPI documentation to be much better, we need to add a few things that Swashbuckle will be able to use to generate the docs.

As I stated previously, Swashbuckle only has access to things that can be discovered through reflection, which means the definition of your function, its parameters and any attributes decorating it, so the following translates to very little information.

[FunctionName("CreateOrder")]
public async Task<IActionResult> Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = "order")]
    HttpRequestMessage req,
    ILogger log)

This doesn’t even have any information about the Order type that is expected in the body of the method, never mind return codes.

Expected Body Type

To document the type expected in the body of the POST, we need to tell Swashbuckle what to expect. We do this by using the RequestBodyType attribute from the AzureFunctions.Extensions.Swashbuckle.Attribute namespace.

Note that this is an additional attribute on the req parameter on top of the existing HttpTrigger attribute.

[FunctionName("CreateOrder")]
public async Task<IActionResult> Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = "order")]
    [RequestBodyType(typeof(Order), "The Order To Create")] // Describes the Body
    HttpRequestMessage req,
    ILogger log)

With this in place, Swashbuckle knows what type the body contains and we now see that the body type is present in the UI:

If you click on the “Schema” link, you will even see the data type names being used:

Note that the Items array is marked nullable: true which is not desirable. We will address that below in the Data Annotations section.

The bottom of the page also shows you all the current objects in the Schema that are known:

This information documents all the details about the DTOs being used in this API. But we need to fix the nullability and other validation-related information.

Add Data Annotations

Above, the Order‘s Items collection was marked as nullable. We want to fix that and other validation information that Swashbuckle can read. To do that, we need to add Data Annotations to the object definitions.

Currently, the Order looks like this:

public class Order
{
  public int OrderId { get; set; }
  public IEnumerable<OrderLineItem> Items { get; set; }
}

In order to tell Swashbuckle (and our clients) that the Items collection is required, we have to mark it [Required] and [NotNull] from the System.Diagnostics.CodeAnalysis namespace.

The NotNull attribute is also needed because OpenAPI, not being language-specific, supports the concept of null along with the lack of presence of the variable. JavaScript developers will relate to this concept with undefined and null keywords.

So, in order to tell clients that fields MUST have a value, you need to add both attributes to the Items property.

  public class Order
  {
    public int OrderId { get; set; }

    [Required, NotNull]
    public IEnumerable<OrderLineItem> Items { get; set; }
  }

The results:

Note the red "*", meaning required, next to the items collection and the lack of the nullable:true.

To properly annotate the objects, we’ll mark the ID, Quantity and SKU all required as well. Additionally, we’ll put rational [Range] and other appropriate restrictions as well:

public class Order
{
  // if we get to 2 billion orders, we'll all be retired anyway
  [Required, NotNull, Range(1, int.MaxValue)]
  public int OrderId { get; set; }

  [Required, NotNull, MinLength(1)]
  public IEnumerable<OrderLineItem> Items { get; set; }
}

public class OrderLineItem
{
  [Required, NotNull, Range(1, 1000)]
  public int Quantity { get; set; }

  [Required, NotNull, MinLength(1)]
  public string SKU { get; set; }
}

So, the final schema looks like this:

Now your clients know the simple data validations for these objects, but don’t know what the return payloads and HTTP codes are.

Potential Return Types and Codes

By default, Swashbuckle will tell the clients to expect a 200 (Success) HTTP result with no payload.

This doesn’t include any information about any payload sent back to the user and is most likely incorrect or at least not the whole story.

If we know our Function is going to return multiple HTTP codes with different payloads, we need to tell Swashbuckle by using the [ProducesResponseType] attribute on the Function itself.

Assuming we return the following:

• 200/Success with a string message payload
• 400/BadRequest with a collection of error messages

We decorate our function like this:

[ProducesResponseType(typeof(string), StatusCodes.Status200OK)]
[ProducesResponseType(typeof(IEnumerable<string>), StatusCodes.Status400BadRequest)]
HttpStatusCode.BadRequest)]
[FunctionName("CreateOrder")]
public async Task<IActionResult> Run(

This results in

So, we’ve now exposed the input and output types, but we haven’t been able to add any additional information to describe objects or fields to our clients. To do that, we need to add XML comments to the output as well.

To make this information even better, we can comment our code properly. Of course, you were already doing that, right? RIGHT?

Better Comments in the OpenAPI Spec

One thing that you may notice is that, at the top of the function, there is very little information about the method except the name (e.g. “CreateOrder”). We should add a summary about the method.

Now, I need to apologize because I lied to you. Previously, when I said “Swashbuckle only has access to things that can be discovered through reflection”, I was lying (Forgive me!). To give client devs more information about the methods being exposed by an API, we can add C# XML Documentation information to the code and, if configured for it, Swashbuckle will incorporate that too, which can be invaluable.

Add XML Comments

We now add comments like this to our C# code (Functions and DTOs)

/// <summary>
/// Creates an Order that will be shipped to the Warehouse for fulfillment.
/// </summary>
/// <param name="req">the HTTP request</param>
/// <param name="log">the logger</param>
/// <returns>a success message or a collection of error messages</returns>
/// <response code="200">
///   Indicates success and returns a user-friendly message
/// </response>
/// <response code="400">
///   Indicates a data validation issue and will return a list of data validation errors
/// </response>
[ProducesResponseType(typeof(string), StatusCodes.Status200OK)]
[ProducesResponseType(typeof(IEnumerable<string>), StatusCodes.Status400BadRequest)]
HttpStatusCode.BadRequest)]
[FunctionName("CreateOrder")]
public async Task<IActionResult> Run() {
// ...
}

/// <summary>
/// An Order sent from the Shipping Division to be sent to the Warehouse
/// </summary>
public class Order
{
//...
}

The Swagger UI won’t have changed yet. In order for the Swashbuckle library to read this information, you need to tell the C# compiler to generate the documentation in an XML file and tell Swashbuckle about it.

Generate XML Doc file

At the top of the csproj file, add the following line to the first you see.

<DocumentationFile>Bmazon.xml</DocumentationFile>

If you are using Visual Studio, you can access this setting from the Build tab on the Project settings

Now tell Swashbuckle about the XML file

Currently, we’re configuring Swashbuckle in the StartUp.cs file with:

builder.AddSwashBuckle(Assembly.GetExecutingAssembly());

Replace this with

builder.AddSwashBuckle(Assembly.GetExecutingAssembly(), opts => {
  opts.XmlPath = "Bmazon.xml";
});

Now, when you rerun the app, the final result will be a page with the new title and the order schema will have much more detail.

The users of your service will thank you for documenting your APIs this thoroughly. Additionally, they won’t have to ask you questions about the details about how to use the APIs. They can even generate client code with various tools as well.

Next Steps

Now that you have really descriptive documentation for your APIs being automatically generated, your security folks may not like you sending all your API documentation to every single client, regardless of need.

In part three of the series, I will show you how to separate the APIs out into separate groupings and keep your clients all in their own lane.

When migrating existing business services to Azure PaaS as part of an App Modernization project, you may find yourself seriously considering serverless computing using Azure Functions, especially if your target architecture includes MicroServices.

Azure Functions let you focus on what counts — your requirements, your time, and your code — and less about boilerplate code, infrastructure, and processes.

When creating new APIs in any technology, one thing is very important: Documenting those APIs so that others can use them. This is especially important in large enterprises or situations where you are exposing these APIs to the public.

This blog series guides you through creating a C# Function App, creating self-documenting APIs, ensuring the quality of that generated documentation, and separating documentation based on the audience.

The blog series assumes the following:

• You are familiar with C#.
• You have knowledge of software development fundamentals.
• You are comfortable with command-line interfaces.

At AIS, we’ve determined that one of the best approaches to documenting your APIs is to use OpenAPI (formerly Swagger) to have the APIs (nearly) document themselves. This saves time in the long run and even enables API clients to automatically generate client code to interact with your APIs. This helps with shelf life – if 6 months or a year down the road, we decide a better approach is best.

For these articles, I will walk you through the steps for creating well-documented Azure Functions for our fictitious shopping site called “Bmazon” and its modernization effort.

Creating the App

To create the app, we will start with the Azure Functions Core Tools. At the time of this writing, the current version of this library is 3.0.3477

NOTE: This version uses dotnet cli version 3.1 internally, so if your dotnet executable in the path is not that version, it could cause you issues. If you run into errors, this may be fixed by adding global.json file in the current directory with the following content, which will tell the dotnet cli to use whatever 3.1.x version you have installed.

{
  "sdk": {
    "version": "3.1.0",
    "rollForward": "latestMinor"
  }
}

At the PowerShell prompt, we’ll run the following to create our project

C:\dev> func --version
3.0.3477
C:\dev> func init Bmazon --worker-runtime dotnet

Writing C:\dev\Bmazon\.vscode\extensions.json

This will create the shell of a project inside the C:\dev\Bmazon folder.

While creating the app, I’ve copied in an OrderService and the related DTOs from the existing application we’re modernizing to be used by the newly new functions we are creating. You can see the completed code on GitHub. You’ll see a bit more of them in the next article.

Add Functions

We’re going to add 3 different functions to our app.

Shopping API

The Shopping division needs to call HTTP APIs to make an order to the warehouse, so we will add a CreateOrder function that performs this action.

(This can be done interactively by running func new and following prompts, but using the command line parameters is more concise.)

C:\dev\Bmazon> func new --template HttpTrigger --name CreateOrder `
    --authlevel Anonymous
Use the up/down arrow keys to select a template:Function name: CreateOrder

The function "CreateOrder" was created successfully from the 
"HTTPTrigger" template.

Strangely, it outputs a prompt to select the template even when you have passed in the selection as a parameter. You can ignore this.

Warehouse API

Later in our process, the Warehouse team needs to call an HTTP endpoint to send tracking information back to the Shopping division.

We will follow the pattern above and create an API for them to call.

C:\dev\Bmazon> func new --template HTTPTrigger --name OrderShipped `
    --authlevel Anonymous
Use the up/down arrow keys to select a template:Function name: OrderShipped

The function "OrderShipped" was created successfully from the 
"HTTPTrigger" template.

Shared APIs

Since both the Shopping and Warehouse divisions will need to check on the status of an order at various times, there will be a shared function to check status.

C:\dev\Bmazon> func new --template HTTPTrigger --name OrderShippingStatus `
    --authlevel Anonymous
Use the up/down arrow keys to select a template:Function name: OrderShipped

The function "OrderShippingStatus" was created successfully from the 
"HTTPTrigger" template.

Code Cleanup

We’ll do a bit of code cleanup before moving on.

Choose GET or POST

If you look at the code, you’ll notice that, by default, the Functions were created supporting both GET and POST.

public async Task<IActionResult> Run(
   [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)]
   HttpRequest req
...

We can fix that by changing the code on each function by removing either "get" or "post" appropriately (Typically you will have the first 2 operations be POSTs and the latter be GET).

Organizing the Code

The func calls above will create all the Function files in the top folder. We’ll move ours into a Functions folder to keep things cleaner. They all just happened to start with “O”, so we can be terse.

C:\dev\Bmazon> mkdir Functions; mv O*.cs Functions\

Add OpenAPI Document Generation

In order to add OpenAPI to Azure Functions, I chose to use the Swashbuckle library. There are a few other libraries out there to work with .Net and OpenAPI, but I chose Swashbuckle because I’m familiar with it.

Installing the Package

The core Swashbuckle project doesn’t support Azure Functions directly, so I used the AzureExtensions.Swashbuckle package, a nice extension written by Vitaly Bibikov.

To install it:

C:\dev\Bmazon> dotnet add package AzureExtensions.Swashbuckle

  Determining projects to restore...
  Writing C:\Users\XXX\AppData\Local\Temp\tmp69AA.tmp
info : Adding PackageReference for package 'AzureExtensions.Swashbuckle' into project 'C:\dev\Bmazon\Bmazon.csproj'.
info : Restoring packages for C:\dev\Bmazon\Bmazon.csproj...
...
...
info : Committing restore...
info : Generating MSBuild file C:\dev\Bmazon\obj\Bmazon.csproj.nuget.g.props.
info : Writing assets file to disk. Path: C:\dev\Bmazon\obj\project.assets.json
log  : Restored C:\dev\Bmazon\Bmazon.csproj (in 525 ms).

Setting up Swashbuckle

In order to configure Swashbuckle, your Functions App needs a Functions Startup class like the following, which we’ll put in Startup.cs in the Bmazon folder.

using System.Reflection;
using AzureFunctions.Extensions.Swashbuckle;
using Microsoft.Azure.Functions.Extensions.DependencyInjection;

[assembly: FunctionsStartup(typeof(Bmazon.Startup))]
namespace Bmazon
{
  public class Startup : FunctionsStartup
  {
    public override void Configure(IFunctionsHostBuilder builder)
    {
      builder.AddSwashBuckle(Assembly.GetExecutingAssembly());
    }
  }
}

Exposing OpenAPI Endpoints

Your code will also need to expose the OpenAPI JSON and UI endpoints as HTTP-triggered Azure Functions so that client code can load them on demand.

(Adding them in a single OpenApi\OpenApiFunctions.cs file for now)

using System.Net.Http;
using System.Threading.Tasks;
using AzureFunctions.Extensions.Swashbuckle;
using AzureFunctions.Extensions.Swashbuckle.Attribute;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;

namespace Bmazon.OpenApi
{
  public static class OpenApiFunctions
  {
    [SwaggerIgnore]
    [FunctionName("OpenApiJson")]
    public static Task<HttpResponseMessage> RunJson(
        [HttpTrigger(
            AuthorizationLevel.Anonymous, 
            "get", Route = "openapi/json")]
        HttpRequestMessage req,
        [SwashBuckleClient] ISwashBuckleClient swashbuckleClient)
    {
      return Task.FromResult(
            swashbuckleClient.CreateSwaggerJsonDocumentResponse(req));
    }

    [SwaggerIgnore]
    [FunctionName("OpenApiUI")]
    public static Task<HttpResponseMessage> RunUi(
        [HttpTrigger(
            AuthorizationLevel.Anonymous, 
            "get", 
            Route = "openapi/ui")]
        HttpRequestMessage req,
        [SwashBuckleClient] ISwashBuckleClient swashbuckleClient)
    {
      // CreateOpenApiUIResponse generates the HTML page from the JSON results
      return Task.FromResult(
            swashbuckleClient.CreateSwaggerUIResponse(req, "openapi/json"));
    }
  }
}

This sets up 2 new Functions on the openapi/json and openapi/ui URLs to load the JSON file and Swagger UI respectively. The [SwaggerIgnore] attribute causes Swashbuckle to ignore these API methods for document generation purposes.

Generate and View the API Documentation

NOTE: You must have the Azure Storage Emulator or Azurite RUNNING locally in order for this to work properly.

C:\dev\Bmazon> func start
Microsoft (R) Build Engine version 16.8.3+39993bd9d for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  Restored C:\dev\Bmazon\Bmazon.csproj (in 840 ms).
  Bmazon -> C:\dev\Bmazon\bin\output\bin\Bmazon.dll

Build succeeded.

Time Elapsed 00:00:05.60

Azure Functions Core Tools
Core Tools Version:       3.0.3284 Commit hash: 98bc25e668274edd175a1647fe5a9bc4ffb6887d
Function Runtime Version: 3.0.15371.0

[2021-02-27T15:05:33.871Z] Found C:\dev\Bmazon\Bmazon.csproj. Using for user secrets file configuration.

Functions:

  CreateOrder: [POST] http://localhost:7071/api/order
  OpenApiJson: [GET] http://localhost:7071/api/openapi/json
  OpenApiUi: [GET] http://localhost:7071/api/openapi/ui
  OrderShipped: [POST] http://localhost:7071/api/order/shipment
  OrderShippingStatus: [GET] http://localhost:7071/api/order/shipment/{id}

For detailed output, run func with --verbose flag.
[2021-02-27T15:05:41.693Z] Host lock lease acquired by instance ID '000000000000000000000000016514FF'.

If you don’t see that last line after a few seconds, you probably don’t have the storage emulator running

Take note of the list of functions shown with the URLs next to them, especially the ones starting with “OpenApi”.

If you visit the OpenApiUI URL listed above, you will see the following in your browser:

That’s it! You now have a modernized serverless architecture with APIs that are documenting themselves!

If you add any new Functions, they will automatically show up here as well. Your clients can download from the JSON endpoint and import the definitions into Postman or client generators.

Next Steps

Now that you have self-documenting APIs, you may notice that the information in the Swagger UI is rather underwhelming. Due to the nature of Azure Functions, there is very little information that Swagger can glean from the runtime type information it can gather.

In part two of the series, I will show you how to make the documentation MUCH better.