lb: remove "consistent_hashing", default to "rendezvous_hashing" instead

We havn't been using Consistent Hashing (`class HashRing`) ever since we
hadded Rendezvous Hashing, because the latter is superior.  Let's
remove this obsolete feature.

Author: MaxKellermann

debian/changelog

@@ -3,6 +3,7 @@ cm4all-beng-proxy (21.11) unstable; urgency=low
   * spawn: fix memory warning timer bug
   * io_uring: disable the iowait state
   * lb: add whitelist for REJECT_CLIENT and TARPIT_CLIENT
+  * lb: remove "consistent_hashing", default to "rendezvous_hashing" instead
   * call sodium_init() on startup (to select optimized cipher implementations)
   * fix build with glibc 2.43
 

doc/lb.rst

@@ -114,9 +114,7 @@ Example::
 
 - ``sticky_method``: one of:
 
-  - ``consistent_hashing`` (the default)
-
-  - ``rendezvous_hashing``
+  - ``rendezvous_hashing`` (the default)
 
   - ``cache``: an assignment cache. The advantage of that cache is that
     existing clients will not be reassigned when new nodes appear. The
@@ -213,8 +211,8 @@ This requires ``avahi-daemon`` to be installed and running. And, of
 course, it requires the pool members to publish their service.
 
 If ``sticky`` is enabled on the pool, then :program:`beng-lb` uses
-`consistent hashing
-<https://en.wikipedia.org/wiki/Consistent_hashing>`__ to pick a member
+`rendezvous hashing
+<https://en.wikipedia.org/wiki/Rendezvous_hashing>`__ to pick a member
 (to reduce member reassignments).
 
 .. _lb_protocol:

libcommon

@@ -786,7 +786,6 @@ executable(
   'src/lb/Setup.cxx',
   'src/lb/GotoMap.cxx',
   'src/lb/Branch.cxx',
-  'src/lb/MemberHash.cxx',
   'src/lb/Cluster.cxx',
   'src/lb/TranslationHandler.cxx',
   'src/lb/TranslationCache.cxx',

meson.build

@@ -4,7 +4,6 @@
 
 #include "Cluster.hxx"
 #include "ClusterConfig.hxx"
-#include "MemberHash.hxx"
 #include "Context.hxx"
 #include "MonitorStock.hxx"
 #include "MonitorRef.hxx"
@@ -68,9 +67,6 @@ CalculateStickyHash(std::span<const std::byte> source) noexcept
 
 #ifdef HAVE_AVAHI
 
-class LbCluster::StickyRing final
-	: public MemberHashRing<ZeroconfMemberMap::iterator> {};
-
 class LbCluster::ZeroconfMember final : LeakDetector {
 	InetAddress address;
 
@@ -424,29 +420,6 @@ LbCluster::PickNextGoodZeroconf(const Expiry now) noexcept
 					false);
 }
 
-inline LbCluster::ZeroconfMemberMap::const_reference
-LbCluster::PickZeroconfHashRing(Expiry now,
-				sticky_hash_t sticky_hash) noexcept
-{
-	assert(!active_zeroconf_members.empty());
-	assert(sticky_ring != nullptr);
-
-	auto i = sticky_ring->Pick(sticky_hash);
-	assert(i != zeroconf_members.end());
-
-	unsigned retries = active_zeroconf_members.size();
-	while (true) {
-		if (--retries == 0 ||
-		    i->second.GetFailureInfo().Check(now))
-			return *i;
-
-		/* the node is known-bad; pick the next one in the ring */
-		const auto next = sticky_ring->FindNext(sticky_hash);
-		sticky_hash = next.first;
-		i = next.second;
-	}
-}
-
 inline LbCluster::ZeroconfMemberMap::const_reference
 LbCluster::PickZeroconfRendezvous(Expiry now, const Arch arch,
 				  std::span<const std::byte> sticky_source) noexcept
@@ -527,9 +500,6 @@ LbCluster::PickZeroconf(const Expiry now, Arch arch,
 		assert(config.sticky_mode != StickyMode::NONE);
 
 		switch (config.sticky_method) {
-		case LbClusterConfig::StickyMethod::CONSISTENT_HASHING:
-			return &PickZeroconfHashRing(now, sticky_hash);
-
 		case LbClusterConfig::StickyMethod::RENDEZVOUS_HASHING:
 			return &PickZeroconfRendezvous(now, arch, sticky_source);
 
@@ -563,24 +533,6 @@ LbCluster::FillActive() noexcept
 
 	for (auto i = zeroconf_members.begin(); i != zeroconf_members.end(); ++i)
 		active_zeroconf_members.push_back(i);
-
-	switch (config.sticky_method) {
-	case LbClusterConfig::StickyMethod::CONSISTENT_HASHING:
-		if (sticky_ring == nullptr)
-			/* lazy allocation */
-			sticky_ring = std::make_unique<StickyRing>();
-
-		BuildMemberHashRing(*sticky_ring, active_zeroconf_members,
-				    [](ZeroconfMemberMap::const_iterator member) noexcept {
-					    return member->second.GetAddress();
-				    });
-
-		break;
-
-	case LbClusterConfig::StickyMethod::RENDEZVOUS_HASHING:
-	case LbClusterConfig::StickyMethod::CACHE:
-		break;
-	}
 }
 
 class LbCluster::ZeroconfHttpConnect final : StockGetHandler, Lease, Cancellable {

src/lb/Cluster.cxx

@@ -76,13 +76,6 @@ class LbCluster final
 	 */
 	std::unique_ptr<Avahi::ServiceExplorer> explorer;
 
-	class StickyRing;
-
-	/**
-	 * For consistent hashing.  It is populated by FillActive().
-	 */
-	std::unique_ptr<StickyRing> sticky_ring;
-
 	/**
 	 * @see LbClusterConfig::sticky_cache
 	 */
@@ -207,16 +200,6 @@ private:
 						      std::span<const std::byte> sticky_source,
 						      sticky_hash_t sticky_hash) noexcept;
 
-	/**
-	 * Like PickZeroconf(), but pick using Consistent Hashing (via
-	 * #HashRing).
-	 *
-	 * To be called by PickZeroconf(), which has already
-	 * lazy-initialized and verified everything.
-	 */
-	ZeroconfMemberMap::const_reference PickZeroconfHashRing(Expiry now,
-								sticky_hash_t sticky_hash) noexcept;
-
 	/**
 	 * Like PickZeroconf(), but pick using Rendezvous Hashing.
 	 *
@@ -261,7 +244,7 @@ private:
 
 private:
 	/**
-	 * Fill #active_members and #sticky_ring.
+	 * Fill #active_members.
 	 *
 	 * Zeroconf only.
 	 */

src/lb/Cluster.hxx

@@ -99,15 +99,13 @@ struct LbClusterConfig {
 
 #ifdef HAVE_AVAHI
 	enum class StickyMethod : uint_least8_t {
-		CONSISTENT_HASHING,
-
 		RENDEZVOUS_HASHING,
 
 		/**
 		 * Enable the #StickyCache for Zeroconf?
 		 */
 		CACHE,
-	} sticky_method = StickyMethod::CONSISTENT_HASHING;
+	} sticky_method = StickyMethod::RENDEZVOUS_HASHING;
 #endif
 
 	LbSimpleHttpResponse fallback;

src/lb/ClusterConfig.hxx

@@ -567,7 +567,8 @@ static LbClusterConfig::StickyMethod
 ParseStickyMethod(const char *s)
 {
 	if (StringIsEqual(s, "consistent_hashing"))
-		return LbClusterConfig::StickyMethod::CONSISTENT_HASHING;
+		// TODO remove legacy setting
+		return LbClusterConfig::StickyMethod::RENDEZVOUS_HASHING;
 	else if (StringIsEqual(s, "rendezvous_hashing"))
 		return LbClusterConfig::StickyMethod::RENDEZVOUS_HASHING;
 	else if (StringIsEqual(s, "cache"))
@@ -602,7 +603,7 @@ LbConfigParser::Cluster::ParseLine(FileLineParser &line)
 #ifdef HAVE_AVAHI
 		config.sticky_method = line.NextBool()
 			? LbClusterConfig::StickyMethod::CACHE
-			: LbClusterConfig::StickyMethod::CONSISTENT_HASHING;
+			: LbClusterConfig::StickyMethod::RENDEZVOUS_HASHING;
 		line.ExpectEnd();
 #else
 		throw LineParser::Error("Zeroconf support is disabled at compile time");