Troubleshooting an ONNX Model deployment to Azure Functions

Seth Juarez
Seth Juarez8/20/2020

Building awesome machine learning models can be an onerous task. Once all the blood, sweat and tears have been expended creating this magical (and ethical) model sometimes it feels like getting the thing deployed will require severing another limb in the name of releasing software. This post is designed to help you, dear reader, manage the land mines surrounding this complicated task and hopefully make this part of the development cycle painless.

Here's the rundown:

  1. I made a machine learning model in PyTorch that classifies tacos vs burrito images
  2. I exported the model to ONNX
  3. I want to deploy inferencing code to Azure Functions using Python
  4. I've been able to get the function running locally (this can be its own post but I thought the docs were excellent).

What follows is my journey getting this to work (with screenshots to boot). Let's dig in!

Running Locally

Once the Azure Function Core Tools are installed and you've pulled the code running things locally is very straightforward with this handy command:

func start

Running Locally

Notice that you get an endpoint that can easily be called to test everything out (I used a simple curl command to fire the function):

curl http://localhost:7071/api/predict?image=

The image parameter in the querystring tells the function which image to classify:

Calling Local Service

Creating the Function App

Here's the first thing that might trip you up (I made this mistake): make sure the Python version you developed the function with is the same one you use on the Function app.

Local Python Version

As you can see in this particular conda environment the Python version happens to be 3.6: it's basically what I used when I created the inferencing code. When you create your function app make sure to select the right version (turns out 3.6 is not the default - ask me later how I know):

Azure Function Python Version

OK, it's later. I got the weirdest of errors with my mismatched Python versions that took forever to figure out.

Standard Deployment - Remote Build

There's a way to deploy function apps directly from Visual Studio Code. I decided to use the command line to show some of the issues you might run into.

Deploying the app is easily done with the following:

func azure functionapp publish tacosbrain

This initiates something called a remote build. It uploads your code and assets and attempts to build the Azure Function on a remote machine:

Remote Build

In my case it did not work:

Remote Build Error

WHY????????? The answer is actually quite simple - in order to protect the remote environment some constraints are placed on how much space/memory/cpu et al is used to build the app. In my case there's a HUGE pip package that overflowed the memory (my bad). No worries though - there's other options!

Building Locally (First Try)

Turns out that running the func azure functionapp publish tacosbrain command is the same as running func azure functionapp publish tacosbrain --build remote. AND theres a local option as well:

func azure functionapp publish tacosbrain --build local

Since I have at my disposal the full power of my machine this should work!

Local Build


Local Build Error

DOH! Strike two (#AMIRITE?????). There's also a simple answer to this problem as well. Turns out that pip installing the onnxruntime on Windows just doesn't work (you have to use conda in my case). How can we fix this?

Building Locally (with Native Deps)

The answer is always "use docker" (OK not always, but in this case it is). Turns out there's a way to deploy your Azure Function doing a local build with docker:

func azure functionapp publish tacosbrain --build local --build-native-deps

The key here is the --build-native-deps part:

Native Build

Notice how it pulls the appropriate docker file direct from Microsoft's docker registry! This process does take a bit longer but it works. Basically I get to be in control of what resources are used to build the deployment assets.

Native Build Progress

After some time - the build is done and pushed up! It is now ready to be called.

Native Build Complete

It even helpfully gives us the uri to call!

Why So Slow?

Let's curl it up!

Slow Call

I know 3 seconds isn't a long time for regular people - it's ages for a computer though! Why is it so slow???? Turns out:

  1. It needs to warm up, and
  2. Load the model

#2 is what takes a while (I/O is finicky that way).

Better Warm

Here's what I did:

  1. I declared a set of global variables that (if None) are initialized
  2. I use the in memory objects

#1 only happens when it loads the first time.

    global session, transform, classes, input_name

    if session == None or transform == None or classes == None or input_name == None:'=========================>Loading model, transorms and classes')
            model_path = os.path.join(context.function_directory, 'model.onnx')
            classes = ['burrito', 'tacos']
            session = rt.InferenceSession(model_path)
            input_name = session.get_inputs()[0].name
            transform = transforms.Compose([
                    transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
        except Exception as error:
            logging.exception('Python Error')
            response['error'] = { 
                'code': '500',
                'message': f'{type(error).__name__}: {str(error)}'

In this case the things we want in memory are session, transform, classes, and input_name since they can easily be reused for each call. The second call to the API is much faster.

Fast Call

Keeping myself honest here:

Fast Call

Every call after the first load is now much faster: exactly how we want it!


To review:

  • Make sure the Python versions match on your dev machine and the cloud
  • Do a remote build first (this is by far the easiest thing to do)
  • If a remote build fails, do a local build - you will control the computer resources used to create the appropriate artifacts
  • If a standard local build fails (for whatever reason), use the --build-native-deps flag to build locally in a container
  • If it still doesn't deploy correctly - wait, are you sure you got it to run locally?

Hope this helps!!! In the next couple of blog posts I will try to use the API in a client side app (stay tuned for that post)!!


Now for some helpful references:

Your Thoughts?

Let me know if you have a question/thoughts about "Troubleshooting an ONNX Model deployment to Azure Functions"!
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  • Did it help you solve a problem?
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Happy to answer any questions!