core model Overview¶
The core model is an executable coordination model and its syntactic elements are:
States (state machines)
Links (to states)
Barriers
Ports
Connections
The first three syntactic elements (states, links and barriers) are nodes in core model's state machines. Ports are properties of states and connections connect different nodes with each other. States execute code, barriers are used to implement concurrency and links allow to re-use existing state machines.
States¶
States are the central entity for executing code. Hence, they are the only node type that contains script code to be executed. More specifically, states have:
An entry script: the script that is executed once when the state has been activated.
Action scripts: one or many script blocks. Each script block must contain one condition expression. Action scripts are executed while a state is active (until it is deactivated) given their condition expression evaluates to
true.An exit script: the script that is executed once when the state has been deactivated. If the entry script of a state has been executed, its exit script will be executed as well at some point.
States can be parameterized with parameter values. Incomplete parameter values (e.g. structs with missing fields - possibly nested) are allowed in top-level states at upload time to provide default parameter functionality. But they will be rejected when trying to execute them. See the description of Link nodes for details.
In addition, each state can optionally contain child nodes which can also be active as long as the outer state stays active. If the outer state is deactivated, all children are preempted.
The parameter value of a state is available in all state scripts and condition expressions of actions and ports but is read-only. In addition, scripts can set a state's result value which is available in all scripts and expressions of the state itself and also in the parameters of sibling states or action and port conditions of the parent state.
States contain a set of ports which control deactivation of the state and consequential transition to the next state if there is one. In addition, states can be organized hierarchically. Hence, each state also stores (named) child nodes, the parameters of these child nodes and the connections between them.
Links¶
Links reference a different state and behave exactly like actual states, inheriting the ports, children, scripts and parameter values from the linked state. A link can specify connections for its inherited ports and instantiate the linked state with different parameter values.
The parameter value of the linked state acts as a default value for the parameter value of the link. That is, if a parameter value is present in exactly one of link or linked state, that one is used, and if both are present, the parameter value of the link is used. Absence of a parameter is signaled by not including it as field in the struct of parameters, not by means of a null value. A null value behaves like a regular parameter value and can overwrite a default parameter value. If the parameter values of the link and linked state are structs, their fields are merged using the above rule. That is, if fields with the same name exist in both, the one in the link parameter value is used, or they are merged recursively if they are themselves structs. Any fields that occur exclusively in either the link or linked state, according to their name, are used.
Should a link reference another link instead of a state, the final parameter value is computed inside-out: from the state machine to the outer link.
Like states, top-level links with incomplete parameter values can be created, but not executed.
Barriers¶
Barriers implement split and join semantics and are the only possibility to create parallel behavior in core model state machines as a regular state can only activate a single port. They are only activated when all incoming connections are ready and activate all states connected through outgoing connections simultaneously. It must be noted that barriers can only be connected to states and not to other barriers.
Ports¶
Ports control if a state should be deactivated. Ports are defined with
a port name, a port condition and a port priority. The port name is
used to identify a port when it is used in connections while the
priority value is used to determine the order of port evaluation. The
port condition is an expression that, if it evaluates to true,
causes the owning state to be deactivated and a (possibly) connected
state to be activated.
Connections¶
Connections connect states, links and barriers with each other. When connecting two states or links with each other, the connection is defined over the name of the source state or link, the name of a port in the source node and the name of the destination node (can be a state, link or barrier). Connections between barriers and states and between barriers and links are defined with only the name of the source node and the name of the destination node. Connections between two barriers are not allowed.