Backfill

Backfill runs a trigger's schedule over a past range you choose: one firing of its job for each fire time the trigger had in the range. From 4.3.

// The nightly export did not run on the 1st, 2nd and 3rd: run each of those nights now,
// five minutes apart.
BackfillResult result = await scheduler.Backfill(
    new TriggerKey("nightly-export", "reports"),
    from: new DateTimeOffset(2026, 9, 1, 0, 0, 0, TimeSpan.Zero),
    to: new DateTimeOffset(2026, 9, 4, 0, 0, 0, TimeSpan.Zero),
    new BackfillOptions { Spacing = TimeSpan.FromMinutes(5) },
    cancellationToken);

Console.WriteLine($"{result.SlotsFound} slots: {result.Scheduled} scheduled, {result.AlreadyScheduled} already scheduled");

Backfill or misfire

See Misfire Instructions.

Which slots

A slot is a fire time of the trigger in [from, to):

• from is included; to is excluded.
• The trigger's start and end time bound the range. There are no slots before its start time.
• A time the trigger's calendar excludes is not a slot.

A trigger builder sets the start time to the moment it builds unless told otherwise. To backfill from before a cron trigger was scheduled, give it a start time in the past; its first fire time is then past too, and its misfire instruction handles it. A simple trigger stored with a start in the past, or rescheduled by a misfire, starts from then instead.

What is scheduled

One one-shot trigger per slot, for the same job:

A [DisallowConcurrentExecution] job runs its backfilled firings one at a time.

The slot in the job

A backfilled firing runs now, so ScheduledFireTimeUtc is not the slot. GetBackfillSlot() is, and is null on a firing that is not a backfill:

public sealed class NightlyExportJob : IJob
{
    public ValueTask Execute(IJobExecutionContext context, CancellationToken cancellationToken = default)
    {
        // A backfilled firing runs now: the night it is for is its slot.
        DateTimeOffset night = context.GetBackfillSlot() ?? context.ScheduledFireTimeUtc ?? context.FireTimeUtc;
        return Export(night, cancellationToken);
    }

    private static ValueTask Export(DateTimeOffset night, CancellationToken cancellationToken) => ValueTask.CompletedTask;
}

Options

Refusals

Each is raised before anything is scheduled.

Running a range again

• A slot whose trigger is still stored, waiting, paused or firing, is skipped and counted in AlreadyScheduled.
• A slot whose trigger has fired is no longer stored, and is scheduled again.
• The slots are scheduled one call each. A failure or cancellation part way keeps those already scheduled; run the range again for the rest.

Over HTTP and in the dashboard

A mixed 4.2 and 4.3 cluster

The slots are plain one-shot simple triggers, so a 4.2 node fires them. Only the slot key is new: a job on a 4.2 node reads it from context.Trigger.JobDataMap as a string. A 4.2 HTTP host has no backfill route and answers it 404; an HttpScheduler then backfills through the routes 4.2 has, a request per slot.