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In this tutorial, we will demonstrate how to add instrumentation to your application by building a dummy API using Flask—a robust Python web framework for creating RESTful APIs quickly. Our primary focus is on incorporating Prometheus instrumentation rather than delving into the internals of Flask.
This guide shows you how to integrate instrumentation step-by-step into a basic Flask application. Follow along to learn about adding counters to track HTTP requests and exposing metrics via Prometheus.

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Creating a Basic Flask Application

First, create a simple Flask application. In Python, you import the Flask package, initialize a Flask instance, and then create an API endpoint. For example, any GET request to the /cars endpoint will trigger the following function: 
from flask import Flask
app = Flask(__name__)

@app.get("/cars")
def get_cars():
    return ["toyota", "honda", "mazda", "lexus"]

if __name__ == '__main__':
    app.run(port='5001')
This code snippet represents the foundation of our API. At this stage, the application does not include any Prometheus instrumentation.

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Installing the Prometheus Client Library

Before adding instrumentation, install the Prometheus client library: 
pip install prometheus_client
Once installed, you can import and use the Counter object from the Prometheus client to track your application’s metrics. For instance, initialize a counter called http_requests_total to record every HTTP request: 
from prometheus_client import Counter

REQUESTS = Counter('http_requests_total', 'Total number of requests')

app = Flask(__name__)

@app.get("/cars")
def get_cars():
    return ["toyota", "honda", "mazda", "lexus"]

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Incrementing the Counter

To accurately track the number of requests, increment the counter each time the /cars endpoint is hit. You can use the REQUESTS.inc() method, which increases the counter by 1 for each request. Optionally, you can pass a value to inc() if you need a different increment: 
from prometheus_client import Counter

REQUESTS = Counter('http_requests_total', 'Total number of requests')

app = Flask(__name__)

@app.get("/cars")
def get_cars():
    REQUESTS.inc()
    return ["toyota", "honda", "mazda", "lexus"]

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Exposing Metrics via a Separate HTTP Server

Currently, while metrics are recorded, they are not exposed to Prometheus. The simplest method to expose these metrics is to start Prometheus’s built-in HTTP server on a separate port (e.g., port 8000): 
from prometheus_client import Counter, start_http_server

if __name__ == '__main__':
    start_http_server(8000)
    app.run(port='5001')
With this configuration, your Flask API runs on port 5001 while Prometheus metrics are available at port 8000. You can test the metrics endpoint using: 
$ curl 127.0.0.1:8000
# HELP python_gc_objects_uncollectable_total Uncollectable object found during GC
# TYPE python_gc_objects_uncollectable_total counter
python_gc_objects_uncollectable_total{generation="0"} 0.0
python_gc_objects_uncollectable_total{generation="1"} 0.0
python_gc_objects_uncollectable_total{generation="2"} 0.0
# HELP python_gc_collections_total Number of times this generation was collected
# TYPE python_gc_collections_total counter
python_gc_collections_total{generation="0"} 77.0
python_gc_collections_total{generation="1"} 7.0
python_gc_collections_total{generation="2"} 0.0
# HELP python_info Python platform information
# TYPE python_info gauge
python_info{implementation="CPython",major="3",minor="9",patchlevel="5",version="3.9.5"} 1.0
# HELP http_requests_total Total number of requests
# TYPE http_requests_total counter
http_requests_total 5.0
# HELP http_requests_created Total number of requests
# TYPE http_requests_created gauge
http_requests_created 1.6654499091926205e+09

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Integrating Metrics with Flask Middleware

If you prefer to run the API and metrics on the same port, you can expose metrics via a dedicated endpoint by integrating Prometheus with Flask middleware. This method uses make_wsgi_app from the Prometheus client alongside DispatcherMiddleware from Werkzeug: 
from flask import Flask
from prometheus_client import make_wsgi_app
from werkzeug.middleware.dispatcher import DispatcherMiddleware

app = Flask(__name__)

# Add Prometheus middleware to export metrics at /metrics
app.wsgi_app = DispatcherMiddleware(app.wsgi_app, {
    '/metrics': make_wsgi_app()
})
With this setup, your Flask API is still served on port 5001, and you can retrieve Prometheus metrics from the /metrics endpoint.

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Extending the Application with Additional Endpoints

Next, extend the application by adding several routes that handle different HTTP methods. In this example, endpoints are added to return all cars, retrieve a specific car by ID, create a new car, update car details, and delete a car. In a real-world scenario, you should increment the REQUESTS counter for every request: 
@app.get("/cars")
def get_cars():
    REQUESTS.inc()
    return ["toyota", "honda", "mazda", "lexus"]

@app.get("/cars/<int:id>")
def get_car():
    REQUESTS.inc()
    return "Single car"

@app.post("/cars")
def create_cars():
    REQUESTS.inc()
    return "Create Car"

@app.patch("/cars/<int:id>")
def update_cars():
    REQUESTS.inc()
    return "Updating Car"

@app.delete("/cars/<int:id>")
def delete_cars():
    REQUESTS.inc()
    return "Deleting Car"
At this point, your fully instrumented Flask API accurately reflects the total number of HTTP requests via the http_requests_total counter. You can access and monitor your metrics either through the standalone Prometheus HTTP server on port 8000 or directly via the /metrics endpoint if using the middleware approach.
For more detailed information on Prometheus instrumentation, refer to the Prometheus Client Documentation. Additionally, visit the Flask Documentation for further insights into building robust web applications.

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