This project will show how to dynamically instantiate Node.js application processes and have Consul reconfigure and restart HAProxy.

Each of these programs consist of a single binary. You can choose to install them completely or simply download and execute the binary. If doing the latter be sure to adjust relative paths.

• HAProxy
• Consul
• Consul Template

Also, checkout this repository and run npm install to get the required packages.

At any point you can visit http://localhost:8000/admin?stats to view the inner workings of HAProxy.

If we simply want to use a known, finite number of HTTP servers we can use a simple HAProxy configuration and launch the number of processes.

haproxy ./basic.cfg             # Terminal 1
./service-basic-web.js 20001    # Terminal 2
./service-basic-web.js 20002    # Terminal 3
curl http://localhost:8000/     # Terminal 4

Unfortunately any time we scale the number of processes we need mo manually modify basic.cfg. Luckily we can do better.

Start a couple data providers, launch an HTTP server, launch HAProxy. Make a few requests and note that they are routed to a single HTTP server. Launch a new server and notice how requests are split between both. Kill an HTTP server and again see how requests are back down to a single server.

consul agent -dev               # Terminal 1
./service-data.js 30001         # Terminal 2
./service-data.js 30002         # Terminal 3
./service-www.js 20001          # Terminal 4
# Terminal 4
consul-template -template "./advanced.cfg.template:./advanced.cfg:./haproxy-restart.sh"
curl http://localhost:8000      # Terminal 6
curl http://localhost:8000      # Terminal 6
curl http://localhost:8000      # Terminal 6
./service-www.js 20002          # Terminal 7
curl http://localhost:8000      # Terminal 6
curl http://localhost:8000      # Terminal 6
curl http://localhost:8000      # Terminal 6
# Ctrl + C                      # Terminal 4
curl http://localhost:8000      # Terminal 6
curl http://localhost:8000      # Terminal 6
curl http://localhost:8000      # Terminal 6

As the requests run you will see different data returned when you make cURL requests, depending on which servers are running.

If there are two www servers running you'll see the PID values alternate back and forth as HAProxy does round-robin proxying between the services.

If there are two data servers running you'll see the PID values randomly jump as our application randomly routes traffic between the instances.

Of course if you scale up or down the number of either data or www services, the PIDs will shrink or grow accordingly. I'm specifically using PID values to show that different processes are handling the workloads.

{
  "data": {
    "data": 12345678,
    "data_pid": 20001
  },
  "web_pid": 20101
}