Polling vs. Event: Who Asks and Who Announces

Once we understand that an Event is a declaration of a fact, the next step is to understand a more basic question:

how do you even find out that something happened?

There are two fundamental approaches:

ask over and over, or have someone tell you when it happens.

These are Polling and Event.

Polling: Asking Whether Something Changed

Polling is a model where a component in the system: checks over and over whether there’s a change.

For example:

“was a new order created?” “was the file updated?” “is the queue non-empty?”

The check happens:

• At fixed intervals
• Regardless of whether anything actually happened

Polling is seen as “outdated” or “inefficient” - but that’s only a partial view.

Why Polling Exists at All

Polling is simple, predictable, and clear.

Its advantages:

• No intermediary
• No dependency on an external actor
• No need to manage a complex information flow

The component asks - and gets an answer.

Event: Being Told When It Happens

In the Event-driven model, the picture flips.

The component doesn’t ask. It assumes: “when there’s a relevant fact - I’ll be told about it.”

Instead of repeated checks:

• A fact occurs
• An event is created
• And the event is distributed to whoever is listening

So Why Isn’t Event Always “Faster”

Here’s where the mistaken intuition comes in: if you’re not asking all the time - it must be faster.

But Event saves on checks, not on time.

For an event to arrive:

• It needs to be created
• Sent
• Mediated
• And received

Every such step adds delay.

Polling asks early. Event responds later - but more intelligently.

Responsiveness vs. Latency

Polling is wasteful - but predictable. Event is efficient - but adds mediation.

So the real difference isn’t: fast vs. slow,

but:

Polling gives a predictable response at a fixed pace. Event gives a conditional response with no guarantee it will arrive, at a variable arrival time.

It’s a choice between: control over time and savings on work.

An Analogy

Imagine two people:

One: checks every minute whether the door opened.

The other: asks to be called when it happens.

The second one doesn’t get tired - but if someone is slow to notify them, they’ll find out later.

The Bottom Line

Polling and Event aren’t good or bad. They represent different philosophies:

Polling preserves control over time: the system checks repeatedly until there’s an answer. Event gives up control over time: the system doesn’t wait for closure, it moves on and handles what happened when it arrives.

Polling says: it matters to me to know exactly what the state is, even if it costs me in performance. Event says: it matters to me not to get stuck, even if I don’t know when everything will resolve.