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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>
41 lines
1.4 KiB
Plaintext
41 lines
1.4 KiB
Plaintext
; persist/snapshot — checkpoint a projection so a read model rebuilds as
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; snapshot + tail instead of a full replay. A snapshot is just a projection
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; state {:value :seq} stored in the kv facet under a namespaced key. The
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; headline property (tested both ways): snapshot + tail == full replay. Replay
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; is pure — it depends only on the stored snapshot and the log tail, never a
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; clock. Requires: lib/persist/project.sx, lib/persist/kv.sx.
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(define persist/snapshot-key (fn (name) (str "snapshot/" name)))
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; load the stored snapshot for name, or a fresh {:value seed :seq 0} if none
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(define
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persist/snapshot-load
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(fn
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(b name seed)
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(persist/kv-get-or b (persist/snapshot-key name) {:value seed :seq 0})))
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; store a projection state as the snapshot for name; returns the state
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(define
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persist/snapshot-save
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(fn (b name state) (persist/kv-put b (persist/snapshot-key name) state)))
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(define
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persist/snapshot-exists?
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(fn (b name) (persist/kv-has? b (persist/snapshot-key name))))
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; replay = snapshot + tail: load the snapshot then fold events after it
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(define
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persist/replay
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(fn
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(b stream name step seed)
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(persist/project-resume b stream step (persist/snapshot-load b name seed))))
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; replay then persist the new snapshot; returns the updated state
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(define
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persist/checkpoint
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(fn
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(b stream name step seed)
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(let
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((state (persist/replay b stream name step seed)))
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(begin (persist/snapshot-save b name state) state))))
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