Question: I'm trying to implement the default window with the default trigger to evaluate the behavior but it's not emitting any result. 

According to Apache Beam: 

> 
> The default trigger for a PCollection is based on event time, and
>  emits the results of the window when the Beam’s watermark passes the
>  end of the window, and then fires each time late data arrives.
> 
> 
> 

---

> 
> If you are using both the default windowing configuration and
>  the default trigger, the default trigger emits exactly once, and late
>  data is discarded. This is because the default windowing configuration
>  has an allowed lateness value of 0.
> 
> 
> 

my code: 

```
Nb_items = lines | beam.CombineGlobally(beam.combiners.CountCombineFn()).without_defaults() \
                 | 'print' >> beam.ParDo(PrintFn())

```

It only emits the data if I set a trigger 

```
Nb_items = lines | 'window' >> beam.WindowInto(window.GlobalWindows(),
            trigger=trigger.AfterProcessingTime(10),
            accumulation_mode=trigger.AccumulationMode.DISCARDING) \
        | 'CountGlobally' >> beam.CombineGlobally(beam.combiners.CountCombineFn()).without_defaults() \
        | 'print' >> beam.ParDo(PrintFn())

```

How can I observe the default behavior without setting a trigger? 

Is the problem in the combine transform? 

> 
> If your input PCollection
>  uses the default global windowing, the default behavior is to return a
>  PCollection containing one item. That item’s value comes from the
>  accumulator in the combine function that you specified when applying
>  Combine
> 
> 
>

Answer: 
The current issue is that the watermark never reaches the end of the `GlobalWindow`. To have a default trigger, you can use any other window where the watermark can reach the end, e.g.: `'window' >> beam.WindowInto(window.FixedWindows(10))`

As Guillaume rightly asks, if you're running on Batch, triggers are basically ignored.
See Sources Below:

<https://github.com/apache/beam/blob/828b897a2439437d483b1bd7f2a04871f077bde0/examples/java/src/main/java/org/apache/beam/examples/complete/game/LeaderBoard.java#L274>

For more information regarding Google Cloud Dataflow

<https://stackoverflow.com/a/54151029/12149235>