Processing Streaming Events

Forrester defines Streaming Analytics as follows:

The stream processing capabilities of WSO2 SP allow you to capture high volume data flows and process them in real time, and present results in a streaming manner. 

Following are a few stream processing capabilities of WSO2 SP.

Functions

The following functions shipped with Siddhi, consume zero, one or more parameters from streaming events and produce a desired value as output. These functions are executed per event. For more information on Siddhi functions, please refer to Siddhi Query Guide - Functions. More functions are made available as Siddhi Extensions.

• eventTimestamp - Returns the timestamp of the processed event

  eventTimeStamp example

  from fooStream
  select symbol as name, eventTimestamp() as eventTimestamp
  insert into barStream

• UUID - Generates a UUID (Universally Unique Identifier)

  UUID example

  from fooStream
  select UUID() as messageID, messageDetails
  insert into barStream;

• default - Checks if the 'attribute' parameter is null and if so returns the value of the 'default' parameter. The function is given as default(<attribute>, <default value>)

  default example

  from fooStream
  select default(temp, 0.0) as temp, roomNum
  insert into barStream;

• cast - Converts the first parameter according to the cast-to parameter. Incompatible arguments cause Class Cast exceptions if further processed.

  cast example

  from fooStream
  select symbol as name, cast(temp, 'double') as temp
  insert into barStream;

• convert - Converts the first input parameter according to the convert-to parameter

  convert example

  from fooStream
  select convert(temp, 'double') as temp
  insert into barStream;

• ifThenElse - Evaluates the 'condition' parameter and returns value of the 'if.expression' parameter if the condition is true, or returns value of the 'else.expression' parameter if the condition is false. The function is given as ifThenElse(<condition>, <if.expression>, <else.expression>)

  ifThenElse example

  from fooStream
  ifThenElse(sensorValue>35,'High','Low')
  insert into barStream;

• minimum - Returns the minimum value of the input parameters

  minimum example

  from fooStream
  select minimum(price1, price2, price3) as minPrice
  insert into barStream;

• maximum - Returns the maximum value of the input parameters. This function could be used similar to how 'minimum' function is used in a query.

• coalesce - Returns the value of the first input parameter that is not null. All input parameters have to be of the same type.

  coalesce example

  from fooStream
  select coalesce('123', null, '789') as value
  insert into barStream;

• instanceOfBoolean - Returns 'true' if the input is a instance of Boolean. Otherwise returns 'false'. 

  instanceOfBoolean example

  from fooStream
  select instanceOfBoolean(switchState) as state
  insert into barStream;

• instanceOfDouble - Returns 'true'  if the input is a instance of Double. Otherwise returns 'false'. This function could be used similar to how 'instanceOfBoolean' function is used in a query.
• instanceOfFloat - Returns 'true'  if the input is a instance of Float. Otherwise returns 'false'. This function could be used similar to how 'instanceOfBoolean' function is used in a query.
• instanceOfInteger - Returns 'true'  if the input is a instance of Integer. Otherwise returns 'false'. This function could be used similar to how 'instanceOfBoolean' function is used in a query.
• instanceOfLong - Returns 'true'  if the input is a instance of Long. Otherwise returns 'false'. This function could be used similar to how 'instanceOfBoolean' function is used in a query.
• instanceOfString - Returns 'true'  if the input is a instance of String. Otherwise returns 'false'. This function could be used similar to how 'instanceOfBoolean' function is used in a query.

Filters

Filters are applied to input data received in streams to filter information based on given conditions. For more information, see Siddhi Query Guide - Filters.

e.g., Filtering cash withdrawals from an ATM machine where the withdrawal amount is greater tha $100, and the withdrawal data is between 01/12/2017-15/12/2017.

Windows

Windows allow you to capture a subset of events based on a duration or number of events criterion, from an input stream for calculation. Each input stream can only have a maximum of one window.

Criterion - Time windows vs length windows

The subset of events can be captured based on one of the following.

• Time: This involves capturing all the events that arrive during a specific time interval (e.g., writing a query that is applicable to events that occur during a period of 10 minutes).
• Length: This involves capturing a subset of events based on the number of events (e.g., writing a query applicable to each group that consists of 10 events).

Method of processing - Sliding windows vs batch windows

Consider 10 events that have arrived in a stream.

When a sliding length window is included in a Siddhi query, the following event groups are identified:

• Events 1-5
• Events 2-6
• Events 3-7
• Events 4-8
• Events 5-9
• Events 6-10

When a batch window is included in a Siddhi query, the folowing event groups are identified:

• Events 1-5
• Events 6-10

This window feature differs from the Defined Window��concept elaborated in Siddhi Application Overview due to this being specific to a single query only. If a window is to be shared among queries, the Defined Window must be used

For more information about windows, see Siddhi Query Guide - Window.

Aggregate Functions

Aggregation functions allow executing aggregations such as sum, avg, min, etc. on a set of events grouped by a window. If a window is not defined, the aggregation(s) would be calculated by considering all the events arriving at a stream. 

Consider the following events arriving at a stream, where the prices vary from one another.

• Event 1: price = 10.00
• Event 2: price = 20.00
• Event 3: price = 30.00
• Event 4: price = 40.00
• Event 5: price = 50.00

Consider the following two queries, where sum of price is calculated based on a length window of 2, and without a window respectively

from fooStream#window.length(2)
select sum(price) as totalPrice
insert into barStream;

from fooStream
select sum(price) as totalPrice
insert into barStream;

The following output would be generated for Query 1.

• totalPrice = 10.00
• totalPrice = 30.00
• totalPrice = 50.00
• totalPrice = 70.00
• totalPrice = 90.00

The following output would be generated for Query 2.

• totalPrice = 10.00
• totalPrice = 30.00
• totalPrice = 60.00
• totalPrice = 100.00
• totalPrice = 150.00

For more information on aggregate function, please refer to Siddhi Query Guide - Aggregate Functions.

Group By

With the group by  functionality, events could be grouped based on a certain attribute, when performing aggregations. 

Consider the following events, which have a symbol attribute and a price attribute.

• Event 1: symbol = wso2, price = 10.00
• Event 2: symbol = wso2, price = 20.00
• Event 3: symbol = abc, price = 30.00
• Event 4:   symbol = abc,  price = 40.00
• Event 5:   symbol = abc, price = 50.00

When the sum aggregation is calculated for a window of length 3, after grouping by symbol, the given output is generated

from fooStream#window.length(3)
select symbol, sum(price) as totalPrice
group by symbol
insert into barStream;

Output:

• symbol = wso2, totalPrice = 10.00
• symbol = wso2, totalPrice = 30.00
• symbol = abc, totalPrice = 30.00
• symbol = abc, totalPrice = 70.00
• symbol = abc, totalPrice = 120.00

For more information on group by, please refer to Siddhi Query Guide - Group By.

Having

Having allows to filter events after processing the select statement.

This is useful if the filtering is based on some value derived by applying a function/ aggregation. For example, if you want to find all the events where maximum production total across 3 days is less than 1000 units, such filtering could be achieved with a query as follwos

from fooStream
select item, maximum(productionOnDay1, productionOnDay2, productionOnDay3) as maxProduction
having maxProduction < 1000
insert into barStream;

For more information on having clause, please refer to Siddhi Query Guide - Having.

Join

Join is an important feature of Siddhi, which allows combining pair of streams, pair of windows, stream with window, stream/ window with a table and stream/window with an aggregation

The join logic can be defined with 'on' condition as well, which restricts the events combined in a join.

For example, assume that we need to combine a transaction stream with a table containing blacklisted credit card numbers, to identify fraudulent transactions. Following query helps achieve such a requirement.

from transactionStream as t join blacklistedCardsTable as b
on t.cardNumber = b.cardNumber
select t.cardNumber, t.transactionDetails, b.fraudDescription
insert into suspiciousTransactionStream;

For more information on join queries, please refer to Siddhi Query Guide - Join.

Output Rate Limiting

Output rate limiting allows queries to output events periodically based on a specified condition. This helps to limit continuously sending events as output.

For more information on output rate limiting, please refer to Siddhi Query Guide - Output Rate Limiting 

Partitioning

Partitioning in Siddhi allows to logically seperate events arriving at a stream, and to process them separately, in parallel.

For example, assume that the total number of transactions per company needs to be monitored at a stock exchange. However, if all the transactions are arriving at a single stream, we would need to logically seperate them based on the company symbol. The following example depicts how this can be achieved with Siddhi partitioning.

partition with (symbol of stockStream )
begin
    from stockStream
    select symbol, count() as transactionCount
    insert into transactionsPerSymbol;
end;

Partitioning can be done based on an attribute value as above, or based on a condition. For more information on partitioning, please refer to Siddhi Query Guide - Partitioning 

Trigger

Triggers could be used to get events generated by the system itself, based on some time duration.

An example for a trigger definition is as follows.

define trigger FiveMinTriggerStream at every 5 sec;

This would generate an event every 5 seconds. The generated event would contain an attribute of type 'Long' named 'triggered_time', reflecting the time at which event was triggered in milliseconds (epoch time).

Trigger could be defined as a time interval, a cron job or to generate an event when Siddhi is started. For more information on triggers, please refer to Siddhi Query Guide - Trigger

Script

Scripts allow to define function operations in a different programming language. An example is as follows.

define function concatFn[javascript] return string {
    var str1 = data[0];
    var str2 = data[1];
    var str3 = data[2];
    var response = str1 + str2 + str3;
    return response;
};

define stream TempStream(deviceID long, roomNo int, temp double);

from TempStream
select concatFn(roomNo,'-',deviceID) as id, temp 
insert into DeviceTempStream;

For more information on scripts, please refer to Siddhi Query Guide - Script