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