Last event from an Esper window that has time and length

Question

I am trying to get the last event from a time_length_batch but I am at a loss WRT how to do this.

For example ... there is an Inventory Event and would like to get the latest update from each product that is available. The following query mostly provides what is needed:

SELECT product, inventory FROM
InventoryEvents.std:groupwin(name).win:time_length_batch(1 min, 2);

except that within the window I get every event in the last min or 2 events. I am interested in just the last event for that window. (Using 2 to easily test the results.)

As a result, in the UpdateListener, I need to output the last event from the array. But since I have no need for these events and would ultimately like to switch to a subscriber I am looking to get just the last event. In addition, subscribers are much more performant so instead of this:

public static class EveryListener implements UpdateListener {

    public void update(EventBean[] newData, EventBean[] oldData) {

      out.println("* Window received: " + " " +
            newData.length + " " + newData[newData.length - 1].getUnderlying());
    }

    }

Just

 public void update(String product, int inventory) {
        out.println("** Explicit event received: " + " " + product + " " + inventory);
    }

but the results are ...

* Window received:  2 {product=A, inventory=-10}
** Explicit event received:  A 20
** Explicit event received:  A -10

I have tried

SELECT product, inventory FROM
InventoryEvents.std:groupwin(name).win:time_length_batch(1 min, 1000)
  .std:win:lastevent();

but that doesn't give the expected results.

How do I get just the last event of a grouped window?

Answer 1

There is the "last" aggregation function. As it is an aggregation function the group-by clauses must specify the grouping.

So the query is....

SELECT last(inventory) FROM
InventoryEvents.std:groupwin(name).win:time_length_batch(1 min, 2) 
group by name

There is also so "prev" function that may give you the result you are looking for and which does not need the "group by".