Libraries v/s Frameworks

Posted on:September 27, 2023 at 12:13 PM

Libraries v/s Frameworks

I have been professionally developing software for ~4 years now. And never ever I have worked on a project which was complete in itself. Directly or indirectly, there was a dependency on some code written by some kind of OSS developer(s).

While using those dependencies, one thing that intrigued me in my initial days was “Why do people call X a library, and Y a framework?” This post is the answer to that question.

While reading this post, keeping a framework and a library in mind will be helpful. If you’re a Python developer, I’ll suggest flask as a framework and requests as a library. I’ll give examples in Python, wherever required.

Example Code

Let’s define a function that fetches the latest time from a given server.

Library

import requests

def get_latest_server_time(server_url: str):
    resp = requests.get(server_url)
    if resp.status_code == 200:
        return resp.json()
    raise Exception('Unable to get latest time from server')

Simple code, right? Your function utilizes requests “library”, which provides a super-helpful method get, which abstracts the details like creating a connection to a server, sending the request, unmarshalling the response, etc.

Framework

Let’s define a simple API endpoint that returns “Hello World”

from flask import Flask

app = Flask(__name__)

@app.route("/hello")
def hello_world():
    return "Hello World"

if __name__ == '__main__':
    app.run()

Here, we defined a function hello_world and then instructed the flask to call that function if any request hits at the /hello endpoint. Simple enough? Cool, let’s move to the crux of the article

Difference b/w library and framework

Examine the get_latest_server_time function we defined in the Library example. You’re calling the requests.get function. It does something under the hood and returns you a Response object. Then, you process the Response object and take appropriate action. The key thing to note here is

You are controlling the flow of execution. You are responsible for invoking the code written by some 3rd party.

Now, let’s move to our framework example. Let’s assume that we started our API server locally, and it is running on port 5000. What happens when you do the following curl?

$ curl -X GET http://127.0.0.1:5000/hello

You’ll get the following response

Hello World

Let’s walk through step-by-step how we got the response.

The key thing to note here is

The flask server is controlling the flow of execution. It is responsible for invoking the code written by you.

The moment python executes app.run() in our example, the flow of control is transferred to  flask. And after that, flask is responsible for executing the code written by us.

And that folks, is the difference between a framework and a library. It’s all about the flow of control.

While using libraries, you control when a third party module gets invoked.

While using frameworks, the 3rd party module controls when your code gets invoked.

It doesn’t matter which language you use, the above-mentioned distinction will always hold.

Some technical tidbits