For the majority of the past five years, Pushover has run on one physical OpenBSD server. It does have a hot spare hosted with another company in another part of the country, but usually everything has been served from just one machine at a time. Its MariaDB database is replicated in a master-master configuration over a secure tunnel between the servers so that either node can become active at any time.
Shortly after the initial release, I received some great feedback from Chad Etzel, one of the creators of Notifo, the notification service that I used until it was shut down (which prompted me to create Pushover in the first place). Chad asked for Pushover to support sending messages with URLs that can open external apps, and Pushover soon gained supplementary URL support which required changes in the API and on both Android and iOS apps.
On March 7th, 2012, I announced the launch of Pushover, a simple mobile notification service with device clients available for Android and iOS. I kept some notes during the development process, which mostly occurred in the evenings and weekends around my other work.
I had been using Notifo for a year or so to receive push notifications on my phone from my custom network monitor, but last year the free service announced it was shutting down. When I switched back to my Android phone a few months ago, I was unable to download Notifo's Android app which never made it out of beta.