1. A computer-implemented method comprising: transmitting, from a local rule engine to an external rule engine, a schema document that defines an input to and an output of the external rule engine, the schema document being used by the external rule engine to define a rule; exposing, as a call-back service, a function provided by the local rule engine, wherein the function is capable of invoking the rule defined at the external rule engine; receiving, at the call-back service and from the external rule engine, a request to interact with the function; and providing, using the call-back service, an interaction between the external rule engine and the function.