fix(chaos): stop the durability checker from reading a visibility lag as a lost write - #92
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… as a lost write Closes #75. The drill reported a durable write as lost (acked=2034 read=2033 missing=1) on a cluster that had lost nothing. The verify scan stopped at the FIRST empty page of a non-blocking fetch, so a single poll landing inside a normal visibility window ended the scan and every value not yet read was declared missing. That window is a real property, not a bug. A frontend's read horizon advances for the produces it handles and otherwise from a periodic refresh of the vnodes, one second by default, as BrokerServer.reconcile_metadata says in as many words. The checker produces round-robin across hosts and verifies against a single one, wait_healthy only counts Docker health (and even the product's /ready means every vnode answered once, not that a view is current), and the verify runs immediately after the rolling restart. So the last acknowledged write, produced through another node moments earlier, could legitimately be invisible to the node being asked. One missing value, rare, on a healthy cluster, passing on re-run: every detail of the report follows from that. The scan now separates the two things it used to conflate: a write that is gone, and a write not yet visible here. An empty page means nothing right now rather than nothing left, so the drain ends only after pages keep coming back empty for a settle budget, and any page carrying records resets that patience so a long log still drains in one pass. A still-missing set is re-read for a further budget before any verdict: values that turn up were never lost, only late, and the run passes while reporting the observed lag; values that never turn up fail the drill exactly as before. A failed fetch stays fatal rather than becoming a short successful page. This is not the failover-promotion hypothesis the issue opened with. d63d134 (stop answering unreachable with empty on the read path) was already in the failing commit, so a range read that fails is no longer swallowed; what remained was the case where nothing failed and the node simply had not seen the write yet. The design gap in #40 is real, untouched here, and stays open on its own merits. The drain policy is now tested (test/scripts/chaos_checker_test.exs) with an injected fetch, clock and sleep, so it needs no cluster and no wall-clock waiting. Seven cases; four of them fail against the previous stop-at-first-empty policy, including one that reproduces the exact shape of the report. The script skips running a mode under Mix.env :test, so a test can require it without being halted by main/1; the environment already distinguishes the two callers (the drill runs it through mix run in the loadtest image, which is dev), so no flag is invented for something nobody should ever set. Full suite 1269 tests and 0 failures including multinode; format, credo --strict, dialyzer and docs are clean. Coverage is unchanged at 83.3% because the script lives outside lib and ExCoveralls does not measure it, which is why the policy carries its own tests. Also worth noting for whoever hits this next: re-running a chaos job replaces both its artifact and its log, so the evidence of the failure being re-run is destroyed. This diagnosis had to be made from the code because of it.
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📝 WalkthroughWalkthroughThe chaos checker now drains topics through settle-based polling, treats fetch errors as fatal, and revisits missing acknowledged values before declaring failure. Tests use injected fetch, clock, and sleep functions to validate delayed visibility and budget behavior. ChangesChaos checker verification
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to The chaos checker can still run indefinitely on an active topic or report a false durability failure after a timed-out partial scan. These verification defects should be fixed before merge. Sequence Diagram(s)sequenceDiagram
participant ChaosChecker
participant drain
participant revisit
ChaosChecker->>drain: read the topic
drain-->>ChaosChecker: read set and connection
ChaosChecker->>revisit: re-read missing values
revisit-->>ChaosChecker: visibility result or remaining values
ChaosChecker-->>ChaosChecker: report VERIFY OK or halt with status 1
🚥 Pre-merge checks | ✅ 2 | ❌ 3❌ Failed checks (3 warnings)
✅ Passed checks (2 passed)
Full details: Linked Issues checkExplanation The PR fixes a false-positive verification failure for issue Full details: Out of Scope Changes checkExplanation The chaos-checker changes and related tests are in scope. The change to test/malachi/cluster/scrubber_test.exs is unrelated to the linked chaos durability issue and adjusts an existing log-length assertion. ✨ Finishing Touches 💡 1📝 Generate docstrings 💡
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📊 Measured on this branch
Chaos certification: PASSED at RF 3: 4 faults injected, 1,881 acknowledged writes verified Each generator ran on its own runner, server pinned to 3 cores and generator to 1, sweeping These numbers are measured, not committed: a shared runner varies enough between runs that This comment was automatically generated by the results workflow |
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Inline comments:
In `@scripts/chaos_checker.exs`:
- Line 143: Update revisit_loop/5 and drain/4 so draining shares the outer
revisit deadline instead of starting a fresh `@drain_settle_ms` budget; pass the
deadline through the drain policy, cap each drain sleep by the remaining time,
and ensure the retry returns when that deadline is reached even if records
continue arriving. Add a regression test covering a retry that reaches the
deadline while draining.
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…t open Addresses the CodeRabbit review on PR #92. The revisit loop checked its deadline before sleeping and then started a drain with a fresh settle budget of its own, so a retry beginning just before the deadline could still run a whole scan past it. Worse, the settle deadline is reset by every page that carries records, which is exactly what lets a long log drain in one pass: against a topic still being produced to, that reset never stops and the bounded re-read was not bounded at all. In the drill this is a verify step that hangs until the job times out. drain/4 now takes an absolute :deadline alongside the settle budget, defaulting to :infinity so a plain scan is unchanged. The scan returns when the ceiling passes even if pages keep arriving, and a poll that would overshoot it sleeps only the time that is left rather than running one full poll past. The revisit loop passes its own budget down, so the re-read shares it instead of opening a new one, and its own sleep is capped the same way. Four tests cover it: a topic that never stops producing still terminates the scan, a scan starting near the deadline returns instead of spending a full settle window, sleeps are cut to the remaining time (250, 250, 100 against a 600ms ceiling), and a scan with no deadline still behaves as before. Non-vacuity is stronger than usual here: with the ceiling removed the suite does not fail, it hangs, which is the point of the finding. The checker file is 11 tests and 0 failures; the full suite is 1273 tests and 0 failures including multinode, with format, credo --strict, dialyzer and docs clean.
…ongside it Pre-existing flake, unrelated to the chaos checker work on this branch; separate commit for that reason. The test asserting that an unexpected message cannot leak a payload into the log also bounded the whole captured output at 500 characters. `capture_log` collects Logger output process-wide and this file is async, so a line logged by another test at the same moment lands in the capture too. CI failed on PR #92 at 513 characters while the scrubber's own line measures about 120 locally: roughly 400 characters of the failure were other tests talking, not a leak. The assertions that actually guard the payload passed. The absence checks stay on the whole capture, where they belong: the payload must appear nowhere, whichever line it might have reached. The length bound, which is about the scrubber's line, now measures that line, found by the text it must contain. The bound stays at 500 against a line of about 120, so it still catches a line that grew enormous. Verified to still catch a real leak rather than merely stopping the flake: loosening the inspect limits in scrubber.ex so the payload prints fails the test, as it should. The file is 18 tests and 0 failures, and the full suite is 1273 tests and 0 failures.
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Inline comments:
In `@scripts/chaos_checker.exs`:
- Around line 187-188: Update the expired? branch in revisit/5 to return an
explicit deadline-truncated outcome instead of treating the partial acc as a
completed scan; handle that outcome as inconclusive or timed out without
reporting VERIFY FAILED or data loss, and add a regression test covering an
acknowledged value after the fetched prefix.
- Line 180: Update the initial verify scan around drain/4 to create and pass an
absolute verification deadline instead of defaulting to :infinity. Ensure the
deadline is not reset as additional records are processed, allowing the flow to
reach revisit/5 even when the topic continuously produces records.
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…ing as data loss Addresses the second CodeRabbit review on PR #92. Both findings are consequences of the deadline the previous commit introduced, and the second one matters more than it looks. The initial verification scan still passed no ceiling, so it inherited :infinity. Every page carrying records resets the settle budget, which is what lets a long log drain in one pass, so a topic still being produced to could hold that first scan open indefinitely and never reach the re-read at all. It now runs under its own ceiling. The sharper one: a scan cut short by its ceiling returned exactly what a completed scan returns. The caller then computed the missing set from a PREFIX of the topic and printed VERIFY FAILED, reporting data loss for values it had simply not reached yet. That is the same false alarm this whole change set exists to remove, arriving through a door the fix for it opened. drain/4 now reports how it ended, :settled or :timeout, and verdict/2 turns that plus the missing count into one of three outcomes: everything found is a pass however the scan ended; a value missing from a scan that reached the end is the durability alarm; a value missing from a scan that was cut short is INCONCLUSIVE, and says so, naming the ceiling it hit and that this is not evidence of loss. The re-read loop carries the same distinction, so a re-read that runs out of budget is reported as inconclusive rather than as loss. The exit code stays non-zero for the inconclusive case, because a verification that could not finish must not read as a pass; what changes is that the drill's output no longer accuses the cluster of losing data when the checker ran out of time. Three verdict tests pin the rule, including the exact case from the review (a value beyond the truncated prefix), and the existing drain tests now assert the status they end with. Reverting the distinction fails the truncated-scan test, which is the non-vacuity proof. The checker file is 14 tests and 0 failures; the full suite is 1276 tests and 0 failures including multinode, with format, credo --strict, dialyzer and docs clean.
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📝 Description
Reproduces and fixes the intermittent loss of one acknowledged write under the chaos drill's failover sequence.
The drill reported a durable write as lost (acked=2034 read=2033 missing=1) on a cluster that had lost nothing. The verify scan stopped at the FIRST empty page of a non-blocking fetch, so a single poll landing inside a normal visibility window ended the scan and every value not yet read was declared missing.
That window is a real property, not a bug. A frontend's read horizon advances for the produces it handles and otherwise from a periodic refresh of the vnodes, one second by default, as BrokerServer.reconcile_metadata says in as many words. The checker produces round-robin across hosts and verifies against a single one, wait_healthy only counts Docker health (and even the product's /ready means every vnode answered once, not that a view is current), and the verify runs immediately after the rolling restart. So the last acknowledged write, produced through another node moments earlier, could legitimately be invisible to the node being asked. One missing value, rare, on a healthy cluster, passing on re-run: every detail of the report follows from that.
The scan now separates the two things it used to conflate: a write that is gone, and a write not yet visible here. An empty page means nothing right now rather than nothing left, so the drain ends only after pages keep coming back empty for a settle budget, and any page carrying records resets that patience so a long log still drains in one pass. A still-missing set is re-read for a further budget before any verdict: values that turn up were never lost, only late, and the run passes while reporting the observed lag; values that never turn up fail the drill exactly as before. A failed fetch stays fatal rather than becoming a short successful page.
This is not the failover-promotion hypothesis the issue opened with. d63d134 (stop answering unreachable with empty on the read path) was already in the failing commit, so a range read that fails is no longer swallowed; what remained was the case where nothing failed and the node simply had not seen the write yet. The design gap in #40 is real, untouched here, and stays open on its own merits.
The drain policy is now tested (test/scripts/chaos_checker_test.exs) with an injected fetch, clock and sleep, so it needs no cluster and no wall-clock waiting. Seven cases; four of them fail against the previous stop-at-first-empty policy, including one that reproduces the exact shape of the report. The script skips running a mode under Mix.env :test, so a test can require it without being halted by main/1; the environment already distinguishes the two callers (the drill runs it through mix run in the loadtest image, which is dev), so no flag is invented for something nobody should ever set. Full suite 1269 tests and 0 failures including multinode; format, credo --strict, dialyzer and docs are clean. Coverage is unchanged at 83.3% because the script lives outside lib and ExCoveralls does not measure it, which is why the policy carries its own tests.
Also worth noting for whoever hits this next: re-running a chaos job replaces both its artifact and its log, so the evidence of the failure being re-run is destroyed. This diagnosis had to be made from the code because of it.
🔖 Type of Change
✅ Checklist
🧪 How to Test
📸 Screenshots (if applicable)
🔗 Related Issues
Closes #75.
Note about versioning:
patch,minorormajoron the PR[major],[minor]in the titlepatch(0.0.1)Summary by CodeRabbit