Let me parse this carefully. The scenario: A gateway served ds4-flash to some sessions after a provider rotation. Wait — "after a provider rotation" — the rotation presumably changed the default/current model from ds4-flash to something else. Logs show session model rows written BEFORE the rotation still pin ds4-flash, but only some sessions. So after rotation, the gateway served ds4-flash to some sessions — those sessions that had model rows written before the rotation still pin ds4-flash. The task: diagnose which sessions are affected, why the sweep (presumably a migration/sweep job that should have updated or unpinned those rows) missed them, and produce a fix that does not restart healthy sessions. Hmm, this seems like a synthetic infrastructure diagnosis question. Let me think about what's plausible: - There's a gateway that routes model requests to providers. Sessions can have a pinned model (session model rows in some DB). A "provider rotation" happened — maybe rotating provider credentials/instances, or rotating the active model from ds4-flash to ds5 (new model). After rotation, some sessions continued to be served ds4-flash even though they should have moved to the new default. - The logs show session model rows written BEFORE the rotation still pin ds4-flash — but only some sessions. So the question: which sessions are affected? Presumably those whose rows were written in a certain window, or those written by a particular code path (e.g., first-turn session bootstrap), or those with a specific shard/timestamp. - Why did the sweep miss them? A sweep presumably iterates sessions to update pinned model rows after rotation. It might filter by `updated_at > rotation_time` or paginate by `id` with a stale snapshot, or it only scanned rows modified after some checkpoint, or it used a partial index, or it skipped sessions that were "active" (locked), or it processed only rows in a certain keyspace/shard, or it used keyset pagination with a bad cursor so it silently stopped early, or it filtered by `model IS NOT NULL` vs `model IS NULL`... Let me construct a coherent, realistic story: Setup: - Gateway keeps per-session model pin in a `session_models` table (session_id, model, pinned_at, source, updated_at). Sessions get a row written when the session starts ("session model rows"). - A provider rotation happened at time T (rotating model alias from ds4-flash to ds5-flash, or rotating providers; after rotation new sessions should get ds5 or unpinned default). - Post-rotation, gateway still served ds4-flash to some sessions — because their rows still pin ds4-flash. The sweep was supposed to either unpin (set model=NULL so default resolves dynamically) or rewrite pins to the new default. But only some sessions got fixed; the rest still have stale rows. Which sessions are affected? Candidates: 1. Sessions created before T whose rows were written with a specific code path — e.g., rows written at session creation with `source='bootstrap'` and `model` explicitly set, versus rows written lazily on first inference. 2. Sessions whose rows have `updated_at < T` — the sweep only touched rows `updated_at >= T` or `WHERE updated_at < T` — hmm. 3. Sessions on specific shards/replicas. 4. Sessions that were idle during the sweep window — sweep skipped locked/active sessions to avoid disrupting in-flight requests, intending a second pass that never ran (or was gated on a feature flag that only checked new sessions). Why did sweep miss them? The most plausible: the sweep query was something like `UPDATE session_models SET model=NULL WHERE model='ds4-flash' AND updated_at >= :rotation_start` — because they used the deployment-time "last deploy" timestamp or a watermark from a job that had been backfilling, so rows written before rotation were excluded. Or the sweep paginated with `LIMIT` using keyset on `pinned_at` but the cursor was seeded from the wrong offset, or it scanned a read replica with lag and saw rows as already migrated. Or the sweep only iterated sessions that were "active" at sweep time (in-memory session registry), missing sessions that were idle/persisted but not connected — those idle sessions reconnect later and get served ds4-flash again. Actually the phrase "written BEFORE the rotation still pin ds4-flash, but only some sessions" strongly suggests: the sweep targeted rows written before rotation (that's what should be updated), but only some of those got fixed. Or: logs show pre-rotation rows still pinned; the affected sessions are those. Let me design the diagnosis deliverable: 1. Identify affected sessions via a SQL query: sessions whose latest model row has model='ds4-flash' AND pinned_at < T AND (last_activity > T) — i.e., sessions that made requests after rotation and were served ds4-flash. Cross-reference gateway serving logs: requests after T where served_model='ds4-flash' and session had a pinned row. 2. Common root cause patterns to check, and pick the most likely one with evidence: - Sweep watermark mismatch: sweep used `WHERE pinned_at >= X` where X derived from a bad watermark (e.g., previous sweep's checkpoint, deploy timestamp), so pre-rotation rows were skipped. - Keyset pagination bug: sweep did `ORDER BY id LIMIT N` with `WHERE id > cursor`, but cursor initialized to max(id) at rotation (only new rows), so it only processed rows created after rotation; pre-rotation rows never visited. Or cursor stored at epoch 0 but job crashed mid-run and resumed from a checkpoint that skipped a range. - Replica lag / snapshot: sweep read from replica; rows on a lagging shard looked absent or already-null. - Filter on session state: sweep skipped sessions marked `active` or with an open stream to avoid breaking in-flight turns; those sessions stayed connected/streaming across rotation and never got a second pass. - Partial index/filter: sweep used an index on `model` that didn't include NULL-able rows or was filtered by tenant, missing some tenants. - Case/format mismatch: some rows pinned as 'DS4-Flash' or with alias 'ds4-flash-2024' variant; the sweep's WHERE model='ds4-flash' exact match missed alias variants. - Sharded key routing: sessions hashed to a shard whose sweep worker died/failed health check; job marked complete because it doesn't verify shard coverage. Given the question says "logs show session model rows written BEFORE the rotation still pin ds4-flash, but only some sessions" — affected sessions are those with pre-rotation pinned rows that the sweep failed to rewrite/unpin. The distinguishing attribute among pre-rotation rows: something made some rows visible to the sweep and others not. The most defensible, commonly-seen story: the sweep's query filtered on `updated_at > rotation_ts` or used keyset pagination keyed on a timestamp with a watermark initialized from "now", thus only covering rows written after the sweep start; pre-rotation rows with old timestamps were below the watermark. But then it would miss ALL pre-rotation rows, not "some". Unless rows get touched/updated: sessions that got a row update (e.g., turn counter, ttl refresh) after T would have updated_at > T and be caught; sessions idle since before T kept old updated_at and were missed. That elegantly explains "only some sessions": affected = sessions whose session_models rows were not touched after rotation (no activity between rotation and sweep). But wait — those sessions, if idle, wouldn't be served ds4-flash after rotation unless they resumed later. Hmm, "A gateway served ds4-flash to some sessions after a provider rotation" — sessions resumed after rotation (turn after T) with stale rows → served ds4-flash. Sessions active around T got their rows refreshed and the sweep caught them. So affected = sessions with last row update < T and a post-T request served ds4-flash. Alternative elegant story: rows written by two code paths — one writes `model` explicitly (pin) and one writes NULL with the alias resolved at request time. Before rotation, sessions bootstrapped via path A pinned the concrete 'ds4-flash'; path B stored 'default' or the alias. The sweep unpinned rows where source='api-pin' vs missing source='bootstrap'. Hmm. Another very plausible story: the sweep updated rows by re-reading from a cache. Sessions whose rows were in Redis cache at sweep time were skipped ("skip if cache hit because cache is source of truth") — but cache held stale ds4-flash pin; DB updated only for cache-missed sessions. Then cache TTL expired and stale pins rehydrated to DB... hmm, inverted. Let me just pick the cleanest causal chain and present a rigorous diagnostic method that verifies rather than assumes. The question asks to "diagnose which sessions are affected, why the sweep missed them, and produce a fix that does not restart healthy sessions." So the answer should: A. Define affected set precisely: sessions S where (1) latest pre-rotation model row pins ds4-flash (pinned_at < T_rot, model='ds4-flash'), (2) session had activity after T_rot (so the gateway consulted the pin post-rotation), (3) post-rotation requests for that session were served ds4-flash. Provide SQL to enumerate. Also caution about row versioning: use latest row per session (ROW_NUMBER over session_id by pinned_at desc) since table may be append-only. B. Why the sweep missed them: present the mechanism with the telltale evidence to confirm. I'll present the watermark/keyset-pagination hypothesis as primary (with the sub-case: cursor initialized from rotation timestamp or from a per-shard checkpoint; worker crash leaves checkpoint gaps; silent partial completion because job used at-least-once with bad checkpointing). Also mention alternate mechanisms and the discriminating evidence for each, since I can't know their actual system — but the question implies I should produce a definitive diagnosis. Best structure: "Run these three checks; the counts will tell you which failure mode you have; here's the most likely one given 'only some sessions' and 'rows written before rotation'." Hmm. Actually, let me reconsider — maybe the intended scenario is simpler. "Provider rotation" might mean rotating between providers serving the same model (e.g., moving ds4-flash from provider A to provider B). After rotation, the gateway served ds4-flash — which is correct — but to "some sessions" only, i.e., other sessions got a different model?? "Logs show session model rows written BEFORE the rotation still pin ds4-flash, but only some sessions." Hmm, ambiguous. Could be: after rotation, sessions should be re-pinned to new provider/model version; pre-rotation rows still pin old ds4-flash endpoint; sweep should have rewritten them; only some were rewritten. I think the intended reading: a sweep/migration was supposed to update or clear stale ds4-flash pins after rotating the default away from ds4-flash; some sessions remained pinned and got served ds4-flash post-rotation. Diagnose affected sessions, why sweep missed them, fix without restarting healthy sessions. The fix without restarting healthy sessions: don't do a gateway restart (which would drop healthy sessions/streams). Instead: - Stop the bleeding: make the gateway not trust stale pins — add a pin TTL / pin-max-age, or at request time validate pin against rotation epoch: if pin was written before T_rot and model is retired, ignore pin and resolve default (or rewrite on read). - Data fix: run a targeted backfill UPDATE for exactly the affected session ids (the enumerated set), not a broad sweep: `UPDATE session_models SET model=NULL, updated_by='fix-...' WHERE session_id IN (...) AND pinned_at < T_rot AND model='ds4-flash'`. Or set to new default. Use batched updates with small sleeps to avoid lock contention; skip rows locked (SKIP LOCKED / NOWAIT... actually UPDATE WHERE with low batch size). - In-flight streams: for sessions currently streaming on old model, let the stream finish on ds4-flash if provider still serves it during deprecation window; don't kill mid-stream — that's what "don't restart healthy sessions" means: healthy = currently working sessions; we only re-pin at next turn boundary. - Guard: add a request-time shim: `if (pin.model == 'ds4-flash' && pin.pinned_at < T_rot) { rewrite to default; audit log }` — idempotent, self-healing at read path, protects against any stragglers including cache-held pins. Invalidate Redis keys for affected sessions or version-bump the cache namespace so stale cached pins can't rehydrate. - Reconciliation: rerun a verifier query after fix; add a metric/alert for count of pre-rotation pins. Also mention: verify with shadow read of gateway logs — join request log (post-T, served_model='ds4-flash') to session_models; the affected set = sessions appearing in both. Sessions with pre-rotation rows but no post-rotation activity: stale but harmless until they wake — include them in the fix anyway (they'll hit the same bug on next use), i.e., fix is by predicate not by observed serving. Why the sweep missed them — I need to commit. Let me weigh evidence in the prompt: "rows written BEFORE the rotation still pin ds4-flash, but only some sessions." If the sweep had a watermark bug filtering `updated_at >= T_rot`, it would miss all never-touched-since rows. "Only some" implies the sweep caught others. Two clean explanations: (1) Timestamp/watermark: sweep processed rows it considered "recent" (since last checkpoint or since rotation). Rows untouched since before rotation fell below the watermark and were skipped. Sessions active near T_rot had rows touched (refreshed TTL/updated_at) and got cleaned. This is very common. (2) Keyset pagination with cursor on (updated_at, id) or on pinned_at initialized at rotation time — equivalent effect. (3) Snapshot/replica-lag: sweep saw a pre-rotation snapshot... no, replica lag would affect recently written rows, not old ones. (4) The sweep matched `model IN ('ds4-flash')` but some rows stored provider-qualified names like 'ds4-flash@provider-a' vs 'ds4-flash' — after rotation provider qualified rows changed format... The prompt says rows "still pin ds4-flash", implying matching name. (5) The sweep skipped sessions with active connections (to avoid disruption), planning a follow-up for those; follow-up ran before those sessions ended their streams or was skipped when the first pass reported success. Affected = sessions that were connected/streaming during sweep window. Then post-stream, next request still pinned. Evidence: affected sessions correlate with "had an open stream during sweep window" from gateway conn logs. Which to lead with? The prompt: "rows written BEFORE the rotation still pin ds4-flash, but only some sessions" — the emphasis on written-before suggests the failure is about write-time semantics: rows written before rotation vs after. If the sweep's predicate had been purely "model='ds4-flash'", write time wouldn't matter. So the sweep likely used a time bound (watermark) — and time-bound + "before rotation" is exactly the collision. So: the sweep's watermark/predicate excluded rows whose timestamp was before the rotation, i.e., it only fixed rows written after some checkpoint... but then rows "written before the rotation" would ALL be missed unless touched later. The "only some sessions" then = those touched after T (activity) got swept. Good, consistent. Alternatively the sweep DID target pre-rotation rows but paginated with a cursor that started mid-table (cursor from a prior run / default cursor bug), covering rows written after some id — missing older rows. "Only some" = rows with id above cursor. Both are watermark-class bugs. I'll present the watermark hypothesis as primary with the specific signature to confirm (affected rows all have pinned_at/updated_at below a threshold OR ids below a cursor boundary; the boundary should be visible as a hard cutoff in the distribution of affected pinned_at timestamps). That's a nice diagnostic: histogram of affected rows' pinned_at — a sharp cutoff at the watermark timestamp proves the predicate; a gap in id range proves cursor bug. Also possible: the sweep ran on a per-shard basis and one shard's worker failed; affected sessions = sessions hashing to that shard. Evidence: affected session_ids hash consistently to one shard; sweep worker logs show one shard erroring/completing 0 rows. Include as check #3. Deliverable structure: 1. Restate the invariant: post-rotation, ds4-flash is retired; any session served ds4-flash post-T with a pre-T pin is a stale-pin leak. 2. Affected set — precise definition + queries: - Latest row per session (dedupe with window function if append-only). - Condition: model='ds4-flash' AND pinned_at < T_rot AND last_request_at > T_rot (actually activity post rotation; can derive from gateway request logs or session table). - Cross-check with serving logs: count sessions where requests post-T resolved to ds4-flash — should equal the pin-based set; discrepancies indicate either cache-held pins (in serving but DB already clean) or rows on other tables. - Sub-split: (a) actively harmed — served post-rotation; (b) latent — pre-rotation pin, no post-rotation activity yet; both need fixing. 3. Why the sweep missed them: - Reconstruct the sweep predicate from job code/params. Likely watermark `WHERE updated_at >= :from` with `:from` set to rotation time or a checkpoint, or keyset cursor initialized wrong. Because the sweep keyed on write time rather than on the pin's staleness (model + epoch), it conflated "row written recently" with "row needs migration." - Confirm: distribution of affected pinned_at shows hard cutoff at watermark; or sweep logs show rowcount < expected; checkpoint rows in the sweep's bookkeeping table show `from_ts`. - Secondary suspects and how to rule out: shard worker failure (group affected ids by shard), active-session skip list, cache skip (`if cache.exists skip`), alias-case mismatch in WHERE. - The "only some" explained: rows for sessions that had any post-rotation write (TTL touch, turn write, metadata update) got updated_at > watermark → swept; truly idle-since-before-rotation sessions fell below and were skipped. Alternatively sessions connected during sweep were on the skip list. 4. Fix without restarting healthy sessions (the constraint means: no rolling restart of gateway pods; don't kill in-flight streams; don't globally flip config that would disrupt current streams): - Step 1 — Read-path guard (immediate, hot-patchable, no restart): at session resume/pin-resolution, add staleness check: `if pin.model == 'ds4-flash' and pin.pinned_at < T_rot → treat as unpinned`, resolve to current default, and lazily rewrite the row (or enqueue rewrite). This is self-healing and covers cache-held pins. Deploy via config reload / feature-flagged code path if hot reload available; a gateway config reload is not a session restart (reload listeners, keep connections). If code change requires deploy, do a rolling deploy that drains gracefully — rolling deploys don't terminate healthy sessions if done with connection draining; emphasize drain > restart. - Step 2 — Targeted backfill, not broad sweep: enumerate affected ids (from step 2 diagnosis), batched UPDATE (500–1000 per batch, short sleeps, `WHERE ... AND updated_at < NOW() - small` retry), set `model = NULL` (unpin, resolve dynamically) rather than pinning new default — so future rotations don't recur. Or set to new default if product requires explicit pin. Add `pin_epoch` column semantics: write `epoch = rotation_epoch` so future sweeps key on epoch not wall-clock. - Step 3 — Cache hygiene: bump cache namespace/version or delete affected session keys so the stale pin in Redis/memcache can't resurrect; or rely on read-path guard TTL. - Step 4 — In-flight streams: do nothing; they complete on ds4-flash if provider still serves it (ensure deprecation window overlap), or gateway returns stream-end and next turn uses new model. Never mid-stream kill. - Step 5 — Verify: rerun the enumeration query → zero rows in harmed set; watch metric `served_model{model=ds4-flash}` post-rotation → decay to 0 (excluding allowed grace); add alert: count of pins with model=retired and pinned_at 0. - Step 6 — Root-cause hardening: make sweeps idempotent and epoch-based: key migrations on logical epoch (rotation_id) recorded per row (`migrated_epoch`), not timestamp; use full-table keyset pagination from id=0; emit per-shard row counts and assert coverage; make the sweep update-by-read-path-guard the source of truth with the batch job as reconciler. Also, should the fix restart sessions? No — the fix operates on data + read path. Healthy sessions (correctly on new model or actively fine) untouched: our UPDATE predicate only matches ds4-flash pre-rotation rows; sessions already migrated have model!=ds4-flash or epoch>=rot, so they're not touched. Read-path guard only activates on stale pins. No restarts needed at all — that's the point: this is a data/state bug, not a process bug. Edge considerations: - If sessions pin model per-message (rows per turn), latest row wins — mention dedupe. - Timezone/clock skew between gateway hosts writing pinned_at vs the sweep's clock — another way a watermark misfires (rows written "before rotation" by a clock-lagged host look like they're after... actually skew causes the opposite: rows after T look before T and get swept... or rows written before T with skewed forward clocks look after T and get skipped while claiming to be pre-rotation? The log evidence says rows written before rotation still pin — from row timestamps. If a host had +5min skew, its rows at T-1min carry timestamps T+4min → sweep's `>= T` catches them → they're fine; hmm that would make fewer missed, not the observed. If host had -skew, rows after T look before T and get fixed early — fine. Skew matters for defining T_rot precisely: define T_rot as the rotation's monotonic event id/epoch in the config service, not wall clock. Mention briefly.) - Idempotency: the guard + backfill both idempotent → safe to run repeatedly. Now, the format: This is a deep technical diagnosis question. The user seems to be an engineer (possibly testing reasoning about distributed-systems debugging). A structured answer with headers, short paragraphs, code blocks for SQL and pseudocode is appropriate. Length: substantial but not bloated — maybe 500–800 words equivalent with code blocks. Use headers: "Affected set", "Why the sweep missed them", "Fix". Use SQL snippets. Be decisive: commit to the most probable root cause (timestamp watermark/keyset) while giving the discriminating test. Since I can't see their actual logs, I should present the diagnosis method that converges fast, then the likely cause, then fix. But the prompt says "Diagnose" — they want me to actually produce the diagnosis narrative. I'll write it as: here's the affected set definition + how to enumerate; the signature in the data tells you which sweep bug; the one that matches "only some sessions + pre-rotation writes" is the watermark/skip-list class; give both but pick primary. Actually, let me commit more firmly to ONE story to make the answer feel like real diagnosis, with alternates as a short checklist. Primary story: sweep keyed on `updated_at >= rotation_ts` (or keyset cursor initialized at rotation boundary), so rows never touched after rotation (idle sessions) were skipped; sessions active across the rotation had rows touched and were swept. Affected sessions = idle-during-rotation, returning-later sessions. This precisely matches "gateway served ds4-flash to some sessions after the rotation" — those sessions returned after rotation, got stale pin served. "But only some sessions" — the others were either touched (swept) or not yet returned (latent). Hmm wait, but there's a subtlety: if a session is idle from before rotation and returns after, its row's updated_at is still pre-rotation → sweep (with `>= rotation_ts`) skips it → still pinned → served ds4-flash. A session active AT rotation: row touched post-T → swept. A session that never returned: skipped too, but not "served ds4-flash" so invisible in logs — latent. So served-after-rotation set ⊆ skipped set. Good. But also plausible: sweep skipped sessions with open streams. I'll list as alternate #2 with its own fingerprint (affected set correlates with "open stream during sweep window" rather than timestamp cutoff). The timestamp cutoff fingerprint: plot histogram of pinned_at for affected sessions — hard edge at the watermark. The skip-list fingerprint: affected sessions uniformly distributed in time, but all had `state='active'` or an open SSE stream at sweep time. Cursor bug fingerprint: affected ids form a contiguous low range (or gaps between checkpoint intervals). Then the fix section. Key constraints: no restart of healthy sessions. So: 1. Read-path pin validation (config flag, hot-reloaded) — pin older than rotation epoch and equal to retired model → ignore + lazy rewrite. 2. Batched targeted UPDATE for enumerated ids (and by predicate for latent set) — predicate-only is fine since it only matches stale pins; healthy sessions' rows (model=null or new model or epoch>=rot) don't match. Use unpin (NULL) rather than re-pin to avoid the same bug next rotation. 3. Cache invalidation for affected keys (or version bump) — stale cached pin otherwise wins over fixed DB. 4. Don't touch in-flight streams; deprecation overlap window ensures ds4-flash still answers mid-stream; next turn resolves new model. 5. Post-fix verification queries + alert + fix the sweep to be epoch-based/idempotent/full-coverage. Also note: no gateway restart at all; if the guard needs a code deploy, rolling deploy with connection draining preserves sessions; but prefer flag reload. The phrase "does not restart healthy sessions" likely means: don't restart/reconnect client sessions — the fix must be transparent. Emphasize idempotence + non-disruption: updates only rows matching the stale predicate; read-path guard only fires for stale pins; in-flight streams untouched. SQL for affected set (assume table session_models(session_id, model, pinned_at, updated_at, source, shard) and gateway_requests(request_id, session_id, ts, served_model)): Latest pin per session: ```sql WITH latest AS ( SELECT *, ROW_NUMBER() OVER (PARTITION BY session_id ORDER BY pinned_at DESC, id DESC) rn FROM session_models ) SELECT session_id, model, pinned_at, updated_at FROM latest WHERE rn = 1 AND model = 'ds4-flash' AND pinned_at < :T_ROT; ``` Split: - harmed: AND EXISTS (SELECT 1 FROM gateway_requests r WHERE r.session_id = s.session_id AND r.ts > :T_ROT AND r.served_model='ds4-flash') - latent: rest. Fingerprint query: ```sql SELECT date_trunc('minute', pinned_at), count(*) FROM ... harmed ... GROUP BY 1 ORDER BY 1; ``` Hard cutoff at watermark → predicate bug. Compare with sweep job's stored `from_ts`/cursor in migration bookkeeping. Sweep code check: look for `WHERE updated_at >= :watermark` or `WHERE id > :cursor` with `:cursor` init; and skip conditions (`WHERE state != 'active'`, `if cache.get(...): continue`). Why only some: two populations among pre-rotation rows — touched after T (updated_at bumped by turn writes/TTL refresh) → inside watermark → swept; untouched → below watermark → skipped. Alternatively active-stream skip list. Fix SQL: ```sql UPDATE session_models SET model = NULL, pin_epoch = :ROT_EPOCH, updated_at = now(), updated_by = 'rot-fix' WHERE model = 'ds4-flash' AND pinned_at < :T_ROT AND (pin_epoch IS NULL OR pin_epoch < :ROT_EPOCH); ``` Batched by session_id ranges with LIMIT (or SELECT then UPDATE by ids), short sleep between batches, run on primary (avoid replica-based writes), off-peak. Optionally restrict to enumerated ids first (harmed set), then predicate for latent. Read-path guard pseudocode: ```python def resolve_pin(session): pin = cache_or_db_pin(session.id) if pin and pin.model in RETIRED and pin.epoch < CURRENT_ROTATION_EPOCH: audit("stale_pin_ignored", session.id, pin) pin = None # treat as unpinned enqueue_repin(session.id) # or lazily write NULL return pin or current_default(session) ``` Cache: delete keys `pin:{session_id}` for affected ids or bump `PIN_SCHEMA_VERSION` so old cached entries miss. Alerts/verify: ```sql SELECT count(*) FROM session_models WHERE model='ds4-flash' AND pinned_at < :T_ROT; -- expect 0 ``` Metric: gateway served_model counts post-rotation. Sweep hardening: - Key migrations on rotation epoch stored on each row (`pin_epoch`, `migrated_epoch`), not wall clock. - Full keyset scan from id=0 with `(id > cursor)` seeded at 0, checkpoint per shard, emit row counts per shard, alert on worker failure instead of silent completion. - Make the sweep a reconciler that runs until the invariant query returns zero, not a single pass. Also note ordering: deploy guard first (stops harm immediately even if backfill slow), then backfill, then cache cleanup, then fix sweep, then remove guard after invariant holds for N days (or keep guard permanently as defense-in-depth — cheap). One more consideration: "provider rotation" could mean credentials rotation for the ds4-flash provider — in which case serving ds4-flash after rotation might actually be fine, and the bug is something else. But the prompt's framing ("served ds4-flash to some sessions after a provider rotation" + sweep missing rows) implies ds4-flash post-rotation is wrong for those sessions. I'll state the assumed invariant up front in one sentence ("assumption: rotation retired ds4-flash as the default; pins to it must be re-resolved") — good practice to make assumptions explicit. Actually — alternative: rotation moved ds4-flash from provider A to provider B, and gateway kept serving ds4-flash via provider A (old route baked into the pin row: pin includes provider endpoint). Rows pin (model, provider); sweep should rewrite provider for ds4-flash rows; missed ones still route to drained provider A → failures or stale capacity. The diagnosis is structurally identical: stale pin fields not rewritten; sweep watermark bug. I can note this variant briefly: "if the rotation was provider-side, same logic applies with pin.provider instead of pin.model." That covers the ambiguity without derailing. Format plan: - Brief framing line + assumption. - **1. Which sessions are affected** — definition, SQL, split harmed/latent, plus cache caveat. - **2. Why the sweep missed them** — primary mechanism (watermark on write-time), why "only some" (touched vs untouched rows), fingerprint checks, alternates checklist (cursor, shard worker, active-skip, cache-skip, clock skew note). - **3. Fix without restarting healthy sessions** — ordered steps: guard (hot), cache, batched unpin, streams untouched, verify, sweep hardening. Note why healthy sessions untouched (predicate only matches stale rows; guard only fires on stale pins; no process restart, only config reload / graceful rolling deploy if needed). Keep it tight. Use code blocks. Headers with bold or ###. Probably ~600–900 words. Markdown is appropriate (technical, will render). Let me also double check "does not restart healthy sessions" — could also mean the gateway has long-lived sessions (e.g., websocket/streaming) and the naive fix is "restart gateway / recycle sessions to reload pins." Our fix avoids that. State explicitly: no restarts; guard is a config-flagged read-path change (hot reload); data fix is row updates; in-flight streams complete. Write it. Assumption: the rotation retired ds4-flash as the default (or moved it to a new provider), and any session pinning it from before the rotation must be re-resolved. The bug is stale pin state surviving a migration sweep. ## 1. Which sessions are affected Take the **latest** model row per session (if the table is append-only), and intersect with post-rotation serving: ```sql WITH latest AS ( SELECT *, ROW_NUMBER() OVER (PARTITION BY session_id ORDER BY pinned_at DESC, id DESC) AS rn FROM session_models ) SELECT l.session_id, l.pinned_at, l.updated_at, EXISTS (SELECT 1 FROM gateway_requests r WHERE r.session_id = l.session_id AND r.ts > :T_ROT AND r.served_model = 'ds4-flash') AS harmed FROM latest l WHERE l.rn = 1 AND l.model = 'ds4-flash' AND l.pinned_at < :T_ROT; ``` This splits into two populations: - **Harmed**: pre-rotation pin *and* post-rotation requests actually served ds4-flash. These are the ones showing up in your logs. - **Latent**: pre-rotation pin, no post-rotation activity yet. Invisible in serving logs today, but they'll hit the same path the moment they return. Fix both. If the harmed set from serving logs is *larger* than the pin-based set, some sessions are being served ds4-flash with clean DB rows — that means a stale pin is being served from the **cache layer** (Redis/session cache), not the DB. Check that before anything else. ## 2. Why the sweep missed them The phrase "only some sessions, all written before rotation" is the signature of a **watermark bug**: the sweep keyed its work on *write time* instead of *pin staleness*. Look at the sweep code for one of these: ```sql WHERE model = 'ds4-flash' AND updated_at >= :watermark -- primary suspect WHERE id > :cursor ORDER BY id LIMIT N -- keyset variant ``` with `:watermark` initialized to the rotation timestamp (or a previous run's checkpoint), or `:cursor` seeded at the max id at rotation time. The sweep conflated "row was written recently" with "row needs migrating." Why only *some* sessions: rows are touched by more than pin writes — turn writes, TTL refreshes, metadata updates all bump `updated_at`. Sessions **active at or after the rotation** had their row's `updated_at` pushed past the watermark, so the sweep caught them. Sessions **idle across the rotation** kept their pre-rotation timestamp, fell below the watermark, and were skipped. Those are exactly the sessions that later resumed and got served the stale pin. Confirm it in five minutes — this is the discriminating test: ```sql SELECT date_trunc('minute', pinned_at), count(*) FROM GROUP BY 1 ORDER BY 1; ``` A **hard cutoff** in the histogram at the sweep's stored `from_ts`/cursor value proves the predicate. Alternates to rule out if there's no cutoff: - **Shard worker failure**: group harmed session_ids by shard key; if they hash to one shard, check that worker's logs (job "completed" without per-shard coverage assertions). - **Active-session skip list**: sweep skipped sessions with open streams to avoid disruption, and the follow-up pass never ran. Fingerprint: harmed set correlates with "open stream during sweep window," not with a timestamp edge. - **Cache-skip**: sweep treated a cache hit as authoritative and skipped — but the cache held the stale pin. Also sanity-check clock skew: define `T_ROT` from the rotation event's logical epoch in config, not wall clock on gateway hosts. ## 3. The fix — no restarts, healthy sessions untouched This is a state bug, not a process bug, so nothing gets restarted. In order: **a. Read-path guard (deploy first, hot-reloadable flag — stops the harm immediately):** ```python def resolve_pin(session): pin = cache_or_db_pin(session.id) if pin and pin.model in RETIRED_MODELS and pin.epoch < CURRENT_ROTATION_EPOCH: audit("stale_pin_ignored", session.id, pin) pin = None # treat as unpinned enqueue_rewrite(session.id) # lazy, idempotent return pin or current_default(session) ``` Guard only fires on stale pins, so healthy sessions never notice. If the guard needs a code deploy rather than a flag reload, use a rolling deploy with connection draining — that preserves sessions; do not recycle them. **b. Cache hygiene:** delete `pin:{session_id}` for the enumerated set, or bump the pin-cache schema version, so a stale cached pin can't override the fixed DB row or resurrect after the backfill. **c. Targeted backfill (batched, off the replica):** ```sql UPDATE session_models SET model = NULL, pin_epoch = :ROT_EPOCH, updated_by = 'rot-fix' WHERE model = 'ds4-flash' AND pinned_at < :T_ROT AND (pin_epoch IS NULL OR pin_epoch < :ROT_EPOCH); ``` Run in id-batched chunks (~1k rows, small sleep) off-peak. **Unpin (`NULL`) rather than re-pinning the new default** — unpinned sessions resolve dynamically, so the next rotation can't retrigger this. The predicate only matches stale ds4-flash rows; already-migrated rows don't match and are never touched. **d. In-flight streams: do nothing.** Let streams on ds4-flash finish (keep the old provider answerable during a deprecation overlap window). The guard resolves the new model at the *next turn boundary* — killing mid-stream is exactly the disruption we're avoiding. **e. Verify:** the invariant query returns zero; metric `served_model{model="ds4-flash"}` decays to zero post-rotation. Add a permanent alert on `count(pins WHERE model=retired AND pinned_at < epoch)`. **f. Harden the sweep so this can't recur:** - Stamp every row with a logical `pin_epoch`/`migrated_epoch` at rotation time; migrations key off the epoch, never wall-clock timestamps. - Full keyset scan seeded at `id = 0`, checkpointed per shard, with per-shard row counts emitted and a hard alert if any worker reports zero — never silent partial completion. - Reframe the sweep as a **reconciler**: run until the invariant query is empty, not a single pass. Keep the read-path guard permanently as defense-in-depth — it's cheap, idempotent, and catches any future sweep gap the same way.

SCB SCB-stateful_reason-T1 KIMI