7.8.3 Monotonic tabling with external data

Monotonic tables depend on monotonic dynamic predicates. In some situations there is external dynamic data such as a database. One solution is to maintain a shadow copy of all the external data in a dynamic predicate. This wastes resources and introduces maintenance problems. The system allows to use this information directly from the external source. To do this, create a dynamic and monotonic predicate that accesses the data:

:- dynamic my_data/2 as monotonic.

my_data(X, Y) :-
    <access external data>.

Any monotonic table that depends on my_data/2 will be populated correctly and build a dependency. Next, if a new answer is added to the external data the user must call incr_propagate_calls/1 from the Prolog library library(increval). Similarly, when an answer is removed from the external data we use incr_invalidate_calls/1. Both notification calls must be made after the external data has been updated, i.e., my_data/2 must reflect the new situation before calling incr_propagate_calls/1 or incr_invalidate_calls/1.

:- use_module(library(increval)).

on_new_my_data(X, Y) :-
    incr_propagate_calls(my_data(X, Y)).

on_removed_my_data(X,Y) :-
    incr_invalidate_calls(my_data(X, Y)).

incr_propagate_calls(:Answer)

Activate the monotonic answer propagation similarly to when a new fact is asserted for a monotonic dynamic predicate. The Answer term must match a monotonic dynamic predicate. See section 7.8.3 for an example.

Status

Monotonic tabling is experimental and incomplete. Notably support for answer subsumption and call subsumption is probably possible and may greatly improve the application domain and resource usage. Monotonic tabling should work with both shared and private tables. Concurrency issues have not yet been tested though.