This was a little more tricky than might be expected because the
initialization process uses the transactor to bootstrap the database.
Since Clojure doesn't accept mutually recursive modules, this
necessitated a third module, namely "db-factory", which uses both "db"
and "transact". While I was here, I started an "api" module, to paper
over the potentially complicated internal module structure for external
consumers. In time, this "api" module may also grow CLJS-specific JS
transformations.
This agrees with Datomic. DataScript allows tx values, possibly to
allow reconstructing DBs from Datom streams, but appears to handle
user-provided tx values in the transactor inconsistently.
The implementation of :db/tx is special and may need to change over
time. We add it as a special ident, with value the current transaction
entity ID, specified per-transaction. This works well right now but
introduces some (internal) ordering requirements that may need to be
loosened.
Internally, we use SQLite's FTS4 to maintain a fulltext_values table of
unique "text" values. Fulltext indexed datoms have value v that is the
rowid into fulltext_values. We manually maintain the map between rowid
and value in the transactor.
For convenience, we expose two views interpolating the real text values
into the datoms structure.
This version includes SQLite-level unique indexes; these should never be
needed. I've included them as a fail-safe while testing; they'll help
us catch errors in the transaction layer above.
In the future, we might add a layer of indirection, hashing values to
avoid duplicating storage, or sorting URLs, or handling fulltext indexed
values differently, or ...
