On the head and the tail of EDA

Somehow the discussion around "EDA" and "CEP" continues in the Blogsphere - Giles Nilson from Apama has published seven points , out of which I quite agree to the first five. Jack van Hoof, who started this whole thread of discussion, argues that "CEP is not the beginning but finishing of EDA". So what is the answer? head or tail ? the right answer is --- it depends. Some customers work with methodic architecture way, in which first the EDA architecture is being set up, and then CEP tools are invoked on top of this architecture -- this is the case that Jack is talking about. However, in some cases, customers don't apply architectural thinking, but just acquiring some application that is implemented with CEP tool, and thus it introduces a kind of EDA ad-hoc for a specific application, this is the case that Jiles is talking about.

My guess is that most early adopters have applied CEP in ad-hoc way, so it serves as a "head" for more EDA in the future, while recently we see more customers looking at EDA first, since they are taking it from enterprise architecture perspective.

If we'll take the CEP functionality -- finding patterns on multiple events, it may not be implemented on EDA at all -- since the same technique may fit to "non event" environment.

More - Later.

On EDA, CEP and disruptive technology

This is the second time in the last few months that the term "disruptive technology" is being used, this time by Mark Palmer who adds his voice to the EDA vs. CEP discussion. Mark acknowledges the fact that EDA and CEP are not synonyms, but asserting that CEP is the disruptive part of EDA.

Recall that in the Oracle Blog the "disruptive" word was used recently and I have discussed it at that time; Richard Veryard has answered my response claiming that "disruptive" is often not better, at least not in the short run.

Anyway, I have noticed in the recent few weeks, two IBM customers who made plans to move to EDA (neither will use CEP, at least not in the short run). The shift to EDA is a fundamental change in the architecture thinking with the introduction of the decoupled event-driven thinking. Is it new ? - not really. Is it new for these customers - yes, not only new, but a significant change in thinking, it is an indication that there is a beef in EDA, even without introducing CEP, that the enterprise should digest. While thinking in events is natural in the daily life, it is still not natural for enterprise architecture and programming paradigm.

Bottom line -- while CEP certainly has its merits (if the customer has mature enough to digest it), EDA seems to be a more fundamental change in thinking relative to the alternative, and has its own beef. More - Later.

On EDA and maturity model

This is the first postings I am writing on my new Laptop, Lenovo T61p. New laptops are a trauma, since with the new laptop one gets new versions of all software, that somehow not always behave in a familiar way. I still did not succeed to convince the new laptop work with my home wireless network (it succeeds to connect, and then disconnects after a few seconds), so I have arranged workplace near the router and works wired, catching up in reading Blogs.

Jack Wan Hoof in his Blog claims that the market does not understand EDA, since people when talking about EDA are really talking about CEP, in a previous posting he makes a distinction between EDA that is about business events and CEP which processes messages.

It is true that people are often mix the terms of EDA and CEP (both TLAs) which are not quite the same, however, the "message" vs. "event" is something the needs some discussion, as well as the relations between all these, and why should a customer of these technology care anyway.

First of all - I agree that EDA is an architectural pattern, where various services/components/agents/artifacts are communicating in a decoupled way by sending events to some mediator which routes it based on subscription or other type of routing decision, such that the source and sink may not know each other, while CEP is a functionality of detecting patterns among multiple events and deriving new (derived/synthetic) events as a result.

To the question of "messages" vs. "events" -- CEP is called CEP since its inputs and outputs are events according to the definition of event as "something that happened" (with all its variations - see the EPTS glossary), message is in this case a way to represent event (not the only one - CEP can also process events represented as objects that are not messages).

The claim that messages in general may not represent events is valid, I can send my picture as a message and of course some smart guy can find event in it, but it is not intended to represent an event. However, if we apply pattern detection on messages that not represent events, then it is not CEP, since CEP assumes, by definition, that it inputs are events. This can be pattern detection on messages, and one can invent some name for it, if it makes sense.

Interestingly, people also realized that the same type of patterns may be useful for data and not only for events, which makes is detection of patterns on data and not CEP again, and we have started to see some works on patterns in the database community, although they are still quite complex.

EDA by itself has a value to the architectural thinking, recently I came across a plan made by an IBM big customer looking at their future architecture as a combination of SOA and EDA, in the sense that services will communicate among themselves in an event-driven way, but they have not yet discovered CEP at all. In fact, observation on various customers that are in different phase of thinking about the "event" issue leads me to think on a kind of maturity model in this area :

• phase I -- simple event processing: determine what events are important, how they can be obtained, who should receive these events, how these event should be transmitted etc.. This in some cases is a major effort, teaching the customer to "think in events", and building the way to emit and consume events, in some cases that by itself can be a big investment, since it can affect running systems, and invest in development of instrumentation/adapters/periodic pull mechanisms. The type of "event processing" that is being provided by basic EDA is typically routing (including subscriptions) and filtering. This phase should have a business benefit by its own right to justify the investment.
• phase II - mediated event processing: to bridge semantic and syntactic gaps between the event producer and consumer, there are additional mediators that provide ESB type functionality: transformation (including split and aggregation), enrichment (from external sources) and validation - this is often needed, and since many of these features are pretty much common, it may make sense to use COTS for it, although in simple cases, hard-coding may be cost-effective enough, this phase is optional, it may make sense to unify it with phase III.
• phase III - complex event processing -- this is a step-function towards looking at "event clouds" instead of events one-by-one, and typically built on top of the phase I, possibly I +II. I have seen cases in which phase III has been implemented as first step and included step I and II -- this makes step III much longer, but makes sense if the customer has a business problem for which it needs step III, but really did not think in event before.
• phase IV (optional) -- This extends the functionality towards intelligent event processing - which I already discussed in the past.

More about maturity models and event processing - later.

More on the semantics of synonyms

Still lazy days of the holiday week, I took advantage of the time to help my mother, who decided that she wants to leave home and move to seniors residence located 10 minutes walk from my house, this requires to deal with many details, so that is what I was doing in the last three days.... In the picture above (taken from the residence site) you can see the front entrance and the view seen from the residence tower, on the right hand side of the upper picture one can see part of the neighborhood we are living in (Ramat Begin) surrounded by pine trees all over.

Now, holiday eve again, and this is a good time to visit the Blog again. Last time I started the discussion in the semantics of synonyms by posing a simple example of conjunction over a bounded time interval (same pattern that Hans Glide referred to in his Blog), and slightly different from the "temporal sequence" pattern.

In the previous posting I have posed the following example:

Detect a pattern that consists of conjunction of two events (order is not important) - e1, e2.
e1 has two attributes = {N, A}; e2 has also two attributes = {N, B} ; the pattern matching is partitioned according to the value of N (on context partitions I'll write another time).
For each detection, create a derived event e3 which includes two attributes = {N, C}; E3 values are derived as: E3.N := E1.N ; E3. C = E1. A * E2. B.

Let's also assume that the relevant temporal context is time-stamps = [1, 5] - and the events of types E1 and E2 that arrived during this period are displayed in the table below:




The question is: how many instances of event E3 are going to be created, and what will be the values of their attributes?

Looking at this example, for N = 2, there is exactly one pair that matches the pattern
E1 that occurs in timestamp 5, and E2 that occurs in timestamp 4, so E3 will have the attributes {N = 2, C = 24}. However, for N = 1 things are more complicated. If we'll take the set oriented approach that looks at it as "join" (Cartesian product), since we have 3 instances of E1 and two instances of E2, we'll get 6 instances of E3 with all combinations. In some cases we may be interested in all combinations, but typically in event processing we are looking for match and not for join -- that is the difference between "event-at-a-time" type of patterns and "set-at-a-time" patterns that is being used by some of the stream processing semantics. So what is the correct answer ? -- there is no single correct answer, thus what is needed is to fine tune the semantics using policies. For those who are hard-coding event processing, or using imperative event processing languages, this entire issue seems a non-issue, since when they develop the code for a particular case they also build (implicitly) the semantics they require for a specific case, however policies are required when using higher level languages (descriptive, declarative, visual etc...), policies are needed to bridge between the fact that semantics is built-in inside higher level abstraction, and the need to fine-tune the semantics in several cases. In our case we can have several types of policies:

Policies based on order of events - example:

For E1 - select the first instance; for E2 - select the last instance.
For E1 - select the last instance; for E2 - select the last instance

Policies based on values - example:

For E1 - select the highest 2 instances for the value of A ; for E2 select the lowest instance for the value of B.

These are examples only -- it is also important to select a reasonable default which satisfies the "typical case", so if the semantics fits this default, no more action is needed.

These have been examples only, in one of the next postings I'll deal with identifying the set of policies required in order to make the semantics precise.

On semantics of synonyms

Holidays time - and I had to spend time in replacing my home wireless router (not of the same firm that is shown in the picture) who did not work well - to get another expensive one, whose signal is not received well in the lower floor well -- I hate wasting my time on restore things to work, this is never-ending story, every time something else need fix... we need more robust appliances..

Well - back from my personal frustrations to event processing thinking. I have posted a couple of postings about the different possible interpretations of "event_1 occurs before event_2", however, different interpretations are not unique to temporal issues, and the specific anomaly of having different temporal semantics. Let's take a case in which time does not matter - the function defined this time as follows:
Detect a pattern that consists of conjunction of two events (order is not important) - e1, e2.
e1 has two attributes = {N, A}; e2 has also two attributes = {N, B} ; the pattern matching is partitioned according to the value of N (on context partitions I'll write another time).
For each detection, create a derived event e3 which includes two attributes = {N, C}; E3 values are derived as: E3.N := E1.N ; E3. C = E1. A * E2. B.

Let's also assume that the relevant temporal context is time-stamps = [1, 5] - and the events of types E1 and E2 that arrived during this period are displayed in the table below:

The question is: how many instances of event E3 are going to be created, and what will be the values of their attributes? Think about it -- I'll discuss it next week.

More on semantics and race conditions

In previous posting I have posed the following sceanario:

Given the simple application shown below:

• There is a single event source (so no clock synchronization issues) which generates events of three types e1, e2, e3.


• Let's also say that in our story there is a single events of each type that is published (so no synonyms issues), the table shows their occurrence time (when they occurred in reality) and detection time (when they have been reported to the system) - each of them has been reported 1 time unit after its occurrence, no re-ordering problem.


• Events e1, e2 serve as an input to an EPA of type "pattern detection" which detects a temporal sequence pattern "e1 before e2", and when this is detected, it derives an event e4 - some function of e1 and e2.


• Events e3 (raw event) and e4 (derived event) serve as input to another EPA of type "pattern detection" which again detects a temporal sequences pattern "e3 before e4", if this pattern is detected - create event e5 which triggers some action in the consumer.

I also asked the question is -- given the above - will the action triggered by e5 occur?, i.e. will the pattern - "e3 before e4" be evaluated to true?

I got a few answers to this and you can read them as comments to the original posting; as promised I am dedicating this postings to analysis of this simple case:

The first thing to discuss is the semantics of "temporal sequence". There are two possible types of semantics for temporal sequence, which I call "detection time semantics" and "occurrence time semantics".

• The detection time semantics is implemented in various languages and means that the temporal order is the order of the time-stamps in which the "event processing platform" detects that this event occurs; if there is a single thread of such detection, then the events are totally ordered, otherwise, there may be several events with the same "detection timestamp".
• The occurrence time semantics also implemented in various languages means that the temporal order is the order of the time-stamps that are provided as part of the event information, and designate - when this event happend in reality. There are some complexity of synchronization of time in multi-producer environment, however, in this example we assume a single producer (I'll write about multi-producer cases in another posting).
• Note that this two order relations may not be identical.
• There is also kind of hybrid solution ("total order semantics") -- the semantics is really "detection time" semantics, but in order to allow events that arrive a bit late to take their proper role, the events are queued at a buffer (and not considered as detected) until time-out to let "out of order" events to arrive and re-order the buffer, and then send the events according to the buffer order.

Getting back to the example - in the small table on the bottom left-hand side of the figure above, there are occurrence and detection times of e1, e2, e3. For e4 there is only detection time - e4 is different from {e1, e2, e3} by the fact that it is a derived event and not raw event like the other three. The question is "what is the occurrence time of a derived event" ? -- there is no clear answer for it - there are several possible answers:

• In the derived event case the occurrence time = detection time, since this event is not real event but a virtual one, thus, its source is the EPA that creates it, and it occurred when created. In our case it means that occurence-time (e4) = 4.
• Its occurence time is the occurrence time of the last event that completed the pattern - since the participating events in the creation of e4 are {e1, e2} and e2 was the last that completed the pattern, occurrence-time (e4) = occurrence-time (e2) = 2
• Interval semantics: The event e4 occurs in the interval in which all the participants occur, which is this case means occurrence-time (e4) = [1, 2].

The phenomenon of multiple semantic interpretations apply to various other semantic decisions in the semantic of event processing language, and the preferred solution is to provide the user with semantic "fine tuning" policies, under which the user can chose the desired semantics, instead of "hard code" a certain semantics (using the most common one as a default), this is one of the benefits of using COTS for event processing, since it is quite difficult to think about such issues when developing EP manuaully using conventional language.

The semantics of the second "temporal sequence" (e3, e4) is thus:

• According to "detection time" semantics -- both have detection-time of 4. As such the sequence condition is not satisfied. However, if we impose total order by a single thread, this may create race conditions between the two events. In this case it is recommended to use a consistent priority policy - either breadth first (the raw event always comes first) or depth first (the derived event always comes first) to ensure deterministic result.
• According to the "occurence time" -- it depends on the policy chosen, but according to all interprerations - e4 occurs before e3 - thus the temporal sequence is not satisfied.

Bottome line: the temporal sequence (e3, e4) is satisfied if:

• The temporal semantics is detection time
• It is implemented by total order
• The total order policy is "breadth first" - namely priority for the raw events.

In all other cases the temporal sequence will not be satisfied and the corollary action will not execute.

On HITC and some small stuff

I have written about a month ago about the Arab Israeli High Tech Center, whose first goal will be to convert Arab Israeli engineers and mathematicians, to work in the very demanding Israeli high-tech industry. Earlier this week there was very impressive ceremony of kick-off for the center (that somehow migrated from AITC to HITC), in the two picture above - the top one is a group photo of the management and industry advisory committee of the center, and in the bottom one you can see me sitting in the crowd, with my eldest daughter, Anat, that decided to come and watch. This project is highly supported by the Israeli High-Tech industry and some of them included their country general managers -- like: HP, Oracle, BMC, EDS and Matrix (and Israeli services company), and other companies like - IBM, Motorola, Intel, Microsoft, Checkpoint and more, were represented by a senior person, there were several hundred people present, and was quite impressive -- studies will start in February 2009, and still a lot of work to be done to make it happen, but people came in feeling of a history in the making.

And back to event processing -- in the next posting I'll talk about the semantic question I have posted last week, and meanwhile just some short comments:

• Mark Tsimlezon from Coral8 tries to define what is "CEP engine" stating that there is some confusion in the market about this. I almost agree with what he has written and wondered if I should react, since my reaction can further confuse people... So I'll just remark that the term "platform" starts to be very popular, but with somewhat different meanings. I'll write more about platforms in the future.
• Marc adler is blogging about MSFT Oslo and his CEP application - without going now to further details I believe that the direction of having the ability to interface in the user's domain terminology and way of thinking, and then map it automatically to an execution language (directly or through intermediate representation) is a correct idea, somewhat beyond the state-of-the-art today; there will probably be several ways to do it, but a good topic to work on.
• Marco from RuleCore is blogging about the pain of their SAAS model and mentions some obstacles, this is a good topic for further discussion, it was also presented in the EPTS meeting by Bob Marcus. Clear, there are some applications that can be served by this model and some (e.g. distributed applications) are not. Will discuss this issue in length in one of the coming postings.

More topics - Later