BPMN 2.0 Core Notation: Activities, Events, Gateways, Sequence Flows
Scope
Core BPMN 2.0: flow objects, connecting objects and the token semantics that decide whether a model means what its author intended.
Core rules
- Structural elements: pools (participants), lanes (roles inside a pool), flow objects (events, activities, gateways), connecting objects (sequence flow, message flow, association), artefacts (data object, annotation, group).
- Sequence flows connect flow objects inside one pool; message flows cross pool boundaries or connect to a black-box pool.
- Activities: task and subprocess, with markers for loop, multi-instance and compensation.
- Events: start, intermediate (catching or throwing) and end, with triggers such as none, message, timer, signal, error, escalation, conditional and link.
- Gateways: exclusive XOR (exactly one), inclusive OR (one or more), parallel AND (all), event-based (race of catching events), complex (arbitrary expression). An XOR split needs conditions and ideally a default flow.
Worked example
An order arrives; if its value exceeds 10,000 a credit check runs, otherwise it is skipped; both paths continue to issue invoice. Model: order received (start) then a check-value? XOR gateway; the true branch goes to credit check, the default branch bypasses it; an XOR gateway merges both into issue invoice. Reasoning: the XOR split emits exactly one token and the XOR merge passes each token through without synchronising, which is safe here because only one branch ever activates. Had the branches been parallel, the merge would need an AND join, or invoices could be raised twice.
Traps that cost marks
Using an XOR merge to synchronise parallel branches. Expecting an AND gateway to evaluate conditions, which it never does. Attaching conditions to flows leaving an event-based gateway. Forgetting that a terminate end event ends the whole instance, not just one branch.