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|Pack pcache -- prolog/cache_rocks.pl|
This module implements persistent caching of answers. The inspiration
comes from tabled execution where tabled answers are kept as a trie of
tries. The outer trie maps goal variants to answer tries and the answer
tries provide the answers to a specific goal variant. The
library(rocksdb) (and library(bdb)) provide a persistent Key-Value store
that can map a term to a term. The term is represented as an external
record, basically a binary alternative to write/read. This binary
representation is a blob for the key-value store. The representation
represents a variant, currently with two limitations:
A = f(X), B = b(A,A)is a different B than you get from
B = b(f(X),f(X)).
A = [a|A]and
A = [a,a|A]are considered different.
Ignoring these two issues (which can be fixed), we can use RocksDB or BDB as the outer trie used in tabling. We could use a trie or similar structure for the set of answers, but in this case a list preserves the original order and is more compact. Our database basically maps call variants to a list of answers.
In addition, it does some book keeping. First of all, it uses
signature.pl to compute a deep hash of the predicate. A deep hash is
an SHA1 hash computed from the clauses of the predicates and all
predicates called by it. The original goal, say m:
p(a1, ...) is
translated into <SHA1>(a1, ...). This implies that changing a predicate
or one of the predicates called by it invalidate the cache. Second, it
keeps track of partially completed goals and fully completed goals.
Re-running a fully completed goal simply retrieves the cached answers.
Re-running a partially completed goal first retrieves the cached answers
and then re-runs the goal with an offset to compute additional answers,
updating the status.
The cache_properties/2 variant returns all properties of a cache in a dict using the above keys.
forget(m:p(_,_))to remove all data cached for p/2. Notably
forget(_:_)will destroy the entire cache.
The following predicates are exported, but not or incorrectly documented.