feat(backend): bound L1 backfill by the server's remaining freshness (LAB-557) - #268
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…(LAB-557) The read response now carries X-CacheKit-Fresh-For (protocol spec/saas-api.md#remaining-freshness). CachekitIO reads parse it (absent = None/legacy; unparseable/negative = 0, the conservative action) and thread (bytes, is_stale, fresh_for) through the freshness chain; L1 backfill uses min(ttl, fresh_for) so an entry read late in its server-side freshness window is never served fresh from L1 past the server's fresh_until. The freshness read path now gates on backend capability, not just configured SWR — the unbounded backfill predates SWR and applied to every CachekitIO read. Revalidation scheduling stays gated on an actually-configured stale window. Post-lock double-check reads share the same bound and stale-exclusion via _l2_double_check.
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Included review availability: 4 reviews are currently available. Your included PR review attempts over the past 7 days set your current allowance at 5 reviews per hour. WalkthroughCachekitIO now reports remaining freshness. Cache handlers propagate this value through synchronous and asynchronous reads. L1 backfills cap their TTL to server freshness, skip stale or expired entries, and retain legacy behaviour when the signal is absent. ChangesFreshness propagation and bounded L1 backfill
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to L1 backfills now honor the server’s remaining freshness window while preserving legacy behavior when no freshness signal is available. No concrete merge-blocking risk remains. Sequence Diagram(s)sequenceDiagram
participant Decorator
participant CacheOperationHandler
participant CachekitIOBackend
participant L1Cache
Decorator->>CacheOperationHandler: Request freshness-aware read
CacheOperationHandler->>CachekitIOBackend: Read value and freshness
CachekitIOBackend-->>CacheOperationHandler: Value, stale status, fresh_for
CacheOperationHandler-->>Decorator: Return value and freshness
Decorator->>L1Cache: Backfill with bounded TTL when eligible
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✅ Passed checks (4 passed)
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Resolves the one conflict in backends/cachekitio/backend.py: LAB-2846 percent-encodes the key in the request path; LAB-557 adds the fresh_for tuple slot to get_with_freshness. Both kept — the freshness read now goes through _encode_key like every other keyed request.
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Inline comments:
In `@docs/configuration.md`:
- Line 195: Update the CachekitIO backend documentation to explicitly describe
the ttl=None behavior: L1 uses its 300-second default lifetime and caps that
lifetime by the server’s remaining freshness. Keep the existing rule for
configured TTL values and pre-signal servers unchanged.
In `@src/cachekit/cache_handler.py`:
- Line 1996: Update both StandardCacheHandler methods around get_with_freshness
and the related method to normalize legacy backend results from (bytes,
is_stale) into the promised three-element tuple, including a None expiry value,
before returning. Preserve already-normalized results and add direct handler
tests covering legacy backends.
In `@src/cachekit/decorators/wrapper.py`:
- Line 677: Update the lazy backend resolution flow in the decorator wrapper so
_l2_swr_backend_capable is recomputed immediately after _backend is assigned.
Defer backend-dependent stale_ttl validation and swr_by_default activation until
after that resolution, ensuring sync reads, async reads, and _l2_double_check
use SWR behavior and preserve the configured fresh_for when backfilling L1.
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.secrets.baselinedocs/configuration.mdsrc/cachekit/backends/cachekitio/backend.pysrc/cachekit/cache_handler.pysrc/cachekit/decorators/wrapper.pysrc/cachekit/l1_cache.pytests/unit/backends/test_cachekitio_swr_transport.pytests/unit/test_swr_decorator.py
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September 7, 2026 09:20
… legacy 2-tuple (LAB-557) CodeRabbit round on #268 — all three findings valid. wrapper: _l2_swr_backend_capable was a decoration-time snapshot. A provider-backed decorator (no backend= argument, e.g. @cache.production with CACHEKIT_API_KEY set, which DefaultBackendProvider resolves to CachekitIOBackend on first call) had _backend=None at decoration, so every read took the plain get, fresh_for stayed None and the L1 backfill used the full ttl — the LAB-557 bug, unfixed on the zero-config path. The three read sites (sync, async, post-lock double-check) now ask _l2_freshness_capable(), which reads the resolved backend at call time. The decoration-time snapshot is kept for stale_ttl validation only: an explicit stale window still fails at decoration on a non-capable or unresolved backend, as documented. cache_handler: StandardCacheHandler.get_with_freshness[_async] returned a legacy backend's 2-tuple unchanged while promising three elements; the handler now pads (bytes, is_stale) -> (bytes, is_stale, None) itself. The operation handler's tolerant unpack stays as defence for non-Standard CacheHandlerStrategy implementations. docs/configuration.md: state the ttl=None rule (L1's 300 s default, capped by remaining). Tests: provider-resolved async decorator with fresh_for=0 must not record L1 (reaches L2 twice); sync twin takes the freshness read; direct handler tests for the legacy 2-tuple. All four verified red without the fix.
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…ne, docs name the decoration-time rule (LAB-557) Expert-panel pass (4 agents, high stakes) on 63fb737. Bug-hunter and security: no findings — gate ordering, concurrent first-call resolution, Mock class-level semantics and the shorten-only bound all verified. Applied the craftsman/catchphrase findings that survived: - The 2->3 tuple pad existed in three hand-synced copies; the operation handler now calls _normalize_freshness_hit and unpacks strictly. Its layer stays because a custom CacheHandlerStrategy built against the v0.18.0 2-tuple signature still reaches it — the comments and the op-handler test now say that instead of "third-party backend", which is padded upstream by StandardCacheHandler since 63fb737. - _l2_swr_backend_capable -> _l2_swr_capable_at_decoration: the snapshot vs call-time split is now in the name at both remaining use sites. - Closure docstring no longer claims "never snapshotted" two lines above the snapshot; both docstrings cut to the WHY. - ConfigurationError for stale_ttl on an unresolved backend names the explicit backend= escape hatch; docs/configuration.md states that the backend must be known at decoration (@cache.io or explicit backend=), and that env-resolved CachekitIO under other presets still gets the remaining-freshness bound on reads. - _LegacyTupleBackend subclasses the existing _SWRBackend fake. Rejected with reason: deleting the op-handler-level legacy test (still guards the custom-strategy path, now through the shared helper) and the sync provider-path test (pins sync/async gate symmetry — the recurring bug class in this wrapper per the LAB-381 panel). Net -16 lines.
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…-557) Kody flagged _LegacyTupleBackend.get_with_freshness (added in 63fb737) for a missing return annotation. Its detector only sees added lines, so the same gap in _SWRBackend, _PlainBackend and _ExplodingBackend went unflagged — all six read-method fakes now declare their returns, matching the fakes in tests/unit/test_swr_decorator.py. _LegacyTupleBackend keeps the deliberately narrower 2-tuple return — the 0.5.x contract the handler pads is the point of the fake — and carries a type: ignore[override] to say so. pyrightconfig excludes tests/, so this documents intent for readers and IDEs rather than gating CI.
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Resolves two conflicts against main@7bd5abf: - src/cachekit/l1_cache.py: LAB-557 (#268) added DEFAULT_L1_TTL_SECONDS at the same insertion point as this branch's redact_key_for_log import. Both kept — import first, then the constant. - .secrets.baseline: generated by detect-secrets; took main's side and let the pre-commit hook regenerate it. Merge (not rebase) so history is append-only — no force-push.
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Bounds L1 backfill by the server's remaining freshness (LAB-557): CachekitIO reads parse the new
X-CacheKit-Fresh-Forresponse header (protocol#51, emitted by saas#325) and L1 backfill usesmin(ttl, fresh_for)— an entry read late in its server-side freshness window is never served fresh from L1 past the server'sfresh_until. Origin: CodeRabbit outside-diff finding on #233 (LAB-506), deferred there because it wasn't fixable SDK-side alone.What ships
CachekitIOBackend.get_with_freshness→(bytes, is_stale, fresh_for); absent header =None(pre-signal server, legacy behavior); unparseable/negative =0(conservative, mirrors unrecognized-freshness → stale; debug-logged so a garbage-emitting proxy is diagnosable). Threaded through the handler/operation-handler chain; a third-party backend still returning the released 2-tuple degrades tofresh_for=Nonevia a length-tolerant unpack instead of a swallowed unpack error turning every hit into a miss.supports_swr— instancehasattrread Mock/__getattr__proxies as capable), not just configured SWR: the unbounded backfill predates SWR and applied to every CachekitIO read. Revalidation scheduling stays gated on an actually-configured stale window._l1_backfill_from_l2holds both invariants at all three backfill sites in lockstep (stale never recorded; fresh bounded); the post-lock double-checks use a freshness-aware read (_l2_double_check) so an L2-read-error + still-live-old-entry double fault can't sneak an unbounded backfill through a side door.ttl=Nonethe bound clamps to L1's ownDEFAULT_L1_TTL_SECONDS(300s) — a long server remainder must never extend local service toward the 30-day cap (DELETE-as-revocation relies on the ≤300s ageout).Tests: regression per the ticket AC (fresh hit with 0s remaining is not L1-recorded; next read reaches L2), bound/legacy/no-SWR/mixed-reader-stale cases, clamp-never-extend, 2-tuple compat, header-parse vectors.
tests/unit/1960 passed; ruff + basedpyright clean; full-suite failure set identical to main modulo timing-flaky perf benchmarks.Expert-panel review (4 agents, high stakes — crypto/protocol gate): FIX-FIRST → applied: ttl=None clamp (CWE-613 — the bound had become an extension), 2-tuple tolerance, backfill-guard dedup, garbage-header debug log, honest
_l2_double_checkdocstring. Rejected: scheduling revalidation from the double-check (spec-permitted asymmetry on a double-fault rarity — documented instead).Docs:
docs/configuration.mdSWR section documents the bound; protocol matrix row stays 🚧 until this ships in a release (matrix verifies released artifacts). Ticket: LAB-557.Summary by CodeRabbit
New Features
Bug Fixes