CHR2 to SQL conversion
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README.md

CHR2 to SQL converter

The library implements Constraint Handling Rules (CHR) solver by converting the rules into set of SQL scripts. So the solver can work with constraints sets. The syntax is similar to the one defined by Peter Van Weert in "Extension and optimizing compilation of constraint handling rules" aka CHRv2.

One of the features CHR systems are considered different to rule systems is their matching algorithm. Rule systems typically use eager matching and CHR utilizes lazy matching. In eager matching (for example RETE algorithm) first all possible matches of rules LHS are built. And only after one of matching rule is selected and fired. Firing means executing matched rule body which in turn adds or removes objects from the set being matched. This invalidates previously eagerly computed match. The matching algorithms are incremental but nevertheless much of work done on matching is wasted. Incrementality maintenance overhead is significant too. On the other hand lazy algorithms (used in CHR systems) only compute matching for one rule to fire. Typically LEAPS is used in CHR systems as matching algorithm. And in papers advocating CHR this is reported to have better performance characteristics. CHR2SQL system is experimental step to even more eager direction. It not only eager in matching it is also applies all matched rules immediately without conflicts resolution and without matched constraint set invalidation. It still uses CHR syntax so that’s why CHR is in its name. This system preserves logical reading of CHR rules, but operational semantics is obviously different to state of art CHR systems.

Usually CHR systems conform to 2 levels of operational semantics, namely abstract and refined. The abstract one is simpler but not deterministic. Typically systems parallelizing solving process conform to just abstract semantics. It is considered not usable in practice. Systems conforming refined semantics are much more usable but they may be considered as usual general programming language programs, and they are hard to parallelize implicitly. There are extensions (utilized in CHRv2) adding notion of priority to rules. That priorities orders and execution of rules but still leaves space for easy parallelization (not implemented as far as I'm aware).

CHR2SQL implements some semantics which is somewhere between abstract and refined ones. And it doesn't implement priorities, but still it sounds quite usable on samples from CHR text books. I don’t have any formal proof and I don’t have any plans to do them. That’s more a kind of empirical vision. Original motivation for such system is from one of my projects utilizing equality saturation technique for some modeling task. It was implemented using Leuven CHR System on SWI-Prolog. But equality saturation requires huge number of constraints. So they not always fit in memory. The obvious solution to this is to use some DBMS. And this library is an attempt to implement such CHR system.

CHR2SQL commits rules actions in batches. It uses INSERT … SELECT statements, UPDATEs for a large number of rows etc. Others CHR system has only one active constraint. It is very similar to just procedure call. And this makes such systems to be a general programming language. SQL version could use single active constraint too, and it would even fix memory problem for large constraints set, but it would lose many advantages SQL provides for batch data processing.

For example a simple solver from CHR2 thesis implementing Dijkstra’s shortest path algorithm.

init @ +source(V) => dist(V,0).
keep_shortest @ +dist(V,D1), -dist(V,D2), D1=< D2.
label(D) @ +dist(V,D), +edge(V,C,U) => dist(U,D+C).

priority keep_shortest > label(_),
label(X) > label(Y) if X < Y.

It displays needs for dynamic priorities in the thesis. The first rule (init) starts the search, the second rule (keep_shortest) removes bigger found distance if 2 of them found to the same node. And the third (label) actually traverses the graph by edges.

If there were no priorities and the system would have abstract semantics because it runs each rule in separate thread and if the third rule thread has bigger priority than the second the solver process may run in infinite loop if graph has cycles. CHR2SQL doesn't implement priorities but it successfully solves the problem and stops. It is because it handles all data in batches. Even if there are two solver threads and the one implementing “label” manages to perform a few steps until “keep_shortest” runs, that “keep_shortest” will remove all the redundant nodes at once. It resembles breadth-first search for solution while original CHR uses depth-first. CHR2SQL does perform some redundant job but it is a kind of little amount of speculative execution, it can be ignored. These just 3 lines of code implements useful task and can cope with quite big data.