What is the difference between Host ID and Instacne ID ?

In Fly.io prometheus cluster the labels host (4 characters host ID) and instance (app instance ID) are used. As I understand it, the instance ID represents the specific VM instance. However, I cannot explain what the HostI D is and how it relates to the Instance ID. Furthermore, I wonder if the host ID can be determined with a flyctl command.

The host ID is more meaningful internally at Fly, it represents the physical server on which your VM is running. I don’t think it should affect the application itself in any observable way, except maybe if you’re trying to check physical separation of instances for high availability setups.

Thank you for the answer. I wasn’t sure if I should use the Host ID when filtering my Grafana Dashbord metrics. According to your explanation, it doesn’t make sense for my purposes. I will use the instance IDs for filtering in addition to the region.