Do Businesses need 63 different types of event?

BPMN2 event typesOne of the drivers of success in the BPM world has been BPMN – the Business Process Modelling Notation – developed under BPMI before it got absorbed into OMG, and probably a bigger success than any other modelling standard – and now as BPMN 2.o (beta, at this time) slightly reconfigured in name as the Business Process Model And Notation. BPMN is used to provide a “process” perspective (versus, say, a data or an event perspective) of a system and shows the flow or orchestration of process tasks and activities. It is made up of (per the excellent bpme.de BPMN2 summary poster):

  • 2 flow subtypes: sequence is the normal flow, with default and conditional conditional subtypes
  • 7 task types: send, receive, user, manual, “business rule” (really decision), service, and script
  • 6 activity markers: sub-process, loop, parallel, sequential, ad-hoc and compensation
  • 7 gateways: exclusive, event-based, parallel, inclusive, exclusive event-based, parallel event-based, and complex
  • 6 types of data: input, output, object, collection, store, and message
    and
  • 63 types of event

OK, “63 types of event” needs a bit of explanation (and justification). These consist of:

  • 13 main event types: untyped, message, timer, escalation, conditional, link, error, cancel, compensation, signal, multiple, parallel multiple, and terminate

… across 8 situations classified by location in a process:

  • Start: top-level, event sub-process interrupting, and event sub-process non-interrupting
  • Intermediate: catching, boundary interrupting, boundary non-interrupting, and throwing
  • End

… but with some situations not requiring certain types of event (e.g. there is no “start cancel process” event) leaving 58 63 event types defined [*1].

Presumably BPMN tools will let the user specifiy the main event type and associate the correct symbol from the context in most cases, leaving us to consider just the 13 main event types.

So let us analyse these event types from an event processing perspective:

  • Message and timer events: these are probably most familiar with to those with an event processing perspective, with a message relating to what we consider either a source or a derived event.
  • Escalation: this is really a deferal to a different process (e.g. a supervisory process), and can be handled by some internal message in event processing.
  • Conditional: this is “reacting to conditions becoming true” – in other words equivalent to a rule-firing (or event query) success. In a rule system this might be handled just by setting some fact or data, which in an inferencing rule system will use some internal event to signal to the rule engine that other rules may “fire” – however there is no need to explicitly model this behavior. I may also of course create an event (or in a query system, insert / update some event), which is again is not explicitly modelled as “conditional” per se…
  • Link: some means of connecting models: this is more a “page break” than a business event!
  • Error: I prefer the term “exception” here, which of course is a local context – an error event in one process is a normal event in an error-handling process, etc.
  • Cancel: this is more a transaction-specific event: if dealing with transactions then some events may indeed be “cancel transaction X” requests, which may or may not be satisfiable, etc.
  • Compensation: this is where some process route may require some compensatory process – for example in relation to a “cancel” event.
  • Signal: signalling information across processes: this is might be used as a control event, such as in controlling a choreography.
  • Multiple: one of a set of events is “caught” or provides the cue to continue the process.
  • Parallel Multiple: all of a set of events is “caught” to continue the process.
  • Untyped events are used for start and state changes. Typically, though, a state change is the main event we are interested in (in event processing)!
  • Terminate event – process complete! From an event perspective this (process end) might never happen of course…

The extra detail might be explained by the fact that in BPMN you are detailing *how* to implement a process, and the “event symbol” is providing you with some specific context for that event. In event processing languages, you usually describing “what* is to be processed, and where all events contribute to some situation or context. It will be interesting to see how these 58 symbols work out in practice, as a hindrance or a help for process designers…

Note:

*1: the table of events above, from the BPMN2 poster from bpmb.de, has 63 event symbols, and is slightly re-organised from the equivalent table in the BPMN2 beta specification (Table 10.93 pp 269-270, in 10-06-04.pdf). Interestingly the specification document also includes a table to detail the event symbols (Table 12.23 ppp 405-411, in 10-06-04.pdf) but this shows only 58 symbols! Let’s just say “lots of event types” in BPMN2 then…