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Why should an app back-off from trying to read from a database that is responding too slowly?

The database could be under heavy load, so backing-off could give it time to recover.

The database server is being upgraded, so retrying can cause the upgrade to fail.

Backing-off means that the app can retry the operation quicker, to get a faster response.

Why is it important that operations are idempotent if they're being retried?

Retrying the operation leads to data being incremented the number of times the operation is processed successfully.

If the operation is going to be retried, and previous operations have all failed, the data can be incorrect.

If the operation is retried and previous operations have been successful, the data isn't duplicated or corrupted.

What's the main difference between permanent and terminal errors?

Permanent errors need to be handled and then quit the app. Terminal errors automatically quit the app.

Permanent errors can be handled by using data from elsewhere. Terminal errors mean the app can't continue and should quit.

Permanent errors are unrecoverable, log the data and quit. Terminal errors shouldn't log data and quit.

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