In the last blog, we looked at the two basic types of Windows in Flink - Sliding and Tumbling windows. In the blog, I will explain you two important concepts that can be used in Flink - Triggers and Evictors.

# Triggers

Assume we have a sliding window (of width 15 seconds, which slides every 10 seconds) and we are collecting items in the window during streaming. A trigger can be used to tell Flink when to evaluate the function on the items in the window. For example, if you want the function to be evaluated on every 5 items that you receive within the window that we defined above, we can use trigger(CountTrigger.of(5)).

Let us consider few scenarios to understand trigger better.

Note 1: I am assuming that we are receiving the same word in the stream. This is done to make the explanation simple. Since there is a keyBy(0) after map, each word will belong to separate logical window grouped by the word.
Note 2: The sliding window used in this example is based on Processing time. Processing time is the time at which an event is processed in the system compared to EventTime which is the time at which event was created. I will be explaining these concepts in the upcoming blogs.

Update : Read about the concept of ProcesingTime and EventTime from this blog.
scenario 1:

This is the basic case, where window 1 received 5 items within its window-width of 15 seconds. Last two items have overlap with window 2, hence it will be present in both windows 1 and 2. But window 2 has only 2 items which is less than the trigger count 5. Whereas window 1 has received 5 items within its window-width and hence the function sum() will be triggered.

scenario 2:

In this case, the items arrived in such a way that, both windows 1 and 2 received 5 items in the region where it overlaps. Hence, both windows will be triggered at the same time.

scenario 3:

This is similar to scenario 2, except that window 1 received 10 items, 5 of which are overlapping with window 2. What do you think will happen in such scenario?

# Evictors

An evictor is used to remove some items from the window before the window function is called. Let us add an evictor to our trigger example.
Update: After FLIP-4, an evictor can evict elements before or after the window function is called.

Here, the evictor is CountEvictor of 3, i.e., it will evict all the items except 3 from the window once the trigger is fired. e.g., Consider the scenario 1 of trigger example and assume we added a CountEvictor of 3 to it.

The function sum will be applied only to the 3 items which are left in the window after eviction.

That concludes this post, you can find the code used in this article in my GitHub. Thanks for reading!