As always „it depends”

- info - when this was expected and system/process is prepared for that (like automatic retry, fallback to local copy, offline mode, event driven with persistent queue etc) - warning - when system/process was able to continue but in degraded manner, maybe leaving decision to retry to user or other part of system, or maybe just relying on someone checking logs for unexpected events, this of course depends if that external system is required for some action or in some way optional - error - when system/process is not able to continue and particular action has been stopped immediately, this includes situation where retry mechanism is not implemented for step required for completion of particular action - fatal - you need to restart something, either manually or by external watchdog, you don’t expect this kind of logs for simple 5xx