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Requests and APIs


Core Module

Before we can get deployment of our models we need to understand concepts such as APIs and requests. The core reason for this is that we need a new abstraction layer on top of our applications that are not Python-specific. While Python is the defacto language for machine learning, we cannot expect everybody else to use it and in particular, we cannot expect network protocols (both locally and external) to be able to communicate with our Python programs out of the box. For this reason, we need to understand requests, in particular HTTP requests and how to create APIs that can interact with those requests.

Requests

When we are talking about requests, we are essentially talking about the communication method used in client-server types of architectures. As shown in the image below, in this architecture, the client (user) is going to send requests to a server (our machine learning application) and the server will give a response. For example, the user may send a request to get the class of a specific image, which our application will do and then send back the response in terms of a label.

Image

Image credit

The common way of sending requests is called HTTP (Hypertext Transfer Protocol). It is essentially a specification of the intermediary transportation method between the client and server. An HTTP request essentially consists of two parts:

  • A request URL: the location of the server we want to send our request to
  • A request Method: describing what action we want to perform on the server

The common request methods are (case sensitive):

  • GET: get data from the server
  • POST/PUT: send data to the server
  • DELETE: delete data on the server

You can read more about the different methods here. For most machine learning applications, GET and POST are the core methods to remember. Additionally, if you want to read more about HTTP in general we highly recommend that you go over this comic strip protocol, but the TLDR is that it provides privacy, integrity and identification over the web.

❔ Exercises

We are going to do a couple of exercises on sending requests using requests package to get familiar with the syntax.

  1. Start by installing the `requests`` package

    pip install requests
    
  2. Afterwards, create a small script and try to execute the code

    import requests
    response = requests.get('https://api.github.com/this-api-should-not-exist')
    print(response.status_code)
    

    As you can see from the syntax, we are sending a request using the GET method. This code should return status code 404. Take a look at this page that contains a list of status codes. Next, let's call a page that exists

    import requests
    response = requests.get('https://api.github.com')
    print(response.status_code)
    

    What is the status code now and what does it mean? Status codes are important when you have an application that is interacting with a server and wants to make sure that it does not fail, which can be done with simple if statements on the status codes

    if response.status_code == 200:
        print('Success!')
    elif response.status_code == 404:
        print('Not Found.')
    
  3. Next, try to call the following

    response=requests.get("https://api.github.com/repos/SkafteNicki/dtu_mlops")
    

    which gives back a payload. Essentially, payload refers to any additional data that is sent from the client to the server or vice-versa. Try looking at the response.content attribute. What is the type of this attribute?

  4. You should hopefully observe that the .content attribute is of type bytes. It is important to note that this is the standard way of sending payloads to encode them into byte objects. To get a more human-readable version of the response, we can convert it to JSON format

    response.json()
    

    Important to remember that a JSON object in Python is just a nested dictionary if you ever want to iterate over the object in some way.

  5. When we use the GET method we can additionally provide a params argument, that specifies what we want the server to send back for a specific request URL:

    response = requests.get(
        'https://api.github.com/search/repositories',
        params={'q': 'requests+language:python'},
    )
    

    Before looking at reponse.json() can you explain what the code does? You can try looking at this page for help.

  6. Sometimes the content of a page cannot be converted into JSON, because as already stated data is sent as bytes. Say that we want to download an image, which we can do in the following way

    import requests
    response = requests.get('https://imgs.xkcd.com/comics/making_progress.png')
    

    Try calling response.json(), what happens? Next, try calling response.content. To get the result in this case we would need to convert from bytes to an image:

    with open(r'img.png','wb') as f:
        f.write(response.content)
    
  7. The get method is the most useful method because it allows us to get data from the server. However, as stated in the beginning multiple request methods exist, for example, the POST method for sending data to the server. Try executing:

    pload = {'username':'Olivia','password':'123'}
    response = requests.post('https://httpbin.org/post', data = pload)
    

    Investigate the response (this is an artificial example because we do not control the server).

  8. Finally, we should also know that requests can be sent directly from the command line using the curl command. Sometimes it is easier to send a request directly from the terminal and sometimes it is easier to do it from a script.

    1. Make sure you have curl installed, or else find instruction on installing it. To check call curl --help` with the documentation on curl.

    2. To execute requests.get('https://api.github.com') using curl we would simply do

      curl -X GET "https://api.github.com"
      curl -X GET -I "https://api.github.com" # if you want the status code
      

      Try it yourself.

    3. Try to redo some of the exercises yourself using curl.

That ends the intro session on requests. Do not worry if you are still not completely comfortable with sending requests, we are going to return to how we do it in practice when we have created our API. If you want to learn more about the requests package you can check out this tutorial and if you want to see more examples of how to use curl you can check out this page

Creating APIs

Requests are all about being on the client side of our client-server architecture. We are now going to move on to the server side where we will be learning about writing the APIs that requests can interact with. An application programming interface (API) is essentially the way for the developer (you) tells a user how to use the application that you have created. The API is an abstraction layer that allows the user to interact with our application in the way we want them to interact with it, without the user even having to look at the code.

We can take the API from github as an example https://api.github.com. This API allows any user to retrieve, integrate and send data to Github without ever having to visit their webpage. The API exposes multiple endpoints that have various functions:

and we could go on. However, there may be functionality that Github is not interested in users having access to and they may therefore choose not to have endpoints for specific features (1).

  1. 🙋‍♂️ Many companies provide public APIs to interact with their services/data. For a general list of public APIs you can check out this page. For the Danes out there, you can check out this list of public and private APIs from Danish companies and organizations.

The particular kind of API we are going to work with is called REST API (or RESTful API). The REST API specify specific constraints that a particular API needs to fulfill to be considered RESTful. You can read more about what the six guiding principles behind REST API on this page but one of the most important to have in mind is that the client-server architecture needs to be stateless. This means that whenever a request is send to the server it needs to be self-contained (all information included) and the server cannot rely on any previously stored information from previous requests.

To implement APIs in practise we are going to use FastAPI. FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.6+ based on standard Python type hints. FastAPI is only one of many frameworks for defining APIs, however, compared to other frameworks such as Flask and django it offers a sweet spot of being flexible enough to do what you want without having many additional (unnecessary) features.

❔ Exercises

The exercises below are a condensed version of this and this tutorial. If you ever need context for the exercises, we can recommend trying to go through these. Additionally, we also provide this solution file that you can look through for help.

  1. Install FastAPI

    pip install fastapi
    

    This contains the functions, modules, and variables we are going to need to define our interface.

  2. Additionally, also install uvicorn which is a package for defining low level server applications.

    pip install uvicorn[standard]
    
  3. Start by defining a small application like this in a file called main.py:

    from fastapi import FastAPI
    app = FastAPI()
    
    @app.get("/")
    def read_root():
        return {"Hello": "World"}
    
    @app.get("/items/{item_id}")
    def read_item(item_id: int):
        return {"item_id": item_id}
    

    Importantly here is the use of the @app.get decorator. What could this decorator refer to? Explain what the two functions are probably doing.

  4. Next lets launch our app. Since we called our script main.py and we inside the script initialized our API with app = FastAPI, our application that we want to deploy can be referenced by main:app:

    uvicorn --reload --port 8000 main:app
    

    this will launch a server at this page: http://localhost:8000/. As you will hopefully see, this page will return the content of the root function, like the image below. Remember to also check the output in your terminal as that will give info on when and how your application is being invoked.

    Image

    1. What webpage should you open to get the server to return 1?

    2. Also checkout the pages: http://localhost:8000/docs and http://localhost:8000/redoc. What does these pages show?

    3. The power of the docs and redoc pages is that they allow you to easily test your application with their simple UI. As shown in the image below, simply open the endpoint you want to test, click the Try it out button, input any values and execute it. It will return both the corresponding curl command for invoking your endpoint, the corresponding URL and response of you application. Try it out.

      Image

    4. You can also checkout http://localhost:8000/openapi.json to check out the schema that is generated which essentially is a json file containing the overall specifications of your program.

    5. Try to access http://localhost:8000/items/foo, what happens in this case? When you specify types in your API, FastAPI will automatically do type validation using pydantic, making sure users can only access your API with the correct types. Therefore, remember to include types in your applications!

  5. With the fundamentals in place let's configure it a bit more:

    1. Lets start by changing the root function to include a bit more info. In particular we are also interested in returning the status code so the end user can easily read that. Default status codes are included in the http built-in python package:

      from http import HTTPStatus
      
      @app.get("/")
      def root():
          """ Health check."""
          response = {
              "message": HTTPStatus.OK.phrase,
              "status-code": HTTPStatus.OK,
          }
          return response
      

      try to reload the app and see what is returned now. You should not have to re-launch the app because we initialized the app with the --reload argument.

    2. When we decorate our functions with @app.get("/items/{item_id}"), item_id is in the case what we call a path parameters because it is a parameter that is directly included in the path of our endpoint. We have already seen how we can restrict a path to a single type, but what if we want to restrict it to specific values? This is often the case if we are working with parameters of type str. In this case we would need to define a enum:

      from enum import Enum
      class ItemEnum(Enum):
          alexnet = "alexnet"
          resnet = "resnet"
          lenet = "lenet"
      
      @app.get("/restric_items/{item_id}")
      def read_item(item_id: ItemEnum):
          return {"item_id": item_id}
      

      Add this API, reload and execute both a valid parameter and a non-valid parameter.

    3. In contrast to path parameters we have query parameters. In the requests exercises we saw an example of this where we were calling https://api.github.com/search/code with the query 'q': 'requests+language:python'. Any parameter in FastAPI that is not a path parameter, will be considered a query parameter:

      @app.get("/query_items")
      def read_item(item_id: int):
          return {"item_id": item_id}
      

      Add this API, reload and figure out how to pass in a query parameter.

    4. We have until now worked with the .get method, but lets also see an example of the .post method. As already described the POST request method is used for uploading data to the server. Here is a simple app that saves username and password in a database (please never implement this in real life like this):

      database = {'username': [ ], 'password': [ ]}
      
      @app.post("/login/")
      def login(username: str, password: str):
          username_db = database['username']
          password_db = database['password']
          if username not in username_db and password not in password_db:
              with open('database.csv', "a") as file:
                  file.write(f"{username}, {password} \n")
              username_db.append(username)
              password_db.append(password)
          return "login saved"
      

      Make sure you understand what the function does and then try to execute it a couple of times to see your database updating. It is important to note that we sometimes in the following exercises use the .get method and sometimes the .post method. For our usage it does not really matter.

  6. We are now moving on to figuring out how to provide different standard inputs like text, images, json to our APIs. It is important that you try out each example yourself and in particular you look at the curl commands that are necessary to invoke each application.

    1. Here is a small application, that takes a single text input

      @app.get("/text_model/")
      def contains_email(data: str):
          regex = r'\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}\b'
          response = {
              "input": data,
              "message": HTTPStatus.OK.phrase,
              "status-code": HTTPStatus.OK,
              "is_email": re.fullmatch(regex, data) is not None
          }
          return response
      

      What does the application do? Try it out yourself

    2. Let's say we wanted to extend the application to check for a specific email domain, either gmail or hotmail. Assume that we want to feed this into our application as a json object e.g.

      {
          "email": "mlops@gmail.com",
          "domain_match": "gmail"
      }
      

      Figure out how to alter the data parameter such that it takes in the json object and make sure to extend the application to check if the email and domain also match. Hint: take a look at this page

    3. Let's move on to an application that requires a file input:

      from fastapi import UploadFile, File
      from typing import Optional
      
      @app.post("/cv_model/")
      async def cv_model(data: UploadFile = File(...)):
          with open('image.jpg', 'wb') as image:
              content = await data.read()
              image.write(content)
              image.close()
      
          response = {
              "input": data,
              "message": HTTPStatus.OK.phrase,
              "status-code": HTTPStatus.OK,
          }
          return response
      

      A couple of new things are going on here: we use the specialized UploadFile and File bodies in our input definition. Additionally, we added the async/await keywords. Figure out what everything does and try to run the application (you can use any image file you like).

    4. The above application actually does not do anything. Let's add opencv as a package and let's resize the image. It can be done with the following three lines:

      import cv2
      img = cv2.imread("image.jpg")
      res = cv2.resize(img, (h, w))
      

      Figure out where to add them in the application and additionally add h and w as optional parameters, with a default value of 28. Try running the application where you specify everything and one more time where you leave out h and w.

    5. Finally, let's also figure out how to return a file from our application. You will need to add the following lines:

      from fastapi.responses import FileResponse
      cv2.imwrite('image_resize.jpg', res)
      FileResponse('image_resize.jpg')
      

      Figure out where to add them to the code and try running the application one more time to see that you get a file back with the resized image.

  7. (Optional) Let's try to figure out how to use FastAPI in a machine learning context. Below is a script that downloads a VisionEncoderDecoder from huggingface . The model can be used to create captions for a given image. Thus calling

    predict_step(['s7_deployment/exercise_files/my_cat.jpg'])
    

    returns a list of strings like ['a cat laying on a couch with a stuffed animal'] (try this yourself). Create a FastAPI application that can do inference using this model e.g. it should take in an image, preferably an optional json object for configuring some of the hyperparameters (like max_length) and should return a string containing the generated caption.

    from transformers import VisionEncoderDecoderModel, ViTFeatureExtractor, AutoTokenizer
    import torch
    from PIL import Image
    
    model = VisionEncoderDecoderModel.from_pretrained("nlpconnect/vit-gpt2-image-captioning")
    feature_extractor = ViTFeatureExtractor.from_pretrained("nlpconnect/vit-gpt2-image-captioning")
    tokenizer = AutoTokenizer.from_pretrained("nlpconnect/vit-gpt2-image-captioning")
    
    device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
    model.to(device)
    
    gen_kwargs = {"max_length": 16, "num_beams": 8, "num_return_sequences": 1}
    def predict_step(image_paths):
        images = []
        for image_path in image_paths:
            i_image = Image.open(image_path)
            if i_image.mode != "RGB":
                i_image = i_image.convert(mode="RGB")
    
            images.append(i_image)
        pixel_values = feature_extractor(images=images, return_tensors="pt").pixel_values
        pixel_values = pixel_values.to(device)
        output_ids = model.generate(pixel_values, **gen_kwargs)
        preds = tokenizer.batch_decode(output_ids, skip_special_tokens=True)
        preds = [pred.strip() for pred in preds]
        return preds
    
  8. As the final step, we want to figure out how to include our FastAPI application in a docker container as it will help us when we want to deploy in the cloud because docker as always can take care of the dependencies for our application. For the following set of exercises you can take whatever previous FastAPI application as the base application for the container

    1. Start by creating a requirement.txt file for your application. You will at least need fastapi and uvicorn in the file and we always recommend that you are specific about the version you want to use

      fastapi>=0.68.0,<0.69.0
      uvicorn>=0.15.0,<0.16.0
      # add anything else you application needs to be able to run
      
    2. Next, create a Dockerfile with the following content

      FROM python:3.9
      WORKDIR /code
      COPY ./requirements.txt /code/requirements.txt
      
      RUN pip install --no-cache-dir --upgrade -r /code/requirements.txt
      COPY ./app /code/app
      
      CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "80"]
      

      The above assumes that your file structure looks like this

      .
      ├── app
      │   ├── __init__.py
      │   └── main.py
      ├── Dockerfile
      └── requirements.txt
      

      Hopefully, all these steps should look familiar if you already went through module M9, except for maybe the last line. However, this is just the standard way that we have run our FastAPI applications in the last couple of exercises, this time with some extra arguments regarding the ports we allow.

    3. Next, build the corresponding docker image

      docker build -t my_fastapi_app .
      
    4. Finally, run the image such that a container is spun up that runs our application. The important part here is to remember to specify the -p argument (p for port) which should be the same number as the port we have specified in the last line of our Dockerfile.

      docker run --name mycontainer -p 80:80 myimage
      
    5. Check that everything is working by going to the corresponding localhost page http://localhost/items/5?q=somequery

  9. (Optional) In module M15 on unittesting you learned how to write unit tests for your data pipeline and model. It should come as no surprise that the same can also be done for your API. Doing so should be able to tell you if your API is working as you expect it to do. The only complication regarding APIs is that you need a server to do testing, and we cannot use uvicorn for this. Check out this page on how to test FastAPI application, and add a file called test_api.py to your tests folder with appropriate tests for your API.

This ends the module on APIs. If you want to go further in this direction we highly recommend that you check out bentoml which is an API standard that focuses solely on creating easy-to-understand APIs and services for ml-applications. Additionally, we can also highly recommend checking out Postman which can help design, document and in particular test the API you are writing to make sure that it works as expected.