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fix(keys): keep a returning metric on its slot, and stop broadcasting - #25
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| E2e Test Quality Review |  |
Blocking error-handling issue remains in the new E2E script. reader:remove() is called without checking its return value at integration/index.lua:54 and :68. KeyIndex:remove() returns an error… |
Check the return from each reader:remove() call and fail the endpoint with context when it is non-nil, for example local err = reader:remove("target", "eviction"); assert(not err, "remove failed: " .. err). Also check the return status … |
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Check name Status Explanation Description Check Check skipped - CodeRabbit’s high-level summary is enabled. Title check The title accurately identifies the main changes: preserving slots for returning metrics and stopping unnecessary broadcasts. Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request. Security Check No security vulnerability matches the seven required categories in the changed code. Category 1: no credentials, authentication headers, or secret-bearing configuration are logged or returned; the new… Full details: Security Check
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Explanation
No security vulnerability matches the seven required categories in the changed code. Category 1: no credentials, authentication headers, or secret-bearing configuration are logged or returned; the new integration endpoint returns only
ok, while changed logs contain key-index names or error text. Category 2: no database persistence. Category 3: no mutating API endpoint or permission check. Category 4: no cross-resource access. Category 5: no TLS or certificate configuration. Category 6: the changes manage metric slots in a shared dictionary and add ownership checks for aliases. Category 7: no environment or secret-manager references. The changed runtime files areprometheus_keys.luaand integration test configuration/code; the remaining changes are tests and documentation.
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Full details: E2e Test Quality Review
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Explanation
Blocking error-handling issue remains in the new E2E script.
reader:remove()is called without checking its return value atintegration/index.lua:54and:68.KeyIndex:remove()returns an error when thedelete_countupdate fails (prometheus_keys.lua:381-387), so the following local-index assertions can pass while explicit deletion synchronization failed. The earlierindexResp.Body.Close()issue is fixed inintegration/test.go:120-122.Resolution
Check the return from each
reader:remove()call and fail the endpoint with context when it is non-nil, for examplelocal err = reader:remove("target", "eviction"); assert(not err, "remove failed: " .. err). Also check the return status from the directd:delete()setup calls, and assert that the peer no longer exposes the deleted metric.
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…dcasts A shared-dict slot is live, expired with its node still present, or reclaimed; only ttl() and get_stale() tell the last two apart. Treat them separately: - Renew a known live slot with get() + expire() without reading the shared counters. Revive an expired node in place, keeping its number. - Give a reclaimed slot's key a new number above every reader's self.last instead of bumping delete_count, so expiry no longer sends every worker through a full sync (apache/apisix#13658). - Discover slots with get_stale(), and forget a slot only when ttl() reports "not found" and get_stale() is nil: ttl() also reports "not found" for a node exactly 5ms past expiry (-5 is NGX_DECLINED). - Track aliases left by concurrent registrations, fall back to another alias when the chosen slot is reclaimed, and delete only aliases still owned by the removed name. - Detect a re-created key_count without the expiry broadcast: the writer whose incr() returns 1 publishes a unique generation token, readers do a full sync when it changes or is missing, read it before key_count, and also resync when key_count < self.last.
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Actionable comments posted: 1
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @integration/test.go:
- Line 120: Update the cleanup at indexResp.Body.Close to check its returned
error and fail the regression test when closing the response body fails, using
the surrounding test’s existing failure-handling mechanism.
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CHANGELOG.mdintegration/README.mdintegration/index.luaintegration/nginx.confintegration/test.goprometheus_keys.luaprometheus_test.lua
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Keep main's order in remove_expired_keys(): scan the tracked slots, then reclaim. A node the flush reclaims is dropped from the local index on the next round. Also fail the integration run if the key index regression response cannot be closed.
Adapt the cleanup changes to the slot states of #25: a due slot is forgotten only when its node is gone, and deadlines are moved on the fast-path renewal too. Drop the batched flush_expired(): it bounds the entries freed per call, not the time the dict lock is held, so prometheus:flush_expired() now calls the dict directly.
Since #25 permanent slots are imported with a zero TTL, which was tracked to catch a slot read at its exact expiry millisecond. Every permanent slot another worker registered was then checked on every round: 100k series cost 100k ttl() calls per round per worker. Restore the zero-TTL rule of main, and stop tracking a slot that turns permanent.
Problem
A metric with
exptimethat comes back after expiring went through this path:expire()on its old slot fails with"not found".delete_count.The result is
key_countchurn,delete_countbroadcasts, and per-request full syncs (apache/apisix#13658).Design
A shared-dict slot has three states. Only
ttl()/get_stale()tell them apart:get()ttl()expire()> 0nil< 0nil"not found""not found"Writers
get()+expire()and never reads the shared counters.self.last. Incremental sync picks it up, so nothing is broadcast. Explicitdel()/reset()still bumpdelete_count.Readers
get_stale(), so a slot revived without advancingkey_countis still found.ttl()says"not found"andget_stale()isnil.ttl()alone also reports"not found"for a node exactly 5 ms past expiry, because-5isNGX_DECLINED.get(), so expired values are skipped.Aliases
Concurrent registration can leave several slots for one name. A reverse set is allocated only for such names. When the chosen slot is reclaimed, another alias takes over. On explicit deletion, aliases are removed after checking that each slot is still owned by this name.
key_countre-creationkey_countis an ordinary dict node. If it is evicted, it restarts at 1, and the low numbers are reissued below every reader'sself.last. Without a broadcast, readers need another signal:incr()returns 1 publishes a uniquegenerationtoken, and readers do a full sync when it changes.key_countagain after publishing.sync()reads the token beforekey_count, so a re-creation in between is seen on the next sync.key_count < self.lastalso forces a full sync.Results
Hot path, 57 keys per request with
exptime:Without
exptime, both are 0.Soak: 8 workers, metrics read by a privileged agent that serves no traffic (APISIX topology), 20k rps. Measured while this PR was stacked on #23; the key index changes are the same.
delete_count> 10kTests
Unit: 72 tests (
testPrintfTablefails on LuaJIT only, as on main). They cover:ttl()−5 ms sentinel, on scan and on first import;key_countre-creation: token lost, publish failed, token read order, reader-published token.24 of them fail against main's
prometheus_keys.lua.Integration:
integration/index.luaruns on a realngx.shared.DICT. It covers a fresh reader finding a revived slot, an expiry between the fast-path read and renewal, alias recovery and ownership, and the public API end to end. Against main it fails with "revived slot was not discovered".Fuzz: a randomized multi-instance check on a real shared dict, 120 seeds (half with token publishes failing 50% of the time), with random eviction of
key_count, the token and slots. It finds 0 violations.Interleaving check: every dict op of the reader and writer is a scheduling point, with injected eviction and token-publish failures. It finds 0 violations over 400 seeds.
CI: Lua 5.2 and integration jobs; luacheck clean.
Trade-offs
key_countgrows ~1.9× faster than main under returning-metric churn: a reclaimed slot is never reused.remove_expired_keys()keeps main's order (scan, then flush), so a reference is dropped on the round after the one that reclaims its node. The churn soak measured +~25% with the opposite order; with this order up to one more interval of names is kept.ttl()reads exactly 0 when a worker first imports it is treated as permanent and never rechecked; if it was the expiry millisecond, that worker keeps its local reference.incr(key_count) == 1and publishing the token leaves that re-creation unannounced.key_countover a slot that is not written yet, so a reader that has already passed it misses it until the name gets a duplicate slot. main heals this through its frequent broadcasts; with those gone, it heals more slowly here.Summary by CodeRabbit
Bug Fixes
Tests
Documentation