Real-Time vs Polling Calendar Sync: What's the Difference?
If you've ever subscribed to a calendar and wondered why a new event took half a day to show up, you've met polling. And if you've used a sync that updates almost as fast as you can edit, you've met real-time. They're solving the same problem with opposite philosophies, and the difference decides whether the sync is something you can actually rely on.
This page lays the two side by side - how each works, and where each one helps or hurts.
Polling: check on a timer
Polling means asking, on a schedule, "has anything changed?" A polling sync wakes up every few minutes or hours, downloads the calendars, compares them to what it saw last time, and applies any differences. The familiar ICS subscription built into Google and Outlook is a polling approach: your app re-fetches the calendar file on its own (often slow) timer, and it's read-only.
The trade-offs:
- Latency equals the interval. If it polls every four hours, a change can take up to four hours to appear. Shorten the interval and you hammer the provider with requests; lengthen it and the data goes stale.
- Wasteful. The vast majority of polls find nothing new but still download everything to check.
- Often one-way. ICS subscriptions in particular are read-only - you can't edit across the link.
Real-time: react to a notification
Real-time sync flips the model. Instead of asking, it waits to be told. The provider (Google or Microsoft) sends a notification - a webhook - the instant a calendar changes, and the service reacts immediately, fetching just that change and writing it to the other calendar.
The trade-offs go the other way:
- Low latency, always. Changes propagate in about a minute regardless of how often they happen, because there's no interval to wait for.
- Efficient. The service fetches only the change it was notified about, not the whole calendar, every time.
- Naturally two-way. Both providers can notify, so changes flow in either direction.
Side by side
| Polling / ICS subscription | Real-time (webhook) sync | |
|---|---|---|
| How it learns of changes | Asks on a fixed timer | Provider notifies it instantly |
| Typical latency | Minutes to hours | About a minute |
| Data moved | Whole calendars, repeatedly | Only the specific change |
| Direction | Often one-way / read-only | Two-way |
| Reliable for double-booking? | Risky - view can be stale | Yes - stale window is small |
When polling is actually fine
Polling isn't evil; it's just slow. If all you want is to glance at another calendar inside your main one and a few hours of lag won't cause harm, an ICS subscription is zero-setup and free. The trouble starts the moment you depend on the synced view to be current - to prevent a double-booking, to reschedule from whichever app is open, to keep a scheduling link honest. There, polling's lag turns into real missed appointments.
Why real-time is the better default
For anything you act on, real-time wins on every axis that matters: speed, two-way flow, efficiency, and - because it moves only the changed data - a lighter touch on your information. That last point connects to privacy: less data moving around, over official APIs you control, as covered in is calendar sync safe and private?.
Calendar Family uses the real-time, webhook-driven approach end to end. To see the mechanism up close, read how calendar webhook sync works; for the bigger picture, how real-time calendar sync works. Calendar Family is free.