# Agent Note: A config hot-reload must not kill or degrade a live app

Status: implemented

English | [中文](2026-07-20-config-hot-reload-resilience.zh.md)

## Problem

An invalid `cordis.yml` edit must not kill a running agent, but preserving the process is insufficient when a valid-looking update partially replaces the Loader tree before a later entry fails. Callers also need to observe a rejected live update without treating the same error as an unhandled boot failure. Personal configuration adds a second requirement: HMR must observe one exact file outside its module roots, including a file or parent directory created after startup.

## Decision

The vendored Cordis lifecycle and Loader plugins provide an awaited, compensating config transaction, logged as local modifications 6, 8, and 9 in [vendor/README.md](../../../../vendor/README.md).

`Fiber.update()` returns its `internal/update` waterfall result. Config validation remains synchronous, while the default continuation returns the restart promise. Loader entry updates can therefore distinguish validation, import, application, and rollback failure from successful lifecycle settlement. `EntryTree.await()` rechecks service-gated fibers after Loader tasks drain and rejects settled failures; a fiber waiting on an absent service remains a valid pending entry rather than making settlement hang.

Loader imports a changed module name before disposing the active fiber. Candidate application is awaited; a failure disposes candidate effects and restores the prior plugin or config. Group reconciliation starts candidates concurrently, awaits every outcome, and restores changed entries, additions, removals, and moves before rejecting. Persistence occurs only after successful programmatic mutation. This is a compensating transaction: lifecycle effects may be briefly visible, and a failed rollback is reported as an `AggregateError` rather than misrepresented as a retained tree.

Include reads and validates detached candidate content, applies patches to a clone, reconciles the Loader tree, and only then commits cached content and parsed data. `refresh()` rejects to its caller after a parse, validation, application, or rollback failure. Initial load remains fail-loud; only an absent file may use `initial`. A non-array YAML/JSON result is invalid, and both file refresh and Include-config update re-apply patches without mutating the cached parse.

HMR contains live refresh rejection. Its `registerConfig(filename, refresh)` method watches one exact path from the nearest existing ancestor, serializes and coalesces refreshes, and returns an async disposer that closes the watcher and drains active work. Both exact-path and ordinary config-file refreshes use that queue. A failure is normalized to `Error`, logged, and broadcast through the parallel `hmr/config-update-failed(filename, error)` event; rejecting observers are logged without stopping later refreshes. Creation, change, and removal are observed.

## Alternatives considered

**Contain failures inside `Include.refresh()`.** Rejected because it prevents an HMR host from broadcasting the failure and still permits Loader reconciliation to hide partial application. Include owns candidate parsing and commit; HMR owns containment and observation.

**Restart the process for every config edit.** Rejected because Cordis effects already provide reversible plugin lifecycle, and a syntax error or failed optional plugin must not discard live sessions merely to recover the prior composition.

**Promise invisible atomic replacement.** Rejected because arbitrary plugin effects cannot be snapshotted. Awaited application plus explicit compensation provides a stable final result without claiming that observers cannot see intermediate lifecycle transitions.

## Consequences

- A failed live refresh rejects internally, retains or restores the last-good tree when compensation succeeds, and broadcasts one typed failure without becoming an unhandled rejection.
- A rollback failure is visible and may leave an entry unavailable; the event and log do not claim otherwise.
- Fibers waiting on declared dependencies remain valid pending entries: lifecycle settlement means no current work failed, not that every dependency exists.
- Exact config watchers add filesystem resources only for registered paths and release them with their owning HMR fiber.
- The vendored Loader, Include, HMR, and core event typing diverge further from upstream; the complete divergence is maintained in the vendor manifest.

## Testing

`packages/boot/app-boot/tests/config-reload.spec.ts` boots real temporary Loader/Include trees and covers parse and shape rejection, import-before-dispose, plugin/config restoration, multi-entry rollback, ancestor disablement, overlay convergence, option identity, failed direct-update persistence, and failed programmatic moves. `packages/boot/app-boot/tests/hmr-config.spec.ts` covers existing and missing exact paths, add/change/removal, serialized coalescing, disposal drainage, non-`Error` normalization, failure broadcast, and rejecting-observer containment. `packages/host/webserver/tests/webserver.spec.ts` proves a service-gated startup failure rejects Loader composition with its bind diagnostic, and `packages/typert/loader/tests/loader.spec.ts` exercises awaited programmatic removal through a real Loader consumer.
