How to use window.frequent? - wso2

Question

Anybody can give me a example about how to use window.frequent?
For example，
I write a test,
"define stream cseEventStream (symbol string, price float, time long);" +
"" +
"#info(name = 'query1') " +
"from cseEventStream[700 > price]#window.frequent(3, symbol) " +
"select symbol, price, time " +
"insert expired events into outputStream;";
But from the outputStream, i can't find out the rule.
Thanks.

Answer 1

In this particular query 'window.frequent(3, symbol)' will make the query to find the most frequent 3 symbols(or 3 symbols that has the highest number of occurrences). But, when you insert events to outputStream you have inserted only expired events. So that, as the end result this query will output events that are expired from the frequent window.
In a frequent window, expired events are events that are not belonging to a frequent group anymore. In this case events which are the symbol is not among 3 symbols that has the highest number of occurrences.
for an example if you send the following sequence of events,
{"symbolA", 71.36f, 100}
{"symbolB", 72.36f, 100}
{"symbolB", 74.36f, 100}
{"symbolC", 73.36f, 100}
{"symbolC", 76.36f, 100}
{"symbolD", 76.36f, 100}
{"symbolD", 76.36f, 100}
The query will output {"symbolA", 71.36f, 100}.
When you send the events with 'symbolD'. SymbolA will not be among the top3 symbols with highest number of occurrences anymore so that event with symbolA is expired and {"symbolA", 71.36f, 100} is emitted.

Answer 2

For every event which has a price > 700, this window will retain most frequent 3 items based on symbol and since the output type is 'expired events' you will only receive output once an event loose it's position as a frequent event.
Ex: for frequent window of size 2
Input
WSO2 1000 1
WSO2 1000 2
ABC 700 3
XYZ 800 4
Output
ABC 700 3
ABC event was in the frequent window and was expired upon receiving of XYZ event. If you use default output which is 'current events' it will output all incoming events which are selected as frequent events and put into the window.
Implementation is based on Misra-Gries counting algorithm.
Documentation : https://docs.wso2.com/display/CEP400/Inbuilt+Windows#InbuiltWindows-frequent
Test cases : https://github.com/wso2/siddhi/blob/master/modules/siddhi-core/src/test/java/org/wso2/siddhi/core/query/window/FrequentWindowTestCase.java