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Monday, January 19, 2009

A Transparent New Year

Posted by Louis Lovas

It's been a while since I've put a few thoughts in print on these pages on event processing, but that's not to say I've not been busy. We've wrapped up quite a successful year in the Apama division within Progress software. For this year, 2009 we've got quite a number of new features and capabilities in store for the Apama CEP platform. We will be extending the breadth and depth our event processing language (EPL) to meet the challenges we've seen in the market and a vision we have for the future. Of course you will see announcements in our standard marketing press releases but I (and others) will explore the technical merits of those new features in these blog pages in much more detail. Something we've not done that much of in the past. There are many historical reasons for that not really worth explaining. Suffice to say our intention is to be more transparent in the coming year.

To kick that off, it's worth starting a bit of a tutorial on the Apama EPL, a language we call MonitorScript.  I'll begin with the basics here and in subsequent blogs build upon these main concepts, providing insight into the power and flexibility of our EPL. And as we release new extensions and capabilities it will be easier to explain the benefits of those new features. So without further ado, here is the first installment.

First a few basic concepts...

  • Apama MonitorScript is an imperative programming language with a handful of declarative statements. This is an important consideration and one we highlight as a distinction in our platform against competitive platforms that are based on a declarative programming model. The imperative model provides a more natural style of development similar to traditional languages like java and C++.
  • The language is executed at runtime by our CEP engine called the Correlator.  

Second a few basic terms...

  • A monitor defines the outer most block scope, similar to a class in java or C++. It is the basic building block of Apama applications. A typical application is made up of many monitors. As you might imagine monitors need to interact with one another, I'll explore that capability in a later blog.
  • A event defines a well ordered structure of  data. The EPTS Glossary definition has a handful of great examples of Events
  • A listener, defined by the on statement, declares or registers interest in an event or event pattern. In a large application it's the Correlator runtime engine that will typically process 1,000's or even 10,000's of registered event patterns. In our example below we just have one.
  • An action defines a method or function similar to java or C++. 
  • The onload() action is a reserved name (along with with a few others) that is analogous to main() in a java program. 
To put it all together... "A monitor typically listens for events and takes one or more actions when an event pattern is triggered."

The language sports a number of capabilities that will be familiar to anyone schooled in java, C++ or any traditional imperative programming language. I won't bore with all the nuances of data types and such obvious basics, those are well articulated in our product documentation and customer training courses. I will however, focus on the unique paradigm of event processing.

Apama MonitorScript, a basic monitor.

event StockTrade {
  string symbol;
  float price;
  integer quantity;

monitor StockTradeWatch {

  StockTrade Trade;

    action onload {
      on all StockTrade():Trade processTick;

    action processTick {
      log "StockTrade event received" +
      " Symbol = " + Trade.symbol +
      " Price = "  + Trade.price.toString() +
      " Quantity = " + Trade.quantity.toString() at INFO;

This monitor called StockTradeWatch defines an event of type StockTrade. Events can originate from many sources in the Apama platform, the most obvious would be an adapter connected to a source of streaming data (i.e. stock trades as example shows), but they can come from files, databases, other monitors, even monitors in other Correlators.

The onload action declares a listener for events of type StockTrade (i.e. on all StockTrade). When the monitor StockTradeWatch receives StockTrade events, the action processTick is invoked. As you can see in this example we simply log a message to indicate that it occurred. The obvious intent is that this monitor will receive a continuous stream of StockTrade events, each one will be processed in-turn by the processTick action.  One can be more selective in the event pattern with a listener, such as on all StockTrade(symbol="IBM"). I will explore the details of event patterns and complex listeners later on.

As I mentioned, I've started with a simple example that shows the basics of the Apama EPL, MonitorScript. It demonstrates the simplicity by which one can declare interest in a particular event pattern, receive matching events and act on them in your application (i.e. action).

 In subsequent installments I will demonstrate more complex features highlighting the power of the language.  That's all for now.

You can find the second installment here.


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