Application Errors

Application-level errors represent failures that occur during use case execution.
They are distinct from domain errors (which describe rule violations in the problem
space) and foundation errors (which describe general-purpose failures like invalid
arguments). Application errors signal infrastructure or coordination problems:
a transaction that won't commit, an event store that can't append, a concurrency
conflict between competing updates.

• UnitOfWorkError — Transaction commit or rollback failure.
• EventStoreError — Event append or retrieval failure.
• ConcurrencyError — Optimistic concurrency conflict.
• EventBusError — Event publishing failure.
• TransactionError — Transaction begin, commit, or rollback failure.

All application errors use RuntimeErrorMixin, making them catchable as RuntimeError.

When to use

Raise these errors from application code when a use case fails due to infrastructure or coordination issues. They extend the Foundation Error base class, so they carry structured messages and work with the presentation error pipeline.

On error boundaries

Application errors describe what failed during coordination. They should not encode transport or presentation concerns — those belong to outer layers.