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Balancer

A simple load balancer written in Go.

Features

  • Testing
  • Round Robin Method
  • Health Checks
  • Fewest Active Connections Method
  • Least Latency Method
  • Weighted Round Robin

How it works

Schematic

On start-up, the application creates n backend web servers in the 5000 block on the localhost port from 5000 to 500N. Each backend conforms to the Server interface. The backend superficially responds to GET requests with its address and the response time. Response times are randomly chosen between 0 and 250ms.

Each backend is assigned a single host reverse proxy. The purpose of the proxy in this application is to handle backend failure gracefully within the server pool. In Go, the reverse proxy has an ErrorHandler method which can be customized to retry requests or propagate issues with its assigned backend to the server pool.

Each backend is assigned to the server pool of the load balancer. The pool is responsible for implementing the server rotation algorithms. An HTTP server is created and implements a response handler for the pool that decides which backend to send the request to.

The main function runs a health check on an interval which makes a TCP connection to each backend server to verify it is alive, then sets the status of each backend through the Server interface accordingly.

Usage

Clone the repository and navigate to the root of the project and run go run . The application takes three flags:

--n       The number of backend servers to start. Defaults to 5.
--port    The port number on which to start the load balancer. This will
          be the port you'd call with cURL
--algo    The balancing algorithm to use. Options are:
          "alwaysfirst"   Takes the first server in the slice
          "roundrobin"    Takes the next healthy server sequentially
          "leastlatency"  Takes the server with the lowest average response time
          "fewestconn"    Takes the server with the least active connections

In another window, run the following cURL command:

$ curl http://localhost:8000
(localhost:5000) Returned response in 46(ms)
(localhost:5001) Returned response in 44(ms)
(localhost:5002) Returned response in 175(ms)
(localhost:5003) Returned response in 197(ms)
(localhost:5004) Returned response in 121(ms)
(localhost:5000) Returned response in 103(ms)

Round Robin Algorithm

Schematic

Credits

The design of this application is inspired and informed by the content and code in the online resources below.