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Fix cluster subsetting silent failure when peer URIs are IPv6 (SI-52492) - #1169
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ZKDeterministicSubsettingMetadataProvider compared the host's FQDN identity against URI.getHost()-form peer cluster URIs. When the d2 client materializes peer URIs in IPv6 form (typical with -Djava.net.preferIPv6Addresses=true, default-on across most LinkedIn services), the FQDN never matches the "[ipv6]" form returned by URI.getHost() on JDK 17, so instanceId = -1 and the subsetter fell back to the full host set. The failure was silent because the no-match branch logged at DEBUG. Resolve a Set<String> of candidate identities (FQDN, IPv4, bracketed and unbracketed IPv6, scope ids preserved) at construction via an injectable AddressResolver and match peer URIs with Set.contains. Bump the no-match log to WARN with a once-per-outage guard that re-arms on a successful match so flapping clusters don't spam the log. Public constructor signatures are preserved so consumers (notably container's D2ClientFactory) need no changes. Companion fix to grpc-infra PR #5819. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Inet6Address.getHostAddress() always returns the RFC 4291 fully-expanded
form ("2a04:f547:43:e66a:0:0:0:1a95"), but URI.getHost() returns
whatever was literally in the URI string — could be expanded or
compressed ("[2a04:f547:43:e66a::1a95]"). Java has no public API to
produce the compressed (RFC 5952) form, so if d2 ever materializes a
peer URI in compressed IPv6 the candidate set's expanded form silently
fails to match and the subsetter falls back to all hosts.
Add a private static canonicalizeIpv6IfApplicable that round-trips any
string containing ':' through InetAddress.getByName().getHostAddress().
FQDNs and IPv4 literals short-circuit on the colon check so no DNS work
happens on the hot path. Run both the local-side computeIdentities (so
the candidate set is in canonical form by construction) and the peer-
side getSubsettingMetadata lookup (so peer URI hosts get the same
treatment) through it; this keeps the two sides in lockstep even for
edge cases like IPv4-mapped IPv6 (::ffff:a.b.c.d), which getByName
auto-converts to Inet4Address — brackets are dropped in that case so a
canonicalized peer URI matches a local Inet4Address candidate.
Mirrors the same fix in grpc-infra PR #5819 (per psanglik's review on
that PR and the shared HostIdentities extraction that followed). Companion
to SI-52492.
7/7 ZKDeterministicSubsettingMetadataProviderTest cases pass.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
shivamgupta1
approved these changes
Jun 9, 2026
shivamgupta1
reviewed
Jun 9, 2026
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| if (uriItem.getVersion() != _peerClusterVersion) |
Contributor
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The DNS resolution might be expensive but it only runs when there's a change to the cluster's version.
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Summary
ZKDeterministicSubsettingMetadataProvidercompared the host's FQDN identity againstURI.getHost()-form peer cluster URIs. When the d2 client materializes peer URIs in IPv6 form (typical with-Djava.net.preferIPv6Addresses=true,default-on across most LinkedIn services), the FQDN never matches the
[ipv6]form returned byURI.getHost()on JDK 17, soinstanceId = -1and the subsetter fell back to the full host set. The failure was silent because the no-matchbranch logged at DEBUG.
Set<String>of candidate identities (FQDN, IPv4, bracketed and unbracketed IPv6, scope ids preserved) at construction via an injectableAddressResolver, and match peer URIs withSet.containsinstead ofindexOf._warnedIdentityNotFoundguard that re-arms on a successful match so flapping clusters don't spam the log.D2ClientFactory) need no changes.Testing Done
testCandidateIdentities_includesFqdnIpv4AndIpv6,testCandidateIdentities_dnsFailure_fallsBackToFqdn,testCandidateIdentities_keepsIpv6ScopeId(usesInet6Address.getByAddressfor cross-OS portability),testGetSubsettingMetadata_matchesViaIpv6,testGetSubsettingMetadata_noMatch_warnsOnceThenReArmsOnRecovery.testGetSubsettingMetadata(FQDN match path) still passes../gradlew :d2:test --tests com.linkedin.d2.balancer.subsetting.ZKDeterministicSubsettingMetadataProviderTest(JDK 11, Gradle 6.9.4).RelativeLoadBalancerStrategy.<svc>-https-LoadBalancerStrategy.TotalHostsCountto drop from full cluster size to ~_minClusterSubsetSizefor clusters with_enableClusterSubsetting=true.