How does a backup alert reach the right people?

Choose a topic, which is a named feed for a particular alert. A backup job publishes a short message to that feed. The people who need the result subscribe in a compatible app. Use a plain result such as “Backup needs attention” and keep the files and recovery details in your own system.

Can someone read or post to the topic just by knowing its name?

Yes on an ordinary public topic. A long or unusual name does not enforce access. Hosted plans with reservations let you set how others can read or write to a reserved topic. On a self-hosted server, start with access denied by default and grant only the read and write access each account needs.

Give the backup job permission to publish and the operator permission to subscribe. Keep posting credentials with the job and use a separate account for the reader. An app subscription is how someone follows a feed. It does not replace your access rules.

How can you spot a bad topic rule before connecting the backup job?

Draw a fictional topic and three roles on paper. Do not create, subscribe to or post to a real topic.

Write an allowed path
Give the backup job permission to publish and the operator permission to read. Mark that the operator should see the result when they check the app.
Try both denied paths
Give an anonymous visitor neither permission. Mark both reading and publishing as denied. Also mark publishing denied for the operator and reading denied for the backup job.
Separate acceptance from a person seeing it
Write down what the job records when the server accepts a message. Put receipt on the operator’s device in a separate column. An accepted post leaves device delivery and reading untested.

What changes with hosted plans or your own server?

ntfy.sh’s standard hosted limit is 250 messages per publishing visitor in that visitor’s own 24-hour period. The allowance is shared across topics that visitor publishes to. Paid plans offer larger allowances, and Pro lists 10 reserved topics. Self-hosted limits are configurable. You take on server access, updates and storage when you host it yourself.

For instant alerts on the stock iOS app, the self-hosted setup uses a connected upstream service to tell the phone to fetch the alert from your server. Check this dependency before choosing where to run it.

Keep the sender configuration and access rules so you can rebuild topics. ntfy.sh’s hosted privacy policy says message text is cached temporarily by default. Store the backup result and any history you need in your own app.