🌦 How I Deployed a Flask Weather App on AWS Using Docker, ECS, and ALB

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Building is fun. Deploying it live? Even more satisfying — especially when your app predicts the weather! ☁️

In this post, I’ll walk you through how I deployed a simple Flask weather application to AWS using Docker, ECS (Elastic Container Service), and ALB (Application Load Balancer). I'll also share a common bug I hit and how I fixed it.

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🛠️ What I Built

A Flask web app that:

• Takes a city name as input.

• Calls the OpenWeatherMap API to get current weather + 5-day forecast.

• Shows details like temperature, wind speed, and conditions.

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🧱 Architecture Breakdown

Here’s the flow:

1. Local Dev

   • Pulled code from GitHub.

   • Built a Docker image using a Dockerfile.

2. Container Registry

   • Pushed the image to AWS Elastic Container Registry (ECR).

3. ECS Deployment

   • Created an ECS Cluster.

   • Configured ECS service to pull the image from ECR.

   • Deployed the container.

4. Traffic Management

   • Set up an Application Load Balancer.

   • Routed traffic from ALB to the ECS service.

   • Accessed the app via a public DNS URL.

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🚀 Step-by-Step Deployment Guide

1. Clone & Build the App

git clone https://github.com/your-repo/weather-app
cd weather-app
docker build -t weather-app .

2. Push to AWS ECR

aws ecr create-repository --repository-name weather-app
aws ecr get-login-password | docker login --username AWS --password-stdin <your-ecr-url>

docker tag weather-app:latest <your-ecr-url>/weather-app:latest
docker push <your-ecr-url>/weather-app:latest

3. ECS & ALB Setup

• Use the AWS Console to:

  • Create a Cluster → EC2 or Fargate.

  • Create a Task Definition referencing your ECR image.

  • Configure Service to launch tasks.

  • Set up an ALB, add target group & listener.

Done right, AWS will assign a DNS URL like:

http://weather-app-alb-xxxxxxxxx.ap-south-1.elb.amazonaws.com/

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🧑‍🔧 The Bug I Faced (and Fixed)

❌ Error:

KeyError: 0
lat = location_data[0]['lat']

🧠 Root Cause:

The OpenWeather Geocoding API didn’t return data for the city entered, so location_data[0] crashed.

✅ Fix:

Wrap the response parsing in a try-except block:

location_data = location_response.json()
if not location_data or 'lat' not in location_data[0]:
    return redirect(url_for("error"))
lat = location_data[0]['lat']

Also added logs to debug:

print("Geo API Response:", location_response.status_code, location_response.text)

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💡 Final Thoughts

This was a great hands-on way to learn:

• Docker image creation.

• ECR, ECS, and container orchestration.

• Load balancing using ALB.

• Debugging real-world deployment bugs.

Next up: Auto-scaling and HTTPS using ACM!

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🔗 Useful Resources

• 📚 OpenWeatherMap API Docs

• 📘 AWS ECS Docs

• 🐳 Docker Basics

• 🌐 Flask Official Docs