From 7baf378420e94c2ac4f086b54b3c2481ed5950f8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Piotr=20Wo=CC=81jcik?=
 <3044353+pwojcikdev@users.noreply.github.com>
Date: Tue, 27 Aug 2024 20:19:28 +0200
Subject: [PATCH] Implement frontier scanning

---
 nano/core_test/toml.cpp                       |   4 +-
 nano/lib/rate_limiting.hpp                    |   4 +-
 nano/lib/stats_enums.hpp                      |  29 +-
 nano/lib/thread_roles.cpp                     |   5 +-
 nano/lib/thread_roles.hpp                     |   1 +
 nano/node/CMakeLists.txt                      |   8 +-
 nano/node/bootstrap/bootstrap_config.cpp      |   2 +
 nano/node/bootstrap/bootstrap_config.hpp      |  20 +-
 .../bootstrap_ascending/frontier_scan.cpp     | 187 ++++++++++++
 .../bootstrap_ascending/frontier_scan.hpp     |  76 +++++
 nano/node/bootstrap_ascending/service.cpp     | 269 ++++++++++++++++--
 nano/node/bootstrap_ascending/service.hpp     |  20 +-
 nano/node/messages.hpp                        |   2 +-
 13 files changed, 588 insertions(+), 39 deletions(-)
 create mode 100644 nano/node/bootstrap_ascending/frontier_scan.cpp
 create mode 100644 nano/node/bootstrap_ascending/frontier_scan.hpp

diff --git a/nano/core_test/toml.cpp b/nano/core_test/toml.cpp
index eb1cb68ae..1ef6deba9 100644
--- a/nano/core_test/toml.cpp
+++ b/nano/core_test/toml.cpp
@@ -601,7 +601,8 @@ TEST (toml, daemon_config_deserialize_no_defaults)
 
 	[node.bootstrap_ascending]
 	enable = false
-	enable_database_scan = false
+	enable_frontier_scan = false
+	enable_database_scan = true
 	enable_dependency_walker = false
 	channel_limit = 999
 	database_rate_limit = 999
@@ -780,6 +781,7 @@ TEST (toml, daemon_config_deserialize_no_defaults)
 	ASSERT_NE (conf.node.vote_processor.batch_size, defaults.node.vote_processor.batch_size);
 
 	ASSERT_NE (conf.node.bootstrap_ascending.enable, defaults.node.bootstrap_ascending.enable);
+	ASSERT_NE (conf.node.bootstrap_ascending.enable_frontier_scan, defaults.node.bootstrap_ascending.enable_frontier_scan);
 	ASSERT_NE (conf.node.bootstrap_ascending.enable_database_scan, defaults.node.bootstrap_ascending.enable_database_scan);
 	ASSERT_NE (conf.node.bootstrap_ascending.enable_dependency_walker, defaults.node.bootstrap_ascending.enable_dependency_walker);
 	ASSERT_NE (conf.node.bootstrap_ascending.channel_limit, defaults.node.bootstrap_ascending.channel_limit);
diff --git a/nano/lib/rate_limiting.hpp b/nano/lib/rate_limiting.hpp
index f67246787..5e209f4d6 100644
--- a/nano/lib/rate_limiting.hpp
+++ b/nano/lib/rate_limiting.hpp
@@ -64,10 +64,10 @@ class rate_limiter final
 {
 public:
 	// initialize with limit 0 = unbounded
-	rate_limiter (std::size_t limit, double burst_ratio);
+	rate_limiter (std::size_t limit, double burst_ratio = 1.0);
 
 	bool should_pass (std::size_t buffer_size);
-	void reset (std::size_t limit, double burst_ratio);
+	void reset (std::size_t limit, double burst_ratio = 1.0);
 
 private:
 	nano::rate::token_bucket bucket;
diff --git a/nano/lib/stats_enums.hpp b/nano/lib/stats_enums.hpp
index c8f3e5f81..e00adb8b4 100644
--- a/nano/lib/stats_enums.hpp
+++ b/nano/lib/stats_enums.hpp
@@ -60,11 +60,13 @@ enum class type
 	blockprocessor_overfill,
 	bootstrap_ascending,
 	bootstrap_ascending_accounts,
-	bootstrap_ascending_verify,
+	bootstrap_ascending_verify_blocks,
+	bootstrap_ascending_verify_frontiers,
 	bootstrap_ascending_process,
 	bootstrap_ascending_request,
 	bootstrap_ascending_reply,
 	bootstrap_ascending_next,
+	bootstrap_ascending_frontiers,
 	bootstrap_server,
 	bootstrap_server_request,
 	bootstrap_server_overfill,
@@ -117,6 +119,7 @@ enum class detail
 	inserted,
 	erased,
 	request,
+	request_failed,
 	broadcast,
 	cleanup,
 	top,
@@ -423,7 +426,7 @@ enum class detail
 	missing_cookie,
 	invalid_genesis,
 
-	// bootstrap ascending
+	// bootstrap_ascending
 	missing_tag,
 	reply,
 	throttled,
@@ -431,13 +434,18 @@ enum class detail
 	timeout,
 	nothing_new,
 	account_info_empty,
+	frontiers_empty,
 	loop_database,
 	loop_dependencies,
+	loop_frontiers,
+	loop_frontiers_processing,
 	duplicate_request,
 	invalid_response_type,
 	timestamp_reset,
+	process_frontiers,
+	dropped_frontiers,
 
-	// bootstrap ascending accounts
+	// bootstrap_ascending_accounts
 	prioritize,
 	prioritize_failed,
 	block,
@@ -446,11 +454,21 @@ enum class detail
 	dependency_update,
 	dependency_update_failed,
 
+	// bootstrap_ascending_frontiers
+	done,
+	done_range,
+	done_empty,
+	next_by_requests,
+	next_by_timestamp,
+	advance,
+	advance_failed,
+
 	next_none,
 	next_priority,
 	next_database,
 	next_blocking,
 	next_dependency,
+	next_frontier,
 
 	blocking_insert,
 	blocking_erase_overflow,
@@ -461,6 +479,10 @@ enum class detail
 	deprioritize,
 	deprioritize_failed,
 	sync_dependencies,
+	frontiers_processed,
+	frontiers_prioritized,
+	frontiers_outdated,
+	frontiers_pending,
 
 	request_blocks,
 	request_account_info,
@@ -468,6 +490,7 @@ enum class detail
 	// active
 	started_hinted,
 	started_optimistic,
+
 	// rep_crawler
 	channel_dead,
 	query_target_failed,
diff --git a/nano/lib/thread_roles.cpp b/nano/lib/thread_roles.cpp
index 76f467cd0..913c4aaa7 100644
--- a/nano/lib/thread_roles.cpp
+++ b/nano/lib/thread_roles.cpp
@@ -101,7 +101,10 @@ std::string nano::thread_role::get_string (nano::thread_role::name role)
 			thread_role_name_string = "Voting que";
 			break;
 		case nano::thread_role::name::ascending_bootstrap:
-			thread_role_name_string = "Bootstrap asc";
+			thread_role_name_string = "Ascboot";
+			break;
+		case nano::thread_role::name::ascending_bootstrap_worker:
+			thread_role_name_string = "Ascboot work";
 			break;
 		case nano::thread_role::name::bootstrap_server:
 			thread_role_name_string = "Bootstrap serv";
diff --git a/nano/lib/thread_roles.hpp b/nano/lib/thread_roles.hpp
index 5896318c4..9092392a7 100644
--- a/nano/lib/thread_roles.hpp
+++ b/nano/lib/thread_roles.hpp
@@ -41,6 +41,7 @@ enum class name
 	bootstrap_server,
 	telemetry,
 	ascending_bootstrap,
+	ascending_bootstrap_worker,
 	bootstrap_server_requests,
 	bootstrap_server_responses,
 	scheduler_hinted,
diff --git a/nano/node/CMakeLists.txt b/nano/node/CMakeLists.txt
index 419e00a72..548d16c0d 100644
--- a/nano/node/CMakeLists.txt
+++ b/nano/node/CMakeLists.txt
@@ -44,13 +44,15 @@ add_library(
   bootstrap/bootstrap.cpp
   bootstrap/bootstrap_server.hpp
   bootstrap/bootstrap_server.cpp
-  bootstrap_ascending/common.hpp
-  bootstrap_ascending/throttle.hpp
-  bootstrap_ascending/throttle.cpp
   bootstrap_ascending/account_sets.hpp
   bootstrap_ascending/account_sets.cpp
   bootstrap_ascending/database_scan.hpp
   bootstrap_ascending/database_scan.cpp
+  bootstrap_ascending/common.hpp
+  bootstrap_ascending/frontier_scan.hpp
+  bootstrap_ascending/frontier_scan.cpp
+  bootstrap_ascending/throttle.hpp
+  bootstrap_ascending/throttle.cpp
   bootstrap_ascending/peer_scoring.hpp
   bootstrap_ascending/peer_scoring.cpp
   bootstrap_ascending/service.hpp
diff --git a/nano/node/bootstrap/bootstrap_config.cpp b/nano/node/bootstrap/bootstrap_config.cpp
index ff1e847da..2264776e0 100644
--- a/nano/node/bootstrap/bootstrap_config.cpp
+++ b/nano/node/bootstrap/bootstrap_config.cpp
@@ -34,6 +34,7 @@ nano::error nano::bootstrap_ascending_config::deserialize (nano::tomlconfig & to
 	toml.get ("enable", enable);
 	toml.get ("enable_database_scan", enable_database_scan);
 	toml.get ("enable_dependency_walker", enable_dependency_walker);
+	toml.get ("enable_frontier_scan", enable_frontier_scan);
 
 	toml.get ("channel_limit", channel_limit);
 	toml.get ("database_rate_limit", database_rate_limit);
@@ -59,6 +60,7 @@ nano::error nano::bootstrap_ascending_config::serialize (nano::tomlconfig & toml
 	toml.put ("enable", enable, "Enable or disable the ascending bootstrap. Disabling it is not recommended and will prevent the node from syncing.\ntype:bool");
 	toml.put ("enable_database_scan", enable_database_scan, "Enable or disable the 'database scan` strategy for the ascending bootstrap.\ntype:bool");
 	toml.put ("enable_dependency_walker", enable_dependency_walker, "Enable or disable the 'dependency walker` strategy for the ascending bootstrap.\ntype:bool");
+	toml.put ("enable_frontier_scan", enable_frontier_scan, "Enable or disable the 'frontier scan` strategy for the ascending bootstrap.\ntype:bool");
 
 	toml.put ("channel_limit", channel_limit, "Maximum number of un-responded requests per channel.\nNote: changing to unlimited (0) is not recommended.\ntype:uint64");
 	toml.put ("database_rate_limit", database_rate_limit, "Rate limit on scanning accounts and pending entries from database.\nNote: changing to unlimited (0) is not recommended as this operation competes for resources on querying the database.\ntype:uint64");
diff --git a/nano/node/bootstrap/bootstrap_config.hpp b/nano/node/bootstrap/bootstrap_config.hpp
index af7b98bcb..cd2c9b8d7 100644
--- a/nano/node/bootstrap/bootstrap_config.hpp
+++ b/nano/node/bootstrap/bootstrap_config.hpp
@@ -22,6 +22,19 @@ public:
 	std::chrono::milliseconds cooldown{ 1000 * 3 };
 };
 
+// TODO: This should be moved next to `frontier_scan` class
+class frontier_scan_config final
+{
+public:
+	// TODO: Serialize & deserialize
+
+	unsigned head_parallelistm{ 128 };
+	unsigned consideration_count{ 4 };
+	std::size_t candidates{ 1000 };
+	std::chrono::milliseconds cooldown{ 1000 * 5 };
+	std::size_t max_pending{ 16 };
+};
+
 // TODO: This should be moved next to `bootstrap_ascending` class
 class bootstrap_ascending_config final
 {
@@ -31,12 +44,14 @@ public:
 
 public:
 	bool enable{ true };
-	bool enable_database_scan{ true };
+	bool enable_database_scan{ false };
 	bool enable_dependency_walker{ true };
+	bool enable_frontier_scan{ true };
 
 	// Maximum number of un-responded requests per channel, should be lower or equal to bootstrap server max queue size
 	std::size_t channel_limit{ 16 };
 	std::size_t database_rate_limit{ 256 };
+	std::size_t frontier_rate_limit{ 100 };
 	std::size_t database_warmup_ratio{ 10 };
 	std::size_t max_pull_count{ nano::bootstrap_server::max_blocks };
 	std::chrono::milliseconds request_timeout{ 1000 * 5 };
@@ -45,6 +60,7 @@ public:
 	std::size_t block_processor_threshold{ 1000 };
 	std::size_t max_requests{ 1024 };
 
-	nano::account_sets_config account_sets;
+	account_sets_config account_sets;
+	frontier_scan_config frontier_scan;
 };
 }
diff --git a/nano/node/bootstrap_ascending/frontier_scan.cpp b/nano/node/bootstrap_ascending/frontier_scan.cpp
new file mode 100644
index 000000000..28fab99de
--- /dev/null
+++ b/nano/node/bootstrap_ascending/frontier_scan.cpp
@@ -0,0 +1,187 @@
+#include <nano/node/bootstrap_ascending/frontier_scan.hpp>
+
+#include <boost/multiprecision/cpp_dec_float.hpp>
+#include <boost/multiprecision/cpp_int.hpp>
+
+nano::bootstrap_ascending::frontier_scan::frontier_scan (frontier_scan_config const & config_a, nano::stats & stats_a) :
+	config{ config_a },
+	stats{ stats_a }
+{
+	// Divide nano::account numeric range into consecutive and equal ranges
+	nano::uint256_t max_account = std::numeric_limits<nano::uint256_t>::max ();
+	nano::uint256_t range_size = max_account / config.head_parallelistm;
+
+	for (unsigned i = 0; i < config.head_parallelistm; ++i)
+	{
+		// Start at 1 to avoid the burn account
+		nano::uint256_t start = (i == 0) ? 1 : i * range_size;
+		nano::uint256_t end = (i == config.head_parallelistm - 1) ? max_account : start + range_size;
+
+		heads.emplace_back (frontier_head{ nano::account{ start }, nano::account{ end } });
+	}
+
+	release_assert (!heads.empty ());
+}
+
+nano::account nano::bootstrap_ascending::frontier_scan::next ()
+{
+	auto const cutoff = std::chrono::steady_clock::now () - config.cooldown;
+
+	auto & heads_by_timestamp = heads.get<tag_timestamp> ();
+	for (auto it = heads_by_timestamp.begin (); it != heads_by_timestamp.end (); ++it)
+	{
+		auto const & head = *it;
+
+		if (head.requests < config.consideration_count || head.timestamp < cutoff)
+		{
+			stats.inc (nano::stat::type::bootstrap_ascending_frontiers, (head.requests < config.consideration_count) ? nano::stat::detail::next_by_requests : nano::stat::detail::next_by_timestamp);
+
+			debug_assert (head.next.number () >= head.start.number ());
+			debug_assert (head.next.number () < head.end.number ());
+
+			auto result = head.next;
+
+			heads_by_timestamp.modify (it, [this] (auto & entry) {
+				entry.requests += 1;
+				entry.timestamp = std::chrono::steady_clock::now ();
+			});
+
+			return result;
+		}
+	}
+
+	stats.inc (nano::stat::type::bootstrap_ascending_frontiers, nano::stat::detail::next_none);
+	return { 0 };
+}
+
+bool nano::bootstrap_ascending::frontier_scan::process (nano::account start, std::deque<std::pair<nano::account, nano::block_hash>> const & response)
+{
+	debug_assert (std::all_of (response.begin (), response.end (), [&] (auto const & pair) { return pair.first.number () >= start.number (); }));
+
+	stats.inc (nano::stat::type::bootstrap_ascending_frontiers, nano::stat::detail::process);
+
+	// Find the first head with head.start <= start
+	auto & heads_by_start = heads.get<tag_start> ();
+	auto it = heads_by_start.upper_bound (start);
+	release_assert (it != heads_by_start.begin ());
+	it = std::prev (it);
+
+	bool done = false;
+	heads_by_start.modify (it, [this, &response, &done] (frontier_head & entry) {
+		entry.completed += 1;
+
+		for (auto const & [account, _] : response)
+		{
+			// Only consider candidates that actually advance the current frontier
+			if (account.number () > entry.next.number ())
+			{
+				entry.candidates.insert (account);
+			}
+		}
+
+		// Trim the candidates
+		while (entry.candidates.size () > config.candidates)
+		{
+			release_assert (!entry.candidates.empty ());
+			entry.candidates.erase (std::prev (entry.candidates.end ()));
+		}
+
+		// Special case for the last frontier head that won't receive larger than max frontier
+		if (entry.completed >= config.consideration_count * 2 && entry.candidates.empty ())
+		{
+			stats.inc (nano::stat::type::bootstrap_ascending_frontiers, nano::stat::detail::done_empty);
+			entry.candidates.insert (entry.end);
+		}
+
+		// Check if done
+		if (entry.completed >= config.consideration_count && !entry.candidates.empty ())
+		{
+			stats.inc (nano::stat::type::bootstrap_ascending_frontiers, nano::stat::detail::done);
+
+			// Take the last candidate as the next frontier
+			release_assert (!entry.candidates.empty ());
+			auto it = std::prev (entry.candidates.end ());
+
+			debug_assert (entry.next.number () < it->number ());
+			entry.next = *it;
+			entry.processed += entry.candidates.size ();
+			entry.candidates.clear ();
+			entry.requests = 0;
+			entry.completed = 0;
+			entry.timestamp = {};
+
+			// Bound the search range
+			if (entry.next.number () >= entry.end.number ())
+			{
+				stats.inc (nano::stat::type::bootstrap_ascending_frontiers, nano::stat::detail::done_range);
+				entry.next = entry.start;
+			}
+
+			done = true;
+		}
+	});
+
+	return done;
+}
+
+std::unique_ptr<nano::container_info_component> nano::bootstrap_ascending::frontier_scan::collect_container_info (std::string const & name)
+{
+	auto collect_progress = [&] () {
+		auto composite = std::make_unique<container_info_composite> ("progress");
+		for (int n = 0; n < heads.size (); ++n)
+		{
+			auto const & head = heads[n];
+
+			boost::multiprecision::cpp_dec_float_50 start{ head.start.number ().str () };
+			boost::multiprecision::cpp_dec_float_50 next{ head.next.number ().str () };
+			boost::multiprecision::cpp_dec_float_50 end{ head.end.number ().str () };
+
+			boost::multiprecision::cpp_dec_float_50 progress = (next - start) * boost::multiprecision::cpp_dec_float_50 (1000000) / (end - start);
+
+			composite->add_component (std::make_unique<container_info_leaf> (container_info{ std::to_string (n), progress.convert_to<std::uint64_t> (), 6 }));
+		}
+		return composite;
+	};
+
+	auto collect_candidates = [&] () {
+		auto composite = std::make_unique<container_info_composite> ("candidates");
+		for (int n = 0; n < heads.size (); ++n)
+		{
+			auto const & head = heads[n];
+			composite->add_component (std::make_unique<container_info_leaf> (container_info{ std::to_string (n), head.candidates.size (), 0 }));
+		}
+		return composite;
+	};
+
+	auto collect_responses = [&] () {
+		auto composite = std::make_unique<container_info_composite> ("responses");
+		for (int n = 0; n < heads.size (); ++n)
+		{
+			auto const & head = heads[n];
+			composite->add_component (std::make_unique<container_info_leaf> (container_info{ std::to_string (n), head.completed, 0 }));
+		}
+		return composite;
+	};
+
+	auto collect_processed = [&] () {
+		auto composite = std::make_unique<container_info_composite> ("processed");
+		for (int n = 0; n < heads.size (); ++n)
+		{
+			auto const & head = heads[n];
+			composite->add_component (std::make_unique<container_info_leaf> (container_info{ std::to_string (n), head.processed, 0 }));
+		}
+		return composite;
+	};
+
+	auto total_processed = std::accumulate (heads.begin (), heads.end (), std::size_t{ 0 }, [] (auto total, auto const & head) {
+		return total + head.processed;
+	});
+
+	auto composite = std::make_unique<container_info_composite> (name);
+	composite->add_component (std::make_unique<container_info_leaf> (container_info{ "total_processed", total_processed, 0 }));
+	composite->add_component (collect_progress ());
+	composite->add_component (collect_candidates ());
+	composite->add_component (collect_responses ());
+	composite->add_component (collect_processed ());
+	return composite;
+}
diff --git a/nano/node/bootstrap_ascending/frontier_scan.hpp b/nano/node/bootstrap_ascending/frontier_scan.hpp
new file mode 100644
index 000000000..382c59b47
--- /dev/null
+++ b/nano/node/bootstrap_ascending/frontier_scan.hpp
@@ -0,0 +1,76 @@
+#pragma once
+
+#include <nano/node/bootstrap/bootstrap_config.hpp>
+#include <nano/node/bootstrap_ascending/common.hpp>
+
+#include <boost/multi_index/hashed_index.hpp>
+#include <boost/multi_index/mem_fun.hpp>
+#include <boost/multi_index/member.hpp>
+#include <boost/multi_index/ordered_index.hpp>
+#include <boost/multi_index/random_access_index.hpp>
+#include <boost/multi_index/sequenced_index.hpp>
+#include <boost/multi_index_container.hpp>
+
+#include <chrono>
+#include <map>
+#include <set>
+
+namespace mi = boost::multi_index;
+
+namespace nano::bootstrap_ascending
+{
+class frontier_scan
+{
+public:
+	frontier_scan (frontier_scan_config const &, nano::stats &);
+
+	nano::account next ();
+	bool process (nano::account start, std::deque<std::pair<nano::account, nano::block_hash>> const & response);
+
+	std::unique_ptr<nano::container_info_component> collect_container_info (std::string const & name);
+
+private: // Dependencies
+	frontier_scan_config const & config;
+	nano::stats & stats;
+
+private:
+	struct frontier_head
+	{
+		frontier_head (nano::account start_a, nano::account end_a) :
+			start{ start_a },
+			end{ end_a },
+			next{ start_a }
+		{
+		}
+
+		// The range of accounts to scan is [start, end)
+		nano::account start;
+		nano::account end;
+
+		nano::account next;
+		std::set<nano::account> candidates;
+
+		unsigned requests{ 0 };
+		unsigned completed{ 0 };
+		std::chrono::steady_clock::time_point timestamp{};
+		size_t processed{ 0 }; // Total number of accounts processed
+	};
+
+	// clang-format off
+	class tag_sequenced {};
+	class tag_start {};
+	class tag_timestamp {};
+
+	using ordered_heads = boost::multi_index_container<frontier_head,
+	mi::indexed_by<
+		mi::random_access<mi::tag<tag_sequenced>>,
+		mi::ordered_unique<mi::tag<tag_start>,
+			mi::member<frontier_head, nano::account, &frontier_head::start>>,
+		mi::ordered_non_unique<mi::tag<tag_timestamp>,
+			mi::member<frontier_head, std::chrono::steady_clock::time_point, &frontier_head::timestamp>>
+	>>;
+	// clang-format on
+
+	ordered_heads heads;
+};
+}
\ No newline at end of file
diff --git a/nano/node/bootstrap_ascending/service.cpp b/nano/node/bootstrap_ascending/service.cpp
index 450767f03..1de0f696c 100644
--- a/nano/node/bootstrap_ascending/service.cpp
+++ b/nano/node/bootstrap_ascending/service.cpp
@@ -29,9 +29,12 @@ nano::bootstrap_ascending::service::service (nano::node_config const & node_conf
 	logger{ logger_a },
 	accounts{ config.account_sets, stats },
 	database_scan{ ledger },
+	frontiers{ config.frontier_scan, stats },
 	throttle{ compute_throttle_size () },
 	scoring{ config, node_config_a.network_params.network },
-	database_limiter{ config.database_rate_limit, 1.0 }
+	database_limiter{ config.database_rate_limit },
+	frontiers_limiter{ config.frontier_rate_limit },
+	workers{ 1, nano::thread_role::name::ascending_bootstrap_worker }
 {
 	// TODO: This is called from a very congested blockprocessor thread. Offload this work to a dedicated processing thread
 	block_processor.batch_processed.add ([this] (auto const & batch) {
@@ -57,6 +60,7 @@ nano::bootstrap_ascending::service::~service ()
 	debug_assert (!priorities_thread.joinable ());
 	debug_assert (!database_thread.joinable ());
 	debug_assert (!dependencies_thread.joinable ());
+	debug_assert (!frontiers_thread.joinable ());
 	debug_assert (!timeout_thread.joinable ());
 }
 
@@ -65,6 +69,7 @@ void nano::bootstrap_ascending::service::start ()
 	debug_assert (!priorities_thread.joinable ());
 	debug_assert (!database_thread.joinable ());
 	debug_assert (!dependencies_thread.joinable ());
+	debug_assert (!frontiers_thread.joinable ());
 	debug_assert (!timeout_thread.joinable ());
 
 	if (!config.enable)
@@ -94,6 +99,14 @@ void nano::bootstrap_ascending::service::start ()
 		});
 	}
 
+	if (config.enable_frontier_scan)
+	{
+		frontiers_thread = std::thread ([this] () {
+			nano::thread_role::set (nano::thread_role::name::ascending_bootstrap);
+			run_frontiers ();
+		});
+	}
+
 	timeout_thread = std::thread ([this] () {
 		nano::thread_role::set (nano::thread_role::name::ascending_bootstrap);
 		run_timeouts ();
@@ -111,10 +124,11 @@ void nano::bootstrap_ascending::service::stop ()
 	nano::join_or_pass (priorities_thread);
 	nano::join_or_pass (database_thread);
 	nano::join_or_pass (dependencies_thread);
+	nano::join_or_pass (frontiers_thread);
 	nano::join_or_pass (timeout_thread);
 }
 
-void nano::bootstrap_ascending::service::send (std::shared_ptr<nano::transport::channel> const & channel, async_tag tag)
+bool nano::bootstrap_ascending::service::send (std::shared_ptr<nano::transport::channel> const & channel, async_tag tag)
 {
 	debug_assert (tag.type != query_type::invalid);
 	debug_assert (tag.source != query_source::invalid);
@@ -125,6 +139,8 @@ void nano::bootstrap_ascending::service::send (std::shared_ptr<nano::transport::
 		tags.get<tag_id> ().insert (tag);
 	}
 
+	on_request.notify (tag, channel);
+
 	nano::asc_pull_req request{ network_constants };
 	request.id = tag.id;
 
@@ -152,6 +168,16 @@ void nano::bootstrap_ascending::service::send (std::shared_ptr<nano::transport::
 			request.payload = pld;
 		}
 		break;
+		case query_type::frontiers:
+		{
+			request.type = nano::asc_pull_type::frontiers;
+
+			nano::asc_pull_req::frontiers_payload pld;
+			pld.start = tag.start.as_account ();
+			pld.count = nano::asc_pull_ack::frontiers_payload::max_frontiers;
+			request.payload = pld;
+		}
+		break;
 		default:
 			debug_assert (false);
 	}
@@ -165,6 +191,8 @@ void nano::bootstrap_ascending::service::send (std::shared_ptr<nano::transport::
 	channel->send (
 	request, nullptr,
 	nano::transport::buffer_drop_policy::limiter, nano::transport::traffic_type::bootstrap);
+
+	return true; // TODO: Return channel send result
 }
 
 std::size_t nano::bootstrap_ascending::service::priority_size () const
@@ -409,6 +437,24 @@ nano::block_hash nano::bootstrap_ascending::service::wait_blocking ()
 	return result;
 }
 
+nano::account nano::bootstrap_ascending::service::wait_frontier ()
+{
+	nano::account result{ 0 };
+
+	wait ([this, &result] () {
+		debug_assert (!mutex.try_lock ());
+		result = frontiers.next ();
+		if (!result.is_zero ())
+		{
+			stats.inc (nano::stat::type::bootstrap_ascending_next, nano::stat::detail::next_frontier);
+			return true;
+		}
+		return false;
+	});
+
+	return result;
+}
+
 bool nano::bootstrap_ascending::service::request (nano::account account, size_t count, std::shared_ptr<nano::transport::channel> const & channel, query_source source)
 {
 	debug_assert (count > 0);
@@ -436,11 +482,7 @@ bool nano::bootstrap_ascending::service::request (nano::account account, size_t
 		tag.start = account;
 	}
 
-	on_request.notify (tag, channel);
-
-	send (channel, tag);
-
-	return true; // Request sent
+	return send (channel, tag);
 }
 
 bool nano::bootstrap_ascending::service::request_info (nano::block_hash hash, std::shared_ptr<nano::transport::channel> const & channel, query_source source)
@@ -451,11 +493,17 @@ bool nano::bootstrap_ascending::service::request_info (nano::block_hash hash, st
 	tag.start = hash;
 	tag.hash = hash;
 
-	on_request.notify (tag, channel);
+	return send (channel, tag);
+}
 
-	send (channel, tag);
+bool nano::bootstrap_ascending::service::request_frontiers (nano::account start, std::shared_ptr<nano::transport::channel> const & channel, query_source source)
+{
+	async_tag tag{};
+	tag.type = query_type::frontiers;
+	tag.source = source;
+	tag.start = start;
 
-	return true; // Request sent
+	return send (channel, tag);
 }
 
 void nano::bootstrap_ascending::service::run_one_priority ()
@@ -549,6 +597,103 @@ void nano::bootstrap_ascending::service::run_dependencies ()
 	}
 }
 
+void nano::bootstrap_ascending::service::run_one_frontier ()
+{
+	wait ([this] () {
+		return !accounts.priority_half_full ();
+	});
+	wait ([this] () {
+		return frontiers_limiter.should_pass (1);
+	});
+	wait ([this] () {
+		return workers.num_queued_tasks () < config.frontier_scan.max_pending;
+	});
+	wait_tags ();
+	auto channel = wait_channel ();
+	if (!channel)
+	{
+		return;
+	}
+	auto frontier = wait_frontier ();
+	if (frontier.is_zero ())
+	{
+		return;
+	}
+	request_frontiers (frontier, channel, query_source::frontiers);
+}
+
+void nano::bootstrap_ascending::service::run_frontiers ()
+{
+	nano::unique_lock<nano::mutex> lock{ mutex };
+	while (!stopped)
+	{
+		lock.unlock ();
+		stats.inc (nano::stat::type::bootstrap_ascending, nano::stat::detail::loop_frontiers);
+		run_one_frontier ();
+		lock.lock ();
+	}
+}
+
+void nano::bootstrap_ascending::service::process_frontiers (std::deque<std::pair<nano::account, nano::block_hash>> const & frontiers)
+{
+	stats.inc (nano::stat::type::bootstrap_ascending, nano::stat::detail::process_frontiers);
+
+	size_t outdated = 0;
+	size_t pending = 0;
+
+	// Accounts with outdated frontiers to sync
+	std::deque<nano::account> result;
+	{
+		auto transaction = ledger.tx_begin_read ();
+
+		auto should_prioritize = [&] (nano::account const & account, nano::block_hash const & frontier) {
+			if (ledger.any.block_exists_or_pruned (transaction, frontier))
+			{
+				return false;
+			}
+			if (auto info = ledger.any.account_get (transaction, account))
+			{
+				if (info->head != frontier)
+				{
+					outdated++;
+					return true; // Frontier is outdated
+				}
+				return false;
+			}
+			if (auto receivable = ledger.any.receivable_lower_bound (transaction, account, { 0 }))
+			{
+				if (receivable->first.account == account)
+				{
+					pending++;
+					return true; // Account doesn't exist but has pending blocks in the ledger
+				}
+				return false;
+			}
+			return false;
+		};
+
+		for (auto const & [account, frontier] : frontiers)
+		{
+			if (should_prioritize (account, frontier))
+			{
+				result.push_back (account);
+			}
+		}
+	}
+
+	stats.add (nano::stat::type::bootstrap_ascending, nano::stat::detail::frontiers_processed, frontiers.size ());
+	stats.add (nano::stat::type::bootstrap_ascending, nano::stat::detail::frontiers_prioritized, result.size ());
+	stats.add (nano::stat::type::bootstrap_ascending, nano::stat::detail::frontiers_outdated, outdated);
+	stats.add (nano::stat::type::bootstrap_ascending, nano::stat::detail::frontiers_pending, pending);
+
+	nano::lock_guard<nano::mutex> guard{ mutex };
+
+	for (auto const & account : result)
+	{
+		accounts.priority_set (account);
+	}
+}
+
 void nano::bootstrap_ascending::service::cleanup_and_sync ()
 {
 	debug_assert (!mutex.try_lock ());
@@ -622,7 +767,7 @@ void nano::bootstrap_ascending::service::process (nano::asc_pull_ack const & mes
 		}
 		bool operator() (const nano::asc_pull_ack::frontiers_payload & response) const
 		{
-			return false; // TODO: Handle frontiers
+			return type == query_type::frontiers;
 		}
 		bool operator() (const nano::empty_payload & response) const
 		{
@@ -664,7 +809,7 @@ void nano::bootstrap_ascending::service::process (const nano::asc_pull_ack::bloc
 	{
 		case verify_result::ok:
 		{
-			stats.inc (nano::stat::type::bootstrap_ascending_verify, nano::stat::detail::ok);
+			stats.inc (nano::stat::type::bootstrap_ascending_verify_blocks, nano::stat::detail::ok);
 			stats.add (nano::stat::type::bootstrap_ascending, nano::stat::detail::blocks, nano::stat::dir::in, response.blocks.size ());
 
 			auto blocks = response.blocks;
@@ -705,7 +850,7 @@ void nano::bootstrap_ascending::service::process (const nano::asc_pull_ack::bloc
 		break;
 		case verify_result::nothing_new:
 		{
-			stats.inc (nano::stat::type::bootstrap_ascending_verify, nano::stat::detail::nothing_new);
+			stats.inc (nano::stat::type::bootstrap_ascending_verify_blocks, nano::stat::detail::nothing_new);
 
 			nano::lock_guard<nano::mutex> lock{ mutex };
 			accounts.priority_down (tag.account);
@@ -717,7 +862,7 @@ void nano::bootstrap_ascending::service::process (const nano::asc_pull_ack::bloc
 		break;
 		case verify_result::invalid:
 		{
-			stats.inc (nano::stat::type::bootstrap_ascending_verify, nano::stat::detail::invalid);
+			stats.inc (nano::stat::type::bootstrap_ascending_verify_blocks, nano::stat::detail::invalid);
 		}
 		break;
 	}
@@ -731,24 +876,69 @@ void nano::bootstrap_ascending::service::process (const nano::asc_pull_ack::acco
 	if (response.account.is_zero ())
 	{
 		stats.inc (nano::stat::type::bootstrap_ascending_process, nano::stat::detail::account_info_empty);
+		return;
 	}
-	else
-	{
-		stats.inc (nano::stat::type::bootstrap_ascending_process, nano::stat::detail::account_info);
 
-		// Prioritize account containing the dependency
-		{
-			nano::lock_guard<nano::mutex> lock{ mutex };
-			accounts.dependency_update (tag.hash, response.account);
-			accounts.priority_set (response.account);
-		}
+	stats.inc (nano::stat::type::bootstrap_ascending_process, nano::stat::detail::account_info);
+
+	// Prioritize account containing the dependency
+	{
+		nano::lock_guard<nano::mutex> lock{ mutex };
+		accounts.dependency_update (tag.hash, response.account);
+		accounts.priority_set (response.account);
 	}
 }
 
 void nano::bootstrap_ascending::service::process (const nano::asc_pull_ack::frontiers_payload & response, const async_tag & tag)
 {
-	// TODO: Make use of frontiers info
+	debug_assert (tag.type == query_type::frontiers);
+	debug_assert (!tag.start.is_zero ());
+
+	if (response.frontiers.empty ())
+	{
+		stats.inc (nano::stat::type::bootstrap_ascending_process, nano::stat::detail::frontiers_empty);
+		return;
+	}
+
 	stats.inc (nano::stat::type::bootstrap_ascending_process, nano::stat::detail::frontiers);
+
+	auto result = verify (response, tag);
+	switch (result)
+	{
+		case verify_result::ok:
+		{
+			stats.inc (nano::stat::type::bootstrap_ascending_verify_frontiers, nano::stat::detail::ok);
+			stats.add (nano::stat::type::bootstrap_ascending, nano::stat::detail::frontiers, nano::stat::dir::in, response.frontiers.size ());
+
+			{
+				nano::lock_guard<nano::mutex> lock{ mutex };
+				frontiers.process (tag.start.as_account (), response.frontiers);
+			}
+
+			// Allow some overfill to avoid unnecessarily dropping responses
+			if (workers.num_queued_tasks () < config.frontier_scan.max_pending * 4)
+			{
+				workers.push_task ([this, frontiers = response.frontiers] {
+					process_frontiers (frontiers);
+				});
+			}
+			else
+			{
+				stats.inc (nano::stat::type::bootstrap_ascending, nano::stat::detail::dropped_frontiers);
+			}
+		}
+		break;
+		case verify_result::nothing_new:
+		{
+			stats.inc (nano::stat::type::bootstrap_ascending_verify_frontiers, nano::stat::detail::nothing_new);
+		}
+		break;
+		case verify_result::invalid:
+		{
+			stats.inc (nano::stat::type::bootstrap_ascending_verify_frontiers, nano::stat::detail::invalid);
+		}
+		break;
+	}
 }
 
 void nano::bootstrap_ascending::service::process (const nano::empty_payload & response, const async_tag & tag)
@@ -816,6 +1006,35 @@ nano::bootstrap_ascending::service::verify_result nano::bootstrap_ascending::ser
 	return verify_result::ok;
 }
 
+nano::bootstrap_ascending::service::verify_result nano::bootstrap_ascending::service::verify (nano::asc_pull_ack::frontiers_payload const & response, async_tag const & tag) const
+{
+	auto const & frontiers = response.frontiers;
+
+	if (frontiers.empty ())
+	{
+		return verify_result::nothing_new;
+	}
+
+	// Ensure frontiers accounts are in ascending order
+	nano::account previous{ 0 };
+	for (auto const & [account, _] : frontiers)
+	{
+		if (account.number () <= previous.number ())
+		{
+			return verify_result::invalid;
+		}
+		previous = account;
+	}
+
+	// Ensure the frontiers are larger or equal to the requested frontier
+	if (frontiers.front ().first.number () < tag.start.as_account ().number ())
+	{
+		return verify_result::invalid;
+	}
+
+	return verify_result::ok;
+}
+
 auto nano::bootstrap_ascending::service::info () const -> nano::bootstrap_ascending::account_sets::info_t
 {
 	nano::lock_guard<nano::mutex> lock{ mutex };
@@ -840,6 +1059,8 @@ std::unique_ptr<nano::container_info_component> nano::bootstrap_ascending::servi
 	composite->add_component (std::make_unique<container_info_leaf> (container_info{ "throttle_successes", throttle.successes (), 0 }));
 	composite->add_component (accounts.collect_container_info ("accounts"));
 	composite->add_component (database_scan.collect_container_info ("database_scan"));
+	composite->add_component (frontiers.collect_container_info ("frontiers"));
+	composite->add_component (workers.collect_container_info ("workers"));
 	return composite;
 }
 
diff --git a/nano/node/bootstrap_ascending/service.hpp b/nano/node/bootstrap_ascending/service.hpp
index 4f3f3668d..652c7b862 100644
--- a/nano/node/bootstrap_ascending/service.hpp
+++ b/nano/node/bootstrap_ascending/service.hpp
@@ -5,13 +5,15 @@
 #include <nano/lib/numbers.hpp>
 #include <nano/lib/observer_set.hpp>
 #include <nano/lib/rate_limiting.hpp>
+#include <nano/lib/thread_pool.hpp>
+#include <nano/lib/timer.hpp>
 #include <nano/node/bootstrap/bootstrap_config.hpp>
 #include <nano/node/bootstrap_ascending/account_sets.hpp>
 #include <nano/node/bootstrap_ascending/common.hpp>
 #include <nano/node/bootstrap_ascending/database_scan.hpp>
+#include <nano/node/bootstrap_ascending/frontier_scan.hpp>
 #include <nano/node/bootstrap_ascending/peer_scoring.hpp>
 #include <nano/node/bootstrap_ascending/throttle.hpp>
-#include <nano/node/fwd.hpp>
 
 #include <boost/multi_index/hashed_index.hpp>
 #include <boost/multi_index/member.hpp>
@@ -63,6 +65,7 @@ namespace bootstrap_ascending
 			blocks_by_hash,
 			blocks_by_account,
 			account_info_by_hash,
+			frontiers,
 		};
 
 		enum class query_source
@@ -71,6 +74,7 @@ namespace bootstrap_ascending
 			priority,
 			database,
 			blocking,
+			frontiers,
 		};
 
 		struct async_tag
@@ -101,8 +105,11 @@ namespace bootstrap_ascending
 		void run_one_database (bool should_throttle);
 		void run_dependencies ();
 		void run_one_blocking ();
+		void run_one_frontier ();
+		void run_frontiers ();
 		void run_timeouts ();
 		void cleanup_and_sync ();
+		void process_frontiers (std::deque<std::pair<nano::account, nano::block_hash>> const & frontiers);
 
 		/* Waits for a condition to be satisfied with incremental backoff */
 		void wait (std::function<bool ()> const & predicate) const;
@@ -122,10 +129,13 @@ namespace bootstrap_ascending
 		/* Waits for next available blocking block */
 		nano::block_hash next_blocking ();
 		nano::block_hash wait_blocking ();
+		/* Waits for next available frontier scan range */
+		nano::account wait_frontier ();
 
 		bool request (nano::account, size_t count, std::shared_ptr<nano::transport::channel> const &, query_source);
 		bool request_info (nano::block_hash, std::shared_ptr<nano::transport::channel> const &, query_source);
-		void send (std::shared_ptr<nano::transport::channel> const &, async_tag tag);
+		bool request_frontiers (nano::account, std::shared_ptr<nano::transport::channel> const &, query_source);
+		bool send (std::shared_ptr<nano::transport::channel> const &, async_tag tag);
 
 		void process (nano::asc_pull_ack::blocks_payload const & response, async_tag const & tag);
 		void process (nano::asc_pull_ack::account_info_payload const & response, async_tag const & tag);
@@ -146,6 +156,7 @@ namespace bootstrap_ascending
 		 * - ok: otherwise, if all checks pass
 		 */
 		verify_result verify (nano::asc_pull_ack::blocks_payload const & response, async_tag const & tag) const;
+		verify_result verify (nano::asc_pull_ack::frontiers_payload const & response, async_tag const & tag) const;
 
 		size_t count_tags (nano::account const & account, query_source source) const;
 		size_t count_tags (nano::block_hash const & hash, query_source source) const;
@@ -158,6 +169,7 @@ namespace bootstrap_ascending
 		nano::bootstrap_ascending::database_scan database_scan;
 		nano::bootstrap_ascending::throttle throttle;
 		nano::bootstrap_ascending::peer_scoring scoring;
+		nano::bootstrap_ascending::frontier_scan frontiers;
 
 		// clang-format off
 		class tag_sequenced {};
@@ -181,6 +193,7 @@ namespace bootstrap_ascending
 		// Requests for accounts from database have much lower hitrate and could introduce strain on the network
 		// A separate (lower) limiter ensures that we always reserve resources for querying accounts from priority queue
 		nano::rate_limiter database_limiter;
+		nano::rate_limiter frontiers_limiter;
 
 		nano::interval sync_dependencies_interval;
 
@@ -190,7 +203,10 @@ namespace bootstrap_ascending
 		std::thread priorities_thread;
 		std::thread database_thread;
 		std::thread dependencies_thread;
+		std::thread frontiers_thread;
 		std::thread timeout_thread;
+
+		nano::thread_pool workers;
 	};
 
 	nano::stat::detail to_stat_detail (service::query_type);
diff --git a/nano/node/messages.hpp b/nano/node/messages.hpp
index 00f1070b0..cba54744d 100644
--- a/nano/node/messages.hpp
+++ b/nano/node/messages.hpp
@@ -724,7 +724,7 @@ public: // Payload definitions
 		static frontier deserialize_frontier (nano::stream &);
 
 	public: // Payload
-		std::vector<frontier> frontiers;
+		std::deque<frontier> frontiers;
 
 	public: // Logging
 		void operator() (nano::object_stream &) const;