The outbox pattern and the crash in between

Sat, 08 Aug 2026 00:00:00 +0000

When you build a calendar application, creating a booking is rarely just a database operation.

Imagine this simple flow:

Bob creates a calendar booking with Alice.

When the booking is created, we need to do at least two things:

Save the booking in the database.
Notify Alice that she has been invited.

At first glance, this sounds straightforward.

But once these operations cross system boundaries, things get surprisingly interesting.

What happens if the booking is saved successfully, but the application crashes before the email is sent?

Every timezone bug is really a calendar bug

Wed, 15 Jul 2026 00:00:00 +0000

Scheduling bugs cluster around two moments in the year, and they are the same two moments for everyone: the second Sunday in March and the last Sunday in October — give or take, depending on which hemisphere and which government.

The assumption that breaks

The bug is almost always this: storing a meeting as a wall-clock time plus an offset rather than as an instant plus a timezone.

{ "startsAt": "2026-10-24T09:00:00+01:00", "recurrence": "WEEKLY" }

That looks unambiguous, and for a single meeting it is. But the recurrence rule has to answer “when is the next one?” — and on 25 October, +01:00 stops being the correct offset for Europe/London. Naively adding seven days gives you 2026-10-31T09:00:00+01:00, an instant that renders as 8:00am to the host. Their weekly standup silently moves an hour earlier, and only for the half of the team in a country that just changed its clocks.

Engineering on BunnyCal Blog

The outbox pattern and the crash in between

Every timezone bug is really a calendar bug

The assumption that breaks