# Handling Errors

Every error ArchitectureKit returns belongs to one of four categories. Use `errors.Is` to check for a category rather than for a concrete error:

- `ErrDomain` means that a business rule rejected the command, as with `NewDomainError`.
- `ErrConflict` means that a precondition did not hold.
- `ErrTransient` means that trying again may help, for example if reading from the database failed.
- `ErrPermanent` means that trying again will not help, for example if an event could not be decoded, or if a subject contains an event type the state has no `Evolve` rule for.

```go
writtenEvents, err := architecturekit.Execute(
  // ...
)

switch {
case errors.Is(err, architecturekit.ErrDomain):
  // A business rule rejected the command.
case errors.Is(err, architecturekit.ErrConflict):
  // A precondition did not hold.
case errors.Is(err, architecturekit.ErrTransient):
  // Trying again may help.
case errors.Is(err, architecturekit.ErrPermanent):
  // Trying again will not help.
}
```

*Note that `ErrConflict` is a special case of `ErrTransient`, so check for it first.*

*Note that `Execute` does not retry. To try again, for example after a conflict, call `Execute` again.*