Saturday, May 24, 2008

On CEP agent in EPN

This logo shows that we are not alone in the world of TLAs - there are other who use this acronyms, once I have made a list of acronyms of EDA which has shown that this is not a very good choice of acronym, but an acronym these days is context-sensitive. Anyway -- talking about "event processing networks", which we now try to clean its definitions - we have talked about SEP (simple event processing) agents, MEP (mediated event processing), CEP (complex event processing) and IEP (Intelligent event processing). The names can still change, I am not happy about any of them, but it seem to be relatively intuitive, and can be explained to non-technical people quite easily. Anyway, this time I'll concentrate on what is CEP agent. CEP agent have two basic phases:

1. Detect patterns over multiple events (these can be either multiple events of the same type, or multiple events from multiple event types - some are doing distinction here, but I don't see the rationale behind it) - this creates a collection of event-instances that satisfy the patterns.

2. Derive events as a function of the collection of events created in phase 1.

Some comments:

  • There can be cases in which one of the two capabilities is used in a degenerated form. The pattern can be trivial (take all events without any pattern), but still a derivation (e.g. count, calculate the average of a certain attribute etc...) is required; on the other hand - the pattern can be complex, however the derivation is trivial (just a concatenation of all participating events).
  • The term for the result of phase 1 - can be called "composite event" or "complex events", the two are not exactly synonyms, according to the glossary, as composite event is a syntactic term that is created by operators from primitive events, whereas complex event is a semantic term that includes aggregation of events that may not be obtained from patterns - however, the definition of phase 1 results is consistent with both.
  • A CEP agent always lives within a context (temporal, spatial, semantic) -- most common type of context is a time-window, but the concept of context is much wider.
  • The semantics of a CEP agent need to be fine-tuned in order to pick the instances of events participating in patterns relative to other possibilities which require to define policies - I'll talk more about semantics fine-tuning is a later postings.
  • The result of a CEP agent is one or more derived events (consistent with other types of agents).

Tuesday, May 20, 2008

On Event Processing Research Challenges

This picture has been taken a year ago on the stairs to the old church in Schloss Dagstuhl, in that meeting we had people both from academia and from industry discussing the state of the art and future of event processing. I have returned to the conclusion of the Dagstuhl Seminar done by my colleague Peter Niblett from IBM recently, after my return to the IBM Research Division from spending several years in the product organization, I was asked about challenges to the research community, as seen by the product development community (or the industry in general), since we had a session about it in Dagstuhl, in which people from the industry expressed their opinions about the same questions, I just had to take it as a basis of a presentation in this area.
In Dagstuhl four major areas has been put as challenge to the Research community (adding my own interpretation and comments)
1. Event Processing Algebra and Meta-Language: Like the database area in the pre-relational era, the first (and probably second) generations of event processing are "engineering based", various vendors are building implementations based on the use cases they see and their innovative ideas, bringing to the table, not only a variety of languages, but also a variety of (typically - implicit) conceptual models behind this implementation. One of the indication for a maturity of an area is the existence of an agreed upon conceptual model. The relational model has done it for databases, the browser concept has done it for the internet, now we need the same for event processing. David Luckham's concept of EPN/EPA is a possible starting point, but more work is needed on this - the challenge is still there; after constructing the "relational model" for event processing we also need the "SQL" (not necessarily SQL extension - but this is a possibility) for it - in term of meta-language that describes the model and can be mapped to various implementation.
2. Software Engineering issues: This is a challenge from a different perspective. Event processing and Event Driven Architectures impose different thinking about computing. We are programmed to think in a certain way in all programming language, and the event based programming is somewhat different paradigm. Even the basic principle of decoupled asynchronous processing is something that is not easy to digest f0r people. We need software engineering tools, methodologies and best practices.
3. Implementation optimization: If we have done the analog of SQL, another analog come to mind - database tuning and query optimization. There are many optimization issues especially when the event processing applications are distributed and may have various QOS requirements - parallel processing, acceleration of hardware, inter-operability, support in illites in general - all are challenges. The goal function in this optimization is not unique, while in some systems it is maximal throughput, in other it may be scalability in number of rules/queries/patterns (there are applications in which the throughput is measured in millions, and others in which the number of rules, the number of producers, or the number of consumers is measured in millions...), the goal function can be a combination of multiple criteria.
4. Variety of "small issues" - examples: uncertain events, out-of-order events, retention and vacuuming of events etc....
It will be interested to look at recent research project to see how much progress has been done on these, and if the list should be modified after a year that has passed. The major research event of the EP community this year is DEBS 2008 , the program has not been published yet, and it will be interesting to compare the accepted papers with this list (I'll do it when surveying the conference itself in July). We also intend to do in the EPTS event processing symposium in September a session with academic people about it - stay tuned for further notice. I am also thinking about a follow-up Dagstuhl Seminar, may be in 2009 or 2010 - to re-discuss event processing in perspective. More - later.