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concurrency.sx: persist/append-expect refuses an append when the stream
advanced past the caller's expected seq, returning {:conflict :expected
:actual} instead of crashing or overwriting. persist/conflict? + accessors.
Phase 2 complete, 54/54.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
25 lines
963 B
Plaintext
25 lines
963 B
Plaintext
; persist/concurrency — optimistic concurrency for the log facet. The caller
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; passes the seq it believes is current (the last-seq it last observed). If the
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; stream has advanced since, the append is refused and a conflict VALUE is
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; returned — never a crash, never a silent overwrite. The caller re-reads the
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; tail and retries. This is the substrate-level answer to "two writers, one
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; stream": the loser gets a result it can act on.
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; Requires: lib/persist/log.sx.
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(define
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persist/append-expect
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(fn
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(b stream expected type at data)
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(let
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((actual (persist/last-seq b stream)))
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(if
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(= actual expected)
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(persist/append b stream type at data)
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{:actual actual :expected expected :conflict true}))))
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(define
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persist/conflict?
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(fn (r) (if (has-key? r :conflict) (get r :conflict) false)))
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(define persist/conflict-expected (fn (r) (get r :expected)))
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(define persist/conflict-actual (fn (r) (get r :actual)))
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