Watch & Reload (Failure / Recovery Contract)

tywrap/dev regenerates wrappers and replaces the runtime bridge during development. Application-level hot module reloading remains the application's job. The helper keeps generated wrappers and the active Python bridge in sync with Python sources while the process stays up.

This page documents the reload lifecycle and failure behavior: what happens when a reload fails, and what stays live so your app keeps working.

Reload is configured in code through tywrap/dev, never in tywrap.config.*. The legacy development block and pythonModules.<module>.watch fields were removed in 0.4.0. Using them now raises an explicit migration error.

The two entry points

Helper	Use it for
startNodeWatchSession(...)	Node-only. Watches the config and local Python sources. After regenerating wrappers, it swaps the active bridge in place.
createBridgeReloader(...)	Cross-runtime manual primitive (e.g. Pyodide). You call reload() yourself. It has no filesystem watcher.

import { startNodeWatchSession } from 'tywrap/dev';
import { NodeBridge } from 'tywrap/node';

const session = await startNodeWatchSession({
  configFile: './tywrap.config.ts',
  createBridge: async config =>
    new NodeBridge({
      pythonPath: config.runtime.node?.pythonPath ?? 'python3',
      timeoutMs: config.runtime.node?.timeout ?? 30000,
    }),
});

// Force a rebuild now (resolves to `true` on success, `false` on failure):
const ok = await session.reloadNow();

// Stop watching and dispose the active bridge:
await session.close();

createBridge receives the freshly resolved config for that reload cycle, so config edits (e.g. a changed timeout) are picked up on the next reload. By default the newly created bridge is published to the global runtime registry, so existing generated wrappers transparently route through the swapped bridge. No imports change and the process never restarts.

Reload lifecycle events

Pass an onEvent callback to observe the session. Events are emitted in this order for a successful cycle:

Event	Meaning
watchPaths	The set of paths the session is currently watching (config file, resolved Python package trees, and extraWatchPaths). Re-emitted whenever the watched set changes.
change	A watched path changed (manual: false), so a reload has been scheduled.
reload-start	A reload cycle began (manual: true for reloadNow() / initial startup).
reload-success	Regeneration and bridge swap completed. Carries written (relative paths of the generated files now on disk) and warnings.
reload-error	The reload failed. Carries the underlying error: Error.

A reload performs these steps atomically with respect to the live state:

1. Generate the next wrapper set into a temporary staging directory.
2. Promote the staged files into the real output directory (writing new files, removing managed files that no longer exist).
3. Warm and activate the next bridge, then dispose the previous one.
4. Commit the refreshed watcher set and emit reload-success.

reloadNow() and filesystem changes are serialized: an in-flight reload finishes before the next one starts, and rapid edits are debounced (debounceMs, default 100).

Failure / recovery contract

If any step of a reload throws, the session stays up. The last-known-good state stays live:

• The previously generated wrappers remain on disk unchanged. Staging happens in a temp directory; the real output directory is only touched once generation succeeds. If promotion itself fails partway, the previous file contents are restored.
• The previously active bridge stays active in the runtime registry and is not disposed. A bridge that was warmed for the failed reload is disposed instead, so you never leak the half-prepared one.
• A structured reload-error event is emitted with the underlying Error, and the failing call (reloadNow() or the debounced auto-reload) resolves to false. The session keeps watching and will attempt the next reload normally.

Generation and bridge construction failures surface through the same event:

Source	What the error looks like
Generation failure (a watched module's IR can't be produced, such as after a syntax error). This is the GenerateFailure path.	error.message begins with Generation failed for N module(s): followed by per-module detail.
Bridge construction failure (your createBridge throws).	The error your factory threw, propagated verbatim.

const session = await startNodeWatchSession({
  configFile: './tywrap.config.ts',
  createBridge,
  onEvent: event => {
    if (event.type === 'reload-error') {
      // Last-good wrappers + bridge are still live here.
      console.error('[tywrap] reload failed, keeping last-good state:', event.error.message);
    }
    if (event.type === 'reload-success') {
      console.log('[tywrap] reloaded:', event.written.join(', '));
    }
  },
});

Startup is the exception. There is no last-known-good state on the very first reload, so if the initial startNodeWatchSession(...) setup fails, the promise rejects (watchers are closed and any partial bridge disposed). After a successful start, subsequent failures keep the last known good state live.

Other notes

• Watched Python package trees are watched per-directory; new nested directories are picked up automatically and __pycache__ / .pytest_cache / .mypy_cache / .ruff_cache churn is ignored.
• Writes into the output directory and .tywrap/{cache,reports} are ignored so generation does not retrigger itself.
• For Pyodide use createBridgeReloader(...) and drive reload() from your own trigger. For the HTTP runtime, reload by restarting/redeploying the remote server. That reload is external to tywrap.