Why does Apache Flink need Watermarks for Event Ti

Can someone explain Event timestamp and watermark properly. I understood it from docs, but it is not so clear. A real life example or layman definition will help. Also, if it is possible give an example ( Along with some code snippet which can explain it ).Thanks in advance

Here's an example that illustrates why we need watermarks, and how they work.

In this example we have a stream of timestamped events that arrive somewhat out of order, as shown below. The numbers shown are event-time timestamps that indicate when these events actually occurred. The first event to arrive happened at time 4, and it is followed by an event that happened earlier, at time 2, and so on:

··· 23 19 22 24 21 14 17 13 12 15 9 11 7 2 4 →

Now imagine that we are trying create a stream sorter. This is meant to be an application that processes each event from a stream as it arrives, and emits a new stream containing the same events, but ordered by their timestamps.

Some observations:

(1) The first element our stream sorter sees is the 4, but we can't just immediately release it as the first element of the sorted stream. It may have arrived out of order, and an earlier event might yet arrive. In fact, we have the benefit of some god-like knowledge of this stream's future, and we can see that our stream sorter should wait at least until the 2 arrives before producing any results.

Conclusion: Some buffering, and some delay, is necessary.

(2) If we do this wrong, we could end up waiting forever. First our application saw an event from time 4, and then an event from time 2. Will an event with a timestamp less than 2 ever arrive? Maybe. Maybe not. We could wait forever and never see a 1.

Conclusion: Eventually we have to be courageous and emit the 2 as the start of the sorted stream.

(3) What we need then is some sort of policy that defines when, for any given timestamped event, to stop waiting for the arrival of earlier events.

This is precisely what watermarks do — they define when to stop waiting for earlier events.

Event-time processing in Flink depends on watermark generators that insert special timestamped elements into the stream, called watermarks.

When should our stream sorter stop waiting, and push out the 2 to start the sorted stream? When a watermark arrives with a timestamp of 2, or greater.

(4) We can imagine different policies for deciding how to generate watermarks.

We know that each event arrives after some delay, and that these delays vary, so some events are delayed more than others. One simple approach is to assume that these delays are bounded by some maximum delay. Flink refers to this strategy as bounded-out-of-orderness watermarking. It's easy to imagine more complex approaches to watermarking, but for many applications a fixed delay works well enough.

If you want to build an application like a stream sorter, Flink's ProcessFunction is the right building block. It provides access to event-time timers (that is, callbacks that fire based on the arrival of watermarks), and has hooks for managing the state needed for buffering events until it's their turn to be sent downstream.