There are two types of workflow engines:
1. Based on Activty. Such as XPDL (which Shark implements on)
2. Based on State. Such as FSM (Which OSWorkflow implements)
There are some means and modeling languages for workflow engines:
FSM 有限状态机。
PetriNet：Petri网模型。
Activity Diagram：UML活动图。
开源引擎：
Shark－Activity Diagram。based on XPDL
Osworkflow－FSM。
Jbpm－FSM+Activity Diagram+PetriNet own modeling language JPDL.
这个规范最早是以WfMC为代表的“业务流程开发商”，他们主要拥护以XPDL作为描述语言来描述业务流程；之后是以OASIS组织为代表的，被IBM,MicroSoft,BEA所拥护的BPEL/BPEL4WS规范；之后向来以规范著称的OMG组织也不甘示弱，联合BPMI组织，独辟蹊径以Notation Specification为入口，首先推出了BPMN规范，进而推出了BPDM，也妄想分一杯业务流程描述规范。
Agile PLM Workflow engine:
1) Entity based
2)Workflow_process, workflow_rules
3)Workflow,state,status
