#Lifecycle

Handlers run synchronously around the scheduler core in the order the middleware was registered. .use(a, b) and .use(a).use(b) register the same order, and each use returns a new definition without changing the original. The scheduler core sits in the middle of the chain, so code before next() prepares the operation and code after it sees the result.

#Order around the core

For .use(a, b), the call order is:

a: before next()
  b: before next()
    scheduler core
  b: after next()
a: after next()

Call next() once to continue the normal scheduling flow. Set inputs such as ctx.candidate.desiredRetention[ctx.input.grade] before it; inspect or change the computed result after it. In review, ctx.scheduledDays is the interval in days. The scheduler applies the configured chrono after the handlers finish to calculate the due time, then validates the result.

Order matters when policies interact: register schedulerScheduledDaysMiddleware before the interval policies so it records their final result on unwind; keep learning steps before the maximum- and monotonic-interval policies so explicit steps retain their delay. Register leech before handlers that set scheduleStatus after next(), so suspension is applied last. The Built-in policies page lists each policy and how DefaultScheduler orders them.

#Review and rollback

Rollback uses its own handlers in the same registration order. A rollback handler can read ctx.config, ctx.input.card, and ctx.input.revlog, and writes only ctx.result.card. Schema declarations alone do not restore custom fields: save the required old values during review and restore them in the rollback handler according to the configured policy, as the streak example on the Overview does.

preview() runs the review handlers for the requested candidate ratings. Keep handlers free of database writes, notifications, and other external effects; persist the selected review result in your application. forget() resets a card with its defaults rather than running a separate handler, so the streak example resets the counter to zero.