What the heck is a UniFi controller exactly?

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So there are two kinds of wireless access points. Stand alone and managed. The unifi wireless access points are technically stand alone. The unifi controller is considered a soft controller because once configured it can be turned off and the APs will continue functioning. The unifi AP’s managed interface is a command line layer that sits on top of Linux. While you can access your AP’s interface through an SSH terminal session. For many non command-line cowboys this is difficult. So, some stand alone wireless AP manufacturers sell units with a web GUI hosted on each AP. From an enterprise perspective, for small deployments managing a half dozen or so wireless APs is fine. In an enterprise environment, once you get passed a dozen or so wireless APs, you really want to manage wireless controllers from a central location. The unifi controller lets you do just this.

For those that don’t want to run a Unifi controller but need a GUI there is a Unifi app on smart devices. Your other alternative is to load it on a computer and run it when you need it.

I get it, It sucks that you don’t have a backup to restore from. Now you know that you need to make backups and put them on in cold storage.

For now you can reset your AP, and use the smart device app to re-setup you AP.

A lot of questions here, let me see if I can answer them.

1. The UniFi Controller is a central hub that your UniFi devices securely communicate with. This controller handles the configuration and maintainence of your devices. When you adopt a device using the controller, that device is paired with that specific controller. The controller saves info about each device. This information is stored on your local filesystem. UniFi devices do not require the controller to function. The controller pushes configuration to each device, and then the device pushes historical stats to the controller.

2. The cloudkey is a specialized piece if hardware that runs the controller. This is an optional piece of equipment.

3. When you reformatted your hard drive, you erased the files that contains your network configuration. When you installed the controller again, it was a fresh new controller that didn't remember any of your devices. Likewise, your devices didn't remember the controller. That's why they didn't let you adopt them into your new controller. That's a good thing from a security perspective. If you want to do this in the future, save a config backup from your controller before you reformat your hard drive. Save that to a device that you aren't reformatting (like a thumb drive). When you reinstall your controller, there's an option to restore from backup. Use it.

Let me know if I can clarify anything.

I learned the same lesson when I built a new pc earlier this year :(

Unifi is based on a Software defines Network (SDN). As such, the configuration is managed and maintained in a centralized location - The Controller. The Security Gateways, APs, and switches are designed to contact their designated controller for configuration details at boot-up. When you change configuration using the controller, the changes are pushed to the appropriate devices which is why the Controller does not have to be on all of the time.

As you noted, the Controller software can be run on a number of different hosts. In addition to the free COntroller software, Ubiquiti also sells the Cloudkey which is nothing more than a purpose built small computer with an instance of the Controller installed. Some people opt to use a Cloudkey rather than install the software. Either way, they perform the same function. Ubiquiti does offer a hosted Cloud Controller service, but it is rather expensive compared to the above options.

All of the configuration details are stored in a database contained on the controller host. It is customary (and recommended) to take backups of the configuration to facilitate recovery and also to ease the migration from one controller instance to another. When you reformatted your Mac, you wiped out the Controller database. if you had taken a backup, you could have easily restored the configuration after reinstalling the COntroller software. The adoption process is primarily used to 'inform' the devices of the Controller network location. Once that is done, the Controller knows which devices it is managing and can maintain the appropriate configuration details.

I run the controller software in a VM, and it controls the small Unifi systems for two houses and one bigger system in a non-profit.

One house has two UAP-AC-Lites, one has two UAP-AC-LR and a few ACMesh.

The non-profit has about a dozen UAPs running staff and guest networks, complete with login portal and logging.

All running on a Debian VM on my home NAS (a Synology DS218+).