Definition
Polling is a way for a device or program to check for news or new data by asking another party again and again at regular time intervals. The other side never has to speak up first, because the asker keeps sending its own periodic questions to stay up to date.
"Are we there yet? Now? How about now?"
Picture a long road trip. A kid in the back seat asks, "Are we there yet?" every minute. The driver keeps saying, "Not yet," and only when the car finally arrives does the answer change to "Yes, we're here."
Programs on your computer and phone often work the same way. Your phone app sends a question to a server, asking whether a new message or notification has arrived, again and again on a fixed schedule. It sets a steady interval, like 1 or 3 seconds, and keeps asking, "Anything new?"
This technique, where a device or program regularly checks the status of something else, is called "polling." It's simple to understand and easy to build, so it has been a basic building block of electronics and software since the early days.
Simple, but it burns energy
The big plus of polling is how simple it is to build. Just set a rule to run a "check the status" command every so often, and it works almost anywhere.
But it keeps asking the same question even when nothing has changed, so it wastes computing power and electricity. Ask a hundred times, and ninety-nine answers are a disappointing "Nothing's happening." Along the way, the CPU and the network get worn out for no good reason.
There's another catch: you can't learn about news right away. If something important happens in the gap between two questions, there's no way to know until the next one goes out. Shorten the interval too much, and your battery drains at lightning speed. Stretch it out, and everything feels slow to respond.
To be more precise: interrupts and push
To be more precise, modern computer systems cut down this waste by also using a "tell me only when needed" approach. Instead of you knocking on the door over and over, the other side rings the doorbell when there's news.
Inside a computer, the CPU often doesn't watch every part one by one. Instead, when data arrives at a network card or a storage drive finishes reading, the part sends a signal to the CPU. This is called an interrupt. That said, USB keyboards and mice are a case of polling: the computer's USB side asks the device for input hundreds to thousands of times per second.
Phone apps and web browsers work like this too. Rather than blindly poking a server, a chat app keeps a connection open with WebSockets or real-time push notifications. That way, your device saves battery and still gets news almost instantly.
Common misconceptions
Polling is inefficient, so it isn't used at all in modern computers or on the web.
It's still used wherever simplicity and reliability matter. For example, when reading the subtle state of a hardware sensor, or in simple embedded devices and batch jobs where building a complex communication channel is hard, polling can be the safest and clearest solution.
Where you meet it
A restaurant reservation app sends a refresh request to the server every 10 seconds to see whether it's your turn yet.
A computer periodically sends a signal to a printer to check whether it has finished printing.
In one sentence
Polling is a communication method where you check for news by asking directly, again and again, at regular intervals.