Key the check-in de-duplication window per account and stop it sliding - #69
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…the last forward The ty 10 gate on usr-activity was dropping legitimate check-ins, not only repeats, and invisibly so: it acks with 201, so the client is told the check-in landed. Two causes. The window was keyed on the caller's network address rather than the account, so accounts sharing one address competed for a single check-in slot. The address buys nothing here: a check-in carries a signed code, so a caller can only check in as an account it controls. The window was also refreshed on absorbed requests while its threshold equalled the client poll interval, leaving 8 ms of headroom. Which of two consecutive polls survived came down to arrival jitter, and once a caller was absorbed the window moved past its next scheduled check-in, so it stayed absorbed until its page reloaded. The window now keys on the account resolved from the signed code. It stays anchored to the last forwarded check-in and sits well below both the client poll interval and the backend's own per-account spacing, so it can only ever absorb a repeat the backend would have refused. The decision moves into CheckinGate, which makes "an absorbed repeat stores nothing" structural rather than a rule the handler has to remember, and makes the window semantics testable. Both defects are covered by tests that fail against the previous behaviour. Closes #68
PR Summary by QodoFix check-in de-dup: per-account keying and fixed (non-sliding) window
AI Description
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High-Level Assessment
Files changed (3)
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Greptile SummaryThe PR replaces address-based, sliding check-in de-duplication with an atomic, account-keyed fixed window.
Confidence Score: 5/5The PR appears safe to merge. No blocking failure remains.
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| Filename | Overview |
|---|---|
| dotnet/EcencyApi/Infrastructure/CheckinGate.cs | Introduces the account-keyed fixed-window decision logic and atomic per-process reservation with documented fail-open behavior. |
| dotnet/EcencyApi/Handlers/PrivateApi.Misc.cs | Replaces address-keyed sliding-window logic with the new account-scoped gate and short-circuits absorbed repeats. |
| dotnet/EcencyApi.Tests/CheckinGateTests.cs | Adds comprehensive tests for timing boundaries, account isolation, malformed anchors, non-sliding behavior, and concurrent reservations. |
Flowchart
%%{init: {'theme': 'neutral'}}%%
flowchart TD
A[Validated check-in request] --> B[Build account-scoped cache key]
B --> C[Lock account stripe]
C --> D[Read cached anchor]
D --> E{Inside absorption window?}
E -- Yes --> F[Return 201 without forwarding]
E -- No --> G{Backend spacing elapsed?}
G -- Yes --> H[Store new anchor]
G -- No --> I[Leave existing anchor unchanged]
H --> J[Forward upstream]
I --> J
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📝 WalkthroughWalkthroughThe check-in gate now deduplicates requests per account with a fixed 780-second window. The API stores timestamps only for forwarded requests. Tests cover timing, cache behavior, account isolation, forwarding, suppression, and fail-open handling. ChangesCheck-in gate
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to Concurrent check-ins for the same account can still be forwarded more than once because the de-duplication decision and timestamp write are not atomic, allowing burst traffic to bypass the intended single-call guarantee. Merge should wait for an atomic reserve-or-suppress operation or explicit owner acceptance of this bounded risk. Sequence Diagram(s)sequenceDiagram
participant Client
participant Activities
participant CheckinGate
participant MemCache
participant Backend
Client->>Activities: submit check-in
Activities->>MemCache: read account timestamp
MemCache-->>Activities: return recorded timestamp
Activities->>CheckinGate: decide forwarding
alt Request is forwarded
Activities->>MemCache: store arrival timestamp with TTL
Activities->>Backend: forward check-in
Backend-->>Activities: return response
else Request is a duplicate
Activities-->>Client: return HTTP 201
end
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🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@dotnet/EcencyApi/Handlers/PrivateApi.Misc.cs`:
- Around line 89-114: The checkin flow around CheckinGate.Decide and
MemCache.Set must atomically reserve each account key before forwarding,
preventing concurrent same-account requests from both observing an empty cache;
add or use a per-key reserve-or-suppress operation and preserve the existing
suppression response for losers. Add a concurrent handler-level test that sends
same-account requests and verifies exactly one reaches Upstream.Pipe.
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dotnet/EcencyApi.Tests/CheckinGateTests.csdotnet/EcencyApi/Handlers/PrivateApi.Misc.csdotnet/EcencyApi/Infrastructure/CheckinGate.cs
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| // refreshing the sliding window, as the branch always intended. | ||
| return; | ||
| } | ||
| MemCache.Set(key, decision.StampToStore!, CheckinGate.CacheTtlSeconds); |
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With the new 780-second window and the documented backend minimum of at least 870 seconds, a second source can arrive at t=800s: the gate forwards it, the backend rejects it as too early, but this line still records t=800s before the upstream call is even made. The regular poll at about t=900s would be eligible relative to the last successful check-in at t=0, yet it is now inside the locally recorded window, so it is dropped with a false 201. This recreates the missed-check-in/streak failure whenever another source lands in the gap between the gate and backend windows; the stamp must not advance for an upstream-rejected attempt.
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Confirmed and fixed in deaf2f0. This was real, and it recreated the original symptom rather than a lesser version of it.
The gate now has two thresholds instead of one. Below WindowMs a repeat is absorbed. Between WindowMs and the new AnchorAfterMs it is forwarded but leaves the anchor alone, which is exactly the band you identified: too far out for the gate to absorb, too close for the backend to credit. At or above AnchorAfterMs it is forwarded and becomes the new anchor. AnchorAfterMs has to be at least the backend's per-account spacing; setting it above only costs one extra forward the backend discards, so the error direction is the safe one.
Decision now carries Forward and StampToStore separately, since a forward no longer always anchors.
Pinned by two tests that fail against the previous commit: AnAttemptTheBackendWillRefuseIsForwardedButDoesNotAnchor, and ASecondSourceNeverDisplacesASteadyPoller run over offsets on both sides of the gap. At 800s and 880s the old behaviour forwards 1 of 20 polls, which is the shape of the production symptom.
Code Review by Qodo
1. Stamp cached before forwarding
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Code Review by Qodo
1. Stamp cached before forwarding
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…redit Review found that the first commit still had a way to displace a steady poller. The window absorbed below 780s but re-anchored on every forward, so a second source arriving in the gap between that threshold and the backend's own per-account spacing was forwarded, refused upstream as too early, and still became the anchor. The caller's own poll ~100s later then landed inside a window that had moved out from under it and was dropped with a false 201. That is the original symptom in a different disguise. The gate now has two thresholds because two clocks matter: the client decides how often a check-in arrives, the backend decides how often one counts. Below WindowMs a repeat is absorbed. Between WindowMs and AnchorAfterMs it is forwarded but leaves the anchor alone. At or above AnchorAfterMs it is forwarded and becomes the new anchor. AnchorAfterMs has to be at least the backend's spacing; setting it too high only forwards once more than needed, which is the harmless direction. Decision carries Forward and StampToStore separately now that a forward does not always anchor. The steady-poller test runs over second-source offsets on both sides of the gap, and fails at 1 of 20 polls against the previous behaviour.
Review found the remaining hole: the gate read the anchor, decided, then wrote it back as three separate steps. The backing store is concurrent but the sequence was not, so check-ins arriving for one account in the same instant could all read an empty anchor and all go upstream, which is the burst the gate exists to collapse. Two tabs opening together is enough to hit it: both fire the ping their page load schedules. DecideAndReserve now does all three under a striped lock, so a concurrent duplicate behaves exactly like a sequential one. The loser reads the anchor the winner just wrote and is absorbed, which stays correct in the direction this gate cares about: a check-in milliseconds behind another is one the backend refuses regardless. Striped rather than one global lock so unrelated accounts do not queue behind each other, and nothing is held across an await. Verified by removing the lock: 32 simultaneous check-ins for one account forward 8 to 13 times instead of once.
1 participant
Closes #68.
The
ty === 10gate onusr-activitywas dropping legitimate check-ins rather than only repeats, and doing it invisibly: it acks with 201, so the client is told the check-in landed. Users who are continuously active saw their check-in quest sit at 0 and their streak break.What was wrong
Keyed on the caller's address, not the account.
usernameis resolved from the signed code a few lines above and was not part of the key. Every account behind one NAT, carrier-grade NAT or household therefore shared a single check-in slot. Keying on the address buys nothing here: a check-in carries a signed code, so a caller can only ever check in as an account it controls, and the backend this gate fronts applies its minimum spacing per account.Sliding window whose threshold equalled the client poll interval. The check was
nowMs - recMs < 900000against a client that polls on a1000 * 60 * 15 + 8ms interval, so there were 8 ms of headroom. The stored timestamp was refreshed on every hit including absorbed ones, and on that path it was taken afterawait ctx.SendJson(201, ...)completed, so it landed later still. Two consequences: whether a legitimate poll survived came down to whether its arrival delay happened to be at least 8 ms longer than the previous one's, and a caller that once fell inside the window had it pushed past its next scheduled check-in, so it stayed inside until its page reloaded.Any second check-in source for the same key, a second tab or the ping a page load fires on mount, reset the window mid-cycle and locked the steady poller inside it.
What changed
CheckinGatenow owns the decision:checkin:<username>like the other keys in this cacheDecidereturns a stamp to store only when forwarding, so "an absorbed repeat stores nothing" is structural rather than a rule the handler has to rememberThe gate still collapses a burst from one account to a single upstream call, which is the only thing it was ever meant to do.
Tests
CheckinGateTestspins the window bounds, the key, the fail-open cases and the two protocol properties. Both defects are covered by tests that fail against the previous behaviour:TheWindowClosesWellBeforeAClientPollsAgainandTheWindowNeverOutlastsTheBackendsOwnSpacingfailAbsorbedRepeatsDoNotDisplaceASteadyPollerfails with 1 of 20 polls forwarded, which is the shape of the production symptomFull suite green: 119 passed, build clean with no warnings.
Parity
No
KNOWN_DIVERGENCESentry applies. The gate sits behindValidateCode, and every probe the catalog generates for this route (::min,::pop,::badcode) uses an invalid code and stops at the 401, so the harness cannot reach this branch.Summary by CodeRabbit
New Features
Bug Fixes
Tests