[RegiByte]

Problem

topologicalSort threw Resource "X" depends on "Y" which doesn't exist in the system config for every declared dependency, optional ones included. An optional dependency exists so a resource can run without it, yet a config that leaves it out could not be started, halted, or reconciled to. In practice: resource x:github declares optional: ['x:reviews']; when reviews are turned off the next config has no x:reviews, and reconcileSystem(prev, system, next) threw instead of restarting x:github without reviews.

Once that is fixed, the opposite direction exposes a second bug. Reconciling to a config that adds an optional dependency restarted its unchanged dependents without halting their previous instances, because the halt set only covered changed and removed resources. Before, this could not happen, since a previous config with a missing dependency could never start.

Fix

• dependencyEdges(deps, resources) in src/dependencies.ts returns the dependencies that are edges in a system: every required one, plus each optional one the system has.
• topologicalSort, buildTopology (and through it formatTopology, toMermaid, toDot, toJSON) and reconcile's withDependents all walk dependencyEdges. A missing optional dependency adds no edge. A missing required one still reaches the existing throw.
• startInOrder is unchanged: it already resolves an absent id to undefined, which is how a failed optional dependency is handled.
• reconcileSystem now halts every restarted resource that was in the previous config, in addition to the changed and removed resources and their dependents.
• README (Graceful Degradation) and CHANGELOG are updated, and the version is bumped to 0.3.1.

Tests

Before: 86 passing. After: 93 passing (7 new). Six of the new specs fail against the old source. The seventh, a missing required dependency beside a missing optional one at the startSystem level, passes on both versions; it locks the existing throw in place.

• topology: the sort orders around a missing optional dependency; a missing required one still throws; no edge, dependent or depth is recorded for the missing one, and the Mermaid and DOT output omit it.
• core: a resource whose optional dependency is absent starts, receives { counter, absent: undefined }, and halts cleanly. A missing required dependency next to a missing optional one still throws.
• reconcile: removing an optional dependency halts the dependent, then halts the dependency, then restarts the dependent without it, with no errors. Adding the dependency halts the dependent, starts the dependency, then restarts the dependent on it.

npm test, npm run type:check and npm run build all pass.