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HAProxy EP 8: Load Balancing with Random Load Balancing
Load balancing distributes client requests across multiple servers to ensure high availability and reliability. One of the simplest load balancing algorithms is Random Load Balancing, which selects a backend server randomly for each client request.
Although this approach does not consider server load or other metrics, it can be effective for less critical applications or when the goal is to achieve simplicity.
What is Random Load Balancing?
Random Load Balancing assigns incoming requests to a randomly chosen server from the available pool of servers. This method is straightforward and ensures that requests are distributed in a non-deterministic manner, which may work well for environments with equally capable servers and minimal concerns about server load or state.
Step-by-Step Implementation with Docker
Step 1: Create Dockerfiles for Each Flask Application
Weβll use the same three Flask applications (app1.py, app2.py, and app3.py) as in previous examples.
Flask App 1 β (app.py)
from flask import Flask
app = Flask(__name__)
@app.route("/")
def home():
return "Hello from Flask App 1!"
@app.route("/data")
def data():
return "Data from Flask App 1!"
if __name__ == "__main__":
app.run(host="0.0.0.0", port=5001)
Flask App 2 β (app.py)
from flask import Flask
app = Flask(__name__)
@app.route("/")
def home():
return "Hello from Flask App 2!"
@app.route("/data")
def data():
return "Data from Flask App 2!"
if __name__ == "__main__":
app.run(host="0.0.0.0", port=5002)
Flask App 3 β (app.py)
from flask import Flask
app = Flask(__name__)
@app.route("/")
def home():
return "Hello from Flask App 3!"
@app.route("/data")
def data():
return "Data from Flask App 3!"
if __name__ == "__main__":
app.run(host="0.0.0.0", port=5003)
Step 2: Create Dockerfiles for Each Flask Application
Create Dockerfiles for each of the Flask applications:
- Dockerfile for Flask App 1 (
Dockerfile.app1):
# Use the official Python image from Docker Hub FROM python:3.9-slim # Set the working directory inside the container WORKDIR /app # Copy the application file into the container COPY app1.py . # Install Flask inside the container RUN pip install Flask # Expose the port the app runs on EXPOSE 5001 # Run the application CMD ["python", "app1.py"]
- Dockerfile for Flask App 2 (
Dockerfile.app2):
FROM python:3.9-slim WORKDIR /app COPY app2.py . RUN pip install Flask EXPOSE 5002 CMD ["python", "app2.py"]
- Dockerfile for Flask App 3 (
Dockerfile.app3):
FROM python:3.9-slim WORKDIR /app COPY app3.py . RUN pip install Flask EXPOSE 5003 CMD ["python", "app3.py"]
Step 3: Create a Dockerfile for HAProxy
HAProxy Configuration file,
global
log stdout format raw local0
daemon
defaults
log global
mode http
option httplog
option dontlognull
timeout connect 5000ms
timeout client 50000ms
timeout server 50000ms
frontend http_front
bind *:80
default_backend servers
backend servers
balance random
random draw 2
server server1 app1:5001 check
server server2 app2:5002 check
server server3 app3:5003 check
Explanation:
- The
balance randomdirective tells HAProxy to use the Random load balancing algorithm. - The
random draw 2setting makes HAProxy select 2 servers randomly and choose the one with the least number of connections. This adds a bit of load awareness to the random choice. - The
serverdirectives define the backend servers and their ports.
Step 4: Create a Dockerfile for HAProxy
Create a Dockerfile for HAProxy (Dockerfile.haproxy):
# Use the official HAProxy image from Docker Hub FROM haproxy:latest # Copy the custom HAProxy configuration file into the container COPY haproxy.cfg /usr/local/etc/haproxy/haproxy.cfg # Expose the port for HAProxy EXPOSE 80
Step 5: Create a docker-compose.yml File
To manage all the containers together, create a docker-compose.yml file:
version: '3'
services:
app1:
build:
context: .
dockerfile: Dockerfile.app1
container_name: flask_app1
ports:
- "5001:5001"
app2:
build:
context: .
dockerfile: Dockerfile.app2
container_name: flask_app2
ports:
- "5002:5002"
app3:
build:
context: .
dockerfile: Dockerfile.app3
container_name: flask_app3
ports:
- "5003:5003"
haproxy:
build:
context: .
dockerfile: Dockerfile.haproxy
container_name: haproxy
ports:
- "80:80"
depends_on:
- app1
- app2
- app3
Explanation:
- The
docker-compose.ymlfile defines the services (app1,app2,app3, andhaproxy) and their respective configurations. - HAProxy depends on the three Flask applications to be up and running before it starts.
Step 6: Build and Run the Docker Containers
Run the following command to build and start all the containers:
docker-compose up --build
This command builds Docker images for all three Flask apps and HAProxy, then starts them.
Step 7: Test the Load Balancer
Open your browser or use curl to make requests to the HAProxy server:
curl http://localhost/ curl http://localhost/data
Observation:
- With Random Load Balancing, each request should randomly hit one of the three backend servers.
- Since the selection is random, you may not see a predictable pattern; however, the requests should be evenly distributed across the servers over a large number of requests.
Conclusion
By implementing Random Load Balancing with HAProxy, weβve demonstrated a simple way to distribute traffic across multiple servers without relying on complex metrics or state information. While this approach may not be ideal for all use cases, it can be useful in scenarios where simplicity is more valuable than fine-tuned load distribution.
