Logic programming, as supported by inference engines, have been recently extended to support the development of Complex Event Processing (CEP) applications. One such an example is the product Drools Fusion.
Let’s investigate logic programming extensions needed for CEP, by grouping them into functional groups:
1. Event definition
The first logical step in supporting CEP is to be able to annotate an object as an event. If one considers the definition of an event as an “observable thing that happens”, a natural mapping in logical programming is a fact; hence events can be modeled as types of facts, which have additional metadata, such as expiration and timestamp. The latter is crucial to be able to support temporal constraints, as we shall see later.
Finally, it is important that the inference engine understands the concept of a clock, so that it can determine what is the current time. This is particularly important when supporting the idea of non-events.
2. Input handling
CEP applications are flooded with events; hence there is a need to limit the events that make it into the working memory of the inference engines.
Drools fusion provides two facilities for doing this, the ability to partition the input set, and the definition of windows of events.
To partition the input set, the developer can define entry points, which function as a named channel, providing an entity that can receive and send events. This is done with the following syntax:
Event from entry-point Channel-Name
For example, the following clause informs the engine that we are interested on an event named Stock coming from the channel Exchange-Market:
Stock() from entry-point “Exchange-Market”
Generally, the semantic of a channel is similar to that of a JMS queue, that is, an event is removed from the channel after a client has consumed it. However, due to the nature of inference engines, this is not the case for entry-points; the event continues to exist until it has been retracted from the working memory, either explicitly by the developer, or implicitly through some other facility, such as the usage of the @expires metadata associated to the event definition, or through the usage of the window function, as we shall see next.
Drools fusion supports the definition of time-based and length-based sliding windows:
Event over window:time(time-argument) Event over window:length(number-argument)
A window defines a workable sub-set of a possibly infinite set of events. A time- based window is defined in terms of time, and a length-based is defined in terms of number of events. A sliding window moves on each progression of time for time- based windows or on each new event for a length-based window. For example, the following clause informs the engine that we are interested only on stocks that happened on the last 1 minute:
Stock() over window:time(1m)
Hence, one can use windows to manipulate the retraction of events from the working memory. However, there is some model impedance when compared to other CEP models. As logic programming is centered on symbols (e.g. variables, functions) under a single namespace sort of speak, the retraction of an event happens across the inference engine’s working memory; that is, it is not limited to a entry-point, or channel, which could have been desired effect. For example, the use-case could be to have different windows for different exchange markets, each exchange market being defined as a separate channel.
Sliding windows is just one of the common patterns for window functions. Other window functions are:
• Time-based and length based batching windows: in this case the progression of the window happens in batches of time or events.
• Partitioned windows: in this case sub-windows are created for each partition of the input events as established by some partitioning attribute of the events
• User-defined windows: these are arbitrarily windows plugged in by the developer.
Does logic programming have any model impedance towards these window functions? This remains an open research question, however intuitively batch windows should not provide to be a problem. The other two types are less clear.
3. Event matching
In logic programming, event matching relates to the unification process, or in simpler terms, the “if condition” of the rules. Events are ground terms (facts), hence inference engines already have a native and powerful mechanism for matching events through first order logic predicates, which include (the non-exhaustive) list of operators: and, or, exists, not, for-all, collect, accumulate, etc.
In particular, collect and accumulate allow for the creation of complex events from simple events, which is one of the primary definitions of CEP.
Furthermore, temporal operators as defined by Allen  have been added: after, before, coincides, during, finishes, includes, meets, overlaps, and starts.
For example, the following clause creates a FireAlarm event after a SmokeDetection event is detected:
FireAlarm(this after SmokeDetection())
The support for these operators are based upon the fact that each event is tagged with a timestamp, which can either be set by the event producer (external) or by the inference engine itself (internal).
Also, as it should have been made clear by the presence of operators such as coincides and during that the time domain is defined as being interval based, rather than a point-in-time. The advantages of an interval-based approach are well explained by , however it is worth noticing that most other CEP solutions are point-in-time.
An important matching function is correlation (i.e. join operator). In logic programming, join is achieved through the unification of terms on common variables . This process has been subject of innumerous researches and optimizations. In particular, most modern inference engines are implemented using the RETE approach . However, at the time of writing, the author has not been able to find relevant work on the performance of join in inference engines when subject to the volume of events and the latency requirements needed in CEP.
4. External data integration
An example of external data integration is to join a stream of customer sales transactions with a remote database containing the customer profile, with the intent of verifying the customer’s risk level for a sale transaction.
In the context of logic programming, data takes the role of facts, hence integration to external data relates to being able to work with remote working memory in a seamless and efficient manner.
However, the common practice for supporting such a scenario is to “copy” the data over to the local working memory of the inference engine that is handling the events. In other words, the external data is inserted into the local inference engine, which then allows the enrichment to proceed normally.
This approach provides good performance, however has the following draw-backs:
• It is only applicable when the size of the external data is reasonably small; it is not viable to copy over entire RDBM tables into the working memory.
• Data integrity is lost, as the data is duplicated and no longer in sync with original. For example, the original data can change and the inference engine would be working with stale data until the change gets propagated.
An alternative approach is to use associative memory, or tuple spaces . Nevertheless, this also assumes that the external data is part of the overall system, even though it is distributed.
The underlying issue is to realize that the external data is owned by external systems, and therefore what is needed is a native bridge in the inference engine, whereby it is able to convert the logic programming paradigm seamlessly into the external system’s model. An example of this approach is Java programs using the JDBC API  to integrate to external database systems.
5. Output handling
Conversely to input handling, output handling is related on how to convert the RHS (i.e. right-hand-side of if-then statement) back into events, or more properly, into streams that flow out through output channels.
Before we address this, it is important to ask why is this needed, as it is not directly clear from our requirements. The reason being is that CEP applications are a component of a larger event processing network (EPN), that is, a CEP application plays the role of an event processing agent (EPA) node that receives events from other upstream nodes in the network and whose output must be send to downstream nodes to it in the network. Please, reference  for the definition of EPNs and EPAs.
This larger context entails that the CEP application must be able to send output that conforms to the expected semantic, as defined by the EPN to facilitate communication.
This semantic, which is similar to that of the input events, is presented here informally:
- Events must be able to be published to different channels. Within each channel, events must be ordered in time forming a stream.
Getting back to our original question, how does one generate stream of events out of the RHS, that is, out of the facts that exist in the working memory?
Consider that the facts (i.e. objects) in the working memory have no ordering constraints, that is, they are not ordered in time by any means. Further, we may be interested on different types of conversions; for example, we may want to notify not only when a new object is inserted into working memory, but also when an object is removed from the working memory.
In general, there is no first-class support for publishing time-ordered events out of an inference engine.
One approach is to support a new send(Object event, String entry- point) verb in the action clause, which guarantees the time-ordering restrictions.
Due to its declarative nature, logic programming is a natural fit for performing event matching. First-order predicates suffice for several complex pattern-matching scenarios, and, as seen, logic programming can be extended to support temporal constraints. Conversely, integration of logic programming inference engines into the large event processing network (EPN) eco-system is less preferable. Finally, future work is needed to research the performance of joins and other complex operators in light of the demanding CEP environment.
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