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persist: snapshots — checkpoint + replay = snapshot + tail + 11 tests
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snapshot.sx: snapshot is a projection state {:value :seq} stored in kv under
snapshot/<name>. persist/checkpoint replays and saves; persist/replay folds
only the tail after the snapshot. Tests assert snapshot+tail == full replay
both ways + determinism. 65/65.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
giles
2026-06-06 18:39:41 +00:00
parent 156d6f12ec
commit b0874b1282
6 changed files with 168 additions and 6 deletions
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114
lib/persist/tests/snapshot.sx Normal file
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; Phase 3 — snapshots + replay. Headline: snapshot + tail == full replay.
(define snap-count (fn (acc e) (+ acc 1)))
(persist-test
"no snapshot loads fresh seed state"
(persist/snapshot-load (persist/open) "feed" 0)
{:value 0 :seq 0})
(persist-test
"snapshot-exists? false initially"
(persist/snapshot-exists? (persist/open) "feed")
false)
(persist-test
"checkpoint stores a snapshot"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "snap" snap-count 0)
(persist/snapshot-exists? b "snap")))
true)
(persist-test
"checkpoint value equals full projection"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/append b "s" "x" 0 {})
(persist/append b "s" "x" 0 {})
(persist/project-value
(persist/checkpoint b "s" "snap" snap-count 0))))
3)
(persist-test
"checkpoint records the last seq"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/append b "s" "x" 0 {})
(persist/project-seq
(persist/checkpoint b "s" "snap" snap-count 0))))
2)
(persist-test
"replay after checkpoint only folds the tail"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "snap" snap-count 0)
(persist/append b "s" "x" 0 {})
(persist/project-value
(persist/replay b "s" "snap" snap-count 0))))
3)
(persist-test
"snapshot + tail == full replay (value)"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "snap" snap-count 0)
(persist/append b "s" "x" 0 {})
(persist/append b "s" "x" 0 {})
(equal?
(persist/project-value
(persist/replay b "s" "snap" snap-count 0))
(persist/project-fold b "s" snap-count 0))))
true)
(persist-test
"snapshot + tail == full replay (whole state)"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "snap" snap-count 0)
(persist/append b "s" "x" 0 {})
(persist/append b "s" "x" 0 {})
(equal?
(persist/replay b "s" "snap" snap-count 0)
(persist/project b "s" snap-count 0))))
true)
(persist-test
"replay determinism: two replays from same snapshot agree"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "snap" snap-count 0)
(persist/append b "s" "x" 0 {})
(equal?
(persist/replay b "s" "snap" snap-count 0)
(persist/replay b "s" "snap" snap-count 0))))
true)
(persist-test
"re-checkpoint advances the snapshot"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "snap" snap-count 0)
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "snap" snap-count 0)
(persist/project-seq (persist/snapshot-load b "snap" 0))))
2)
(persist-test
"snapshots are keyed independently"
(let
((b (persist/open)))
(begin
(persist/append b "s" "x" 0 {})
(persist/checkpoint b "s" "a" snap-count 0)
(persist/snapshot-exists? b "b")))
false)