An example of Flink Windows and Watermarks

This is a simple example of how Flink deals with Windows and Watermarks. The magic trick here is that there isn't anything programmed to run, you'll have to input all elements by yourself -- and then you'll see what happens.

Running

For this, you'll need two terminal windows: One for your input and another to see the result of Flink.

On the "input" window, run nc -l 60000; this will start NetCat on port 60000 on your machine.

On the "output" window, run sbt "run --hostname 127.0.0.1 --port 60000"; this will compile the project and start Flink minicluster.

You can pick any port you want, as long as you run both commands with the same value.

Event Types

SocketFlink will output (on the "output" window) some internal events. You can get more information about those by reading the source code; each event type is based on a Flink function (an operation done in the pipeline) and before each function there is a deeper explanation on what the function does.

The list of event types is:

CREATE

SocketFlink interpreted the value you entered and created an event to be processed.

<dt>WATERMARK</dt>
<dd>The just created event forced the watermark to be moved. In this example, the watermark
will move every time it sees an event that is most recent than any preivous one.</dd>

<dt>REDUCE</dt>
<dd>SocketFlink will group duplicated words in a single element. When this happens, a
REDUCE event will appear.</dd>

<dt>FIRING</dt>
<dd>Flink decided it was time for the event to be expelled from a window.</dd>

<dt>SINK</dt>
<dd>The fired element in the previous event type reached the sink.</dd>

An example run

Input	Output	Explanation
1 window1	CREATE - window1 at 1s seen 1 times WATERMARK - moved to -9	You entered 1 window1; this means "Hey SocketFlink, at the second 1, the word 'window1' appears"; SocketFlink will, then, create the word window1 and, because it is the most recent event, will move the watermark (it's 10 second past the most recent event, so second 1 minus 10 seconds equals second -9)

2 window1 2 window1 20 window1 29 window1 31 window2 45 window2 21 window1

Output

CREATE - window1 at 2s seen 1 times WATERMARK - moved to -8 REDUCE - window1 at 1s seen 1 times + window1 at 2s seen 1 times, now window1 at 1s seen 2 times CREATE - window1 at 2s seen 1 times REDUCE - window1 at 1s seen 2 times + window1 at 2s seen 1 times, now window1 at 1s seen 3 times CREATE - window1 at 20s seen 1 times WATERMARK - moved to 10 REDUCE - window1 at 1s seen 3 times + window1 at 20s seen 1 times, now window1 at 1s seen 4 times CREATE - window1 at 29s seen 1 times WATERMARK - moved to 19 REDUCE - window1 at 1s seen 4 times + window1 at 29s seen 1 times, now window1 at 1s seen 5 times CREATE - window2 at 31s seen 1 times WATERMARK - moved to 21 CREATE - window2 at 45s seen 1 times WATERMARK - moved to 35 REDUCE - window2 at 31s seen 1 times + window2 at 45s seen 1 times, now window2 at 31s seen 2 times FIRING - window1 at 1s seen 5 times to 0, now window1 at 0s seen 5 times SINK - window1 at 0s seen 5 times CREATE - window1 at 21s seen 1 times REDUCE - window1 at 1s seen 5 times + window1 at 21s seen 1 times, now window1 at 1s seen 6 times FIRING - window1 at 1s seen 6 times to 0, now window1 at 0s seen 6 times SINK - window1 at 0s seen 6 times FIRING - window2 at 31s seen 2 times to 30, now window2 at 30s seen 2 times SINK - window2 at 30s seen 2 times