# Failures

Your code says what went wrong, by name, and Jadpo decides how each
failure reaches whoever called: the HTTP status, the response body and
what is logged. Code never throws, never picks an HTTP status and never
builds an error response.

Three kinds of thing can go wrong, and only the first is yours to
declare:

- A **domain failure** is an outcome your app expects, such as a todo
  that doesn't exist. You declare it and say where it can happen.
- A **fault** is a problem outside your code: the database is down, a
  provider timed out, or a write may or may not have been saved. The
  runtime raises it.
- A **defect** is a bug: a number overflowed, the recursion limit was
  reached, or the compiler got something wrong. The runtime raises it
  too.

| | Declared by | Listed in `fails` | The client sees | The log gets |
|---|---|---|---|---|
| **Domain failure**, such as `TodoNotFound` | you, as `type X = Failure` | yes | its code, message and client fields | its code, where it was raised, the path it travelled and its internal fields |
| **Fault**, such as the database being down | the runtime | no | a fixed code for that fault | the cause chain |
| **Defect**, such as an overflow | the runtime | no | `internal_fault` | the cause and stack |