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Review comment: explain why upserts between generational steps don't conflict.

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This commit is contained in:
Nick Alexander 2016-10-13 21:31:03 -07:00
parent caa9d2d7cb
commit 679ab8cf7d
1 changed files with 12 additions and 3 deletions
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15
src/common/datomish/transact.cljc
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@ -506,10 +506,19 @@
(let [{:keys [upserted resolved upserts-ev allocations-ev allocations-e allocations-v entities]} evolution]
(merge-with
concat
{:tempids id->e ;; TODO: ensure we handle conflicting upserts across generations correctly here.
:upserted upserted
{:upserted upserted
:resolved resolved
:entities entities}
:entities entities
;; The keys of the id->e map are unique between generation steps, so we can simply
;; concat tempids. Suppose that id->e and id->e* are two such mappings, resolved on
;; subsequent evolutionary steps, and that id is a key in the intersection of the two
;; key sets. This can't happen: if id maps to e via id->e, all instances of id have
;; been evolved forward (replaced with e) before we try to resolve the next set of
;; :upserts-e. That is, we'll never successfully upsert the same id-literal in more
;; than one generation step. (We might upsert the same id-literal to multiple entids
;; via distinct [a v] pairs in a single generation step; in this case,
;; <resolve-upserts-e will throw.)
:tempids id->e}
(evolve-upserts-ev id->e upserts-ev)
(evolve-upserts-e id->e upserts-e)
(evolve-allocations-ev id->e allocations-ev)