Coordinating Distributed Order Execution

An order processing system operates in conjunction with external systems to process orders received by an online shopping system. The order processing system maintains execution references that indicate that an external system is performing an action with respect to an item in an order. Each execution reference may store data defining the state of the corresponding execution. The external systems may update the stored state according to a state machine, which may be an acyclic directed graph. The order processing system also maintains an execution registry including data specifying the actions performed by external systems that may not be performed in combination. When a request is received to start an action with respect to an item in an order, the order processing system uses the execution references associated with the item and the execution registry to determine if the request may be granted.