Esper

Introduction to Esper

Relational databases or message-based systems such as JMS make it really hard to deal with temporal data and real-time queries. Indeed, databases require explicit querying to return meaningful data and are not suited to push data as it changes. JMS systems are stateless and require the developer to implement the temporal and aggregation logic himself. By contrast, the Esper engine provides a higher abstraction and intelligence and can be thought of as a database turned upside-down: instead of storing the data and running queries against stored data, Esper allows applications to store queries and run the data through. Response from the Esper engine is real-time when conditions occur that match user defined queries. The execution model is thus continuous rather than only when a query is submitted.
Such concepts are a key foundation of EDA, and have been under active research in more than the last 10 years. Awareness of the importance of such systems in real-world architectures has started to emerge only recently.
In Esper, a tailored EPL allows registering queries in the engine. A listener class--which is basically a POJO--will then be called by the engine when the EPL condition is matched as events flow in. The EPL enables to express complex matching conditions that include temporal windows, joining of different event streams, as well as filtering, aggregation, and sorting. Esper statements can also be combined together with "followed by" conditions thus deriving complex events from more simple events. Events can be represented as JavaBean classes, legacy Java classes, XML document or java.util.Map, which promotes reuse of existing systems acting as messages publishers.
A trivial yet meaningful example is as follow: assume a trader wants to buy Google stock as soon as the price goes below some floor value-- not when looking at each tick but when the computation is done over a sliding time window--say of 30 seconds. Given a StockTick event bean with a price and symbol property and the EPL "select avg(price) from StockTick.win:time(30 sec) where symbol='GOOG'", a listener POJO would get notified as ticks come in to trigger the buy order.

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