Session¶

Overview¶

Session window is a type of Unaligned window where the window’s end time keeps moving until there is no data for a given time duration. Unlike fixed and sliding windows, session windows do not overlap, nor do they have a set start and end time. They can be used to group data based on activity.

plot

vertices:
  - name: my-udf
    udf:
      groupBy:
        window:
          session:
            timeout: duration

NOTE: A duration string is a possibly signed sequence of decimal numbers, each with optional fraction and a unit suffix, such as "300ms", "1.5h" or "2h45m". Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".

timeout¶

The timeout is the duration of inactivity (no data flowing in for the particular key) after which the session is considered to be closed.

Example¶

To create a session window of timeout 1 minute, we can use the following snippet.

vertices:
  - name: my-udf
    udf:
      groupBy:
        window:
          session:
            timeout: 60s

The yaml snippet above contains an example spec of a reduce vertex that uses session window aggregation. As we can see, the timeout of the window is 60s. This means we no data arrives for a particular key for 60 seconds, we will mark it as closed.

Let's say, time.now() in the pipeline is 2031-09-29T18:46:30Z as the current time, and we have a session gap of 30s. If we receive events in this pattern:

Event-1 at 2031-09-29T18:45:40Z
Event-2 at 2031-09-29T18:45:55Z   # Notice the 15 sec interval from Event-1, still within session gap
Event-3 at 2031-09-29T18:46:20Z   # Notice the 25 sec interval from Event-2, still within session gap
Event-4 at 2031-09-29T18:46:55Z   # Notice the 35 sec interval from Event-3, beyond the session gap
Event-5 at 2031-09-29T18:47:10Z   # Notice the 15 sec interval from Event-4, within the new session gap

This would lead to two session windows as follows:

[2031-09-29T18:45:40Z, 2031-09-29T18:46:20Z)   # includes Event-1, Event-2 and Event-3
[2031-09-29T18:46:55Z, 2031-09-29T18:47:10Z)   # includes Event-4 and Event-5

In this example, the start time is inclusive and the end time is exclusive. Event-1, Event-2, and Event-3 fall within the first window, and this window closes 30 seconds after Event-3 at 2031-09-29T18:46:50Z. Event-4 arrives 5 seconds later, meaning it's beyond the session gap of the previous window, initiating a new window. The second window includes Event-4 and Event-5, and it closes 30 seconds after Event-5 at 2031-09-29T18:47:40Z, if no further events arrive for the key until the timeout.

Note: Streaming mode is by default enabled for session windows.

Check the links below to see the UDF examples for different languages. Currently, we have the SDK support for Golang and Java.

Golang
Java