# 7.4 The Boot and On-Demand Paths

_Peios / Advanced Peios / peinit / Dependencies_

> The two ways a graph gets built and how they differ beyond scope — including the three places a bad definition can surface.

The two ways a graph gets built differ in more than scope, and the
differences are worth having in one place.

| | Boot | On-demand |
|---|---|---|
| Members | Every boot-triggered root plus its closure | One requested service plus its closure |
| A missing hard-dependency target | Blocks the dependent, in Full mode only | Blocks the dependent |
| A disabled hard-dependency target | Blocks the dependent | Starts it |
| A validation finding | The service is failed, others continue | The start fails |
| Multiple findings on one service | Only the highest-precedence one is reported | — |
| Conflicts | Two boot-triggered conflicting services fail both | The conflicting service is evicted |
| Context | One boot context | One context per explicit start |
| Failure of the whole build | Recovery mode | An error to the caller |

## 7.4.1 Where a definition error lands

The three places a bad definition can be caught behave differently, and
which one catches it depends on what kind of wrong it is:

- **Decoding.** A definition that does not parse — an invalid name, a
  malformed trigger, an unclosed quote, a `registry:` check naming an
  uncacheable key, a duplicate field — is caught when the registry is
  read. At boot it fails that service with `ValidationError` and the
  rest continue; on a reload it rejects the whole reload (§3.2).
- **Graph validation.** A definition that parses but does not fit —
  a cycle, a missing target, a flap-constraint violation, an invalid
  calendar expression — is caught here, and fails that service at boot
  or the whole reload on a reload-config (§7.2).
- **Start.** A definition that parses and fits but whose preconditions
  do not hold — a failed assert, an unresolvable identity, a missing
  binary — is caught when the service actually starts, and fails that
  activation (§5.2, §5.3).

The dividing line between the first two is whether the problem is
visible in one definition on its own. Decoding sees one key at a time;
validation is the first place that can see two definitions together.