Azure Function as an HTTP endpoint

Article three describing my first foray into serverless computing. Introduction here, storage queue triggered function here. TL;DR: I’m using this is as a way to familiarize myself with Azure Functions, and to save some precious family time better spent not trying to remember what we need come Sunday.

In my projects, we always end up integrating against some kind of third party – be it booking systems, news aggregators, external providers of content or something as simple as an image resizer. Common for the integrations, is the need to transform the data received into a format with which we want to work. Sometimes we need to enrich the data  by combining it with other sources, and other times the data exposed by the external API is more than we need.

For this project, I had to get product data from Kolonial.no‘s API by barcode in order to add the product to my cart using their internal ID. Their API provides a product endpoint, but it exposes much more data than I needed. I decided to abstract it away behind a GET endpoint of my own.

This is a philosophy I try to follow in all projects I’m responsible for. It means the business code can trust that the contracts we’ve agreed upon won’t break suddenly because of any external factors, and third parties are kept at the edges of the system. Another advantage in this case is that I might want to cache the product data returned at some point, and with the endpoint (function) in place, I have an easy way to implement said caching without having to modify any business code.

So, an HTTP endpoint was needed, and since I’m working in a serverless architecture, that meant I had to create an HTTP triggered function:

azure-functions-http-trigger.png
The HTTP triggered function is found in the “API & WebHooks” scenario,

Note that you can specify the access/authorization level for the function:

azure-functions-http-authorization-level.png

“Function” means the caller needs to provide a key which is specific to the function in the query string. “Admin” means an app wide key is needed, while “anonymous” will allow anyone who accesses the URL to trigger the function.

Now, the HTTP trigger is not always used as a RESTful endpoint – it can just as well trigger a processing job, write to a storage table or whatever else you need it to do, but in this case I wanted to return a product object on the format determined by the Accept-Encoding of the request.

In order to make the function return an http request, the function needs to have an HTTP output parameter, which is set up by default when you create the function. I left everything as it was, and focused on writing the small function:

Hopefully, the code should be pretty self-explanatory. There are a couple of things which are good to know, though: Newtonsoft.Json is available in azure functions, but it must be imported:

#r "Newtonsoft.Json"

 

And while I very much doubt that I would ever exhaust anything with the small scale of my project, the Microsoft patterns and practices team do recommend that HttpClient is instantiated as few times as possible, and instead kept in memory and re-used. That’s why the static HttpClient is created the first time the function is triggered and kept around:

if(HttpClient == null)
    HttpClient = CreateHttpClient(log);

What’s interesting is that despite the function in principle being serverless, the static variable will hang around (albeit for an unpredictable amount of time), effectively allowing us to follow best practice. You can read more about sharing state in Azure Functions on Mark Heath’s excellent blog.

When it comes to the formatting of the return value, the framework will take of that for you as long as you stick to the request.CreateResponse(…) functions. As an example, this is how the function responds to an Accept: application/json request:

json

While requesting XML, predictably, will return this:

xml

(Both screenshots courtesy of the wonderful Postman Chrome plugin)

And that’s that – a small integration with an external service, contained in a single azure function.

 

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