Application Load Balancer

This repository contains implementation of John Crickett's coding challenge: Build Your Own Load Balancer

Developed with

• .NET 8 LTS
• ASP.NET Core 8 Web API
• C# 12.0

Other open-source libraries used:

• FluentValidation
• Serilog.AspNetCore
• Serilog.Sinks.Console
• Ulid
• Quartz.AspNetCore
• MediatR

Components

• Application: Application server with a GET endpoint, where requests will be redirected to
• Load Balancer: The server which does load balancing among multiple instances of Application

Features

• Load balancer with Registration APIs to register/de-register applications
• Round-robin scheduling among registered applications to balance load
• Background service to periodically monitor health of registered applications and add/remove from the list of available applications
• Handling of applications going offline/online
• Handling of HTTP request to application getting timed-out
• Forwarding of X-Forwarded-* headers to downstream applications

Limitations

• Only HTTP traffic
• Only API controllers (no view, only JSON)
• Application need to register itself to load balancer before getting requests forwarded to them
• Only HTTP GET requests made to load balancer with any header, path, query strings will be forwarded as-it is to registered/downstream applications
• Naturally, all applications should have a GET endpoint that can catch any path value (**catchAll)
• Application must have health check endpoint implemented

Deployment

Configuration

LoadBalancer/appsettings.json

Key	Remarks
LoadBalancer__RegisteredApplicationsHealthCheckInterval	Interval (in sec) after which health check calls will be made

Application/appsettings.json

Key	Remarks
LoadBalancer__Uri	Load balancer URI
LoadBalancer__Timeout	Timeout (in sec) for requests made to load balancer
Application__HealthCheckPath	Application health check path
Application__Timeout	Timeout (in sec) for requests made to registered application(s)

Running load balancer

Open terminal window and change directory to Source/LoadBalancer directory

Run the following command to launch the load balancer on port 1111:

dotnet "./bin/Debug/net8.0/LoadBalancer.dll" --urls="http://+:1111"

Ensure load balancer is running by visiting the following URL: http://localhost:1111/alb-health

Running application(s)

Open terminal window and change directory to Source/Application.

Run the following command to launch the application on port 1112:

dotnet "./bin/Debug/net8.0/Application.dll" --urls="http://+:1112"

Ensure application is running by visiting following URL: http://localhost:1112/<Application__HealthCheckPath>

Register application to load balancer by visiting following URL: http://localhost:1112/register-self

After running the above command, application will get registered to load balancer as a downstream application, from then onwards, any HTTP GET request made to load balancer on path /* will be re-directed to application.

Deregister application to load balancer by running the following URL: http://localhost:1112/deregister-self

In this way, multiple applications can be launched on different ports, registered to load balancer. Traffic can be distributed among them.

To-do

• Dockerization