diff --git a/include/ircd/m/dbs/appendix.h b/include/ircd/m/dbs/appendix.h
index 266c717c0..0c74fac20 100644
--- a/include/ircd/m/dbs/appendix.h
+++ b/include/ircd/m/dbs/appendix.h
@@ -47,6 +47,13 @@ enum ircd::m::dbs::appendix::index
 	/// types are involved.
 	EVENT_REFS,
 
+	/// Involves the event_horizon column which saves the event_id of any
+	/// unresolved event_refs at the time of the transaction. This is important
+	/// for out-of-order writes to the database. When the unresolved prev_event
+	/// is encountered later and finds its event_id in event_horizon it can
+	/// properly complete the event_refs graph to all the referencing events.
+	EVENT_HORIZON,
+
 	/// Involves the event_sender column (reverse index on the event sender).
 	EVENT_SENDER,
 
@@ -101,6 +108,7 @@ namespace ircd::m::dbs
 	string_view _index_room(db::txn &, const event &, const write_opts &);
 	void _index_event_type(db::txn &, const event &, const write_opts &);
 	void _index_event_sender(db::txn &, const event &, const write_opts &);
+	void _index_event_horizon(db::txn &, const event &, const write_opts &, const id::event &);
 	void _index_event_refs_m_room_redaction(db::txn &, const event &, const write_opts &);
 	void _index_event_refs_m_receipt_m_read(db::txn &, const event &, const write_opts &);
 	void _index_event_refs_m_relates_m_reply(db::txn &, const event &, const write_opts &);
diff --git a/include/ircd/m/dbs/dbs.h b/include/ircd/m/dbs/dbs.h
index 98513e287..b9f55c2b8 100644
--- a/include/ircd/m/dbs/dbs.h
+++ b/include/ircd/m/dbs/dbs.h
@@ -36,6 +36,7 @@ namespace ircd::m::dbs
 	extern db::column event_idx;       // event_id => event_idx
 	extern db::column event_json;      // event_idx => full json
 	extern db::index event_refs;       // event_idx | ref_type, event_idx
+	extern db::index event_horizon;    // event_id | event_idx
 	extern db::index event_type;       // type | event_idx
 	extern db::index event_sender;     // host | local, event_idx
 	extern db::index room_head;        // room_id | event_id => event_idx
diff --git a/include/ircd/m/dbs/desc.h b/include/ircd/m/dbs/desc.h
index 6308b48ff..d95330063 100644
--- a/include/ircd/m/dbs/desc.h
+++ b/include/ircd/m/dbs/desc.h
@@ -101,6 +101,14 @@ namespace ircd::m::dbs::desc
 	extern const db::comparator events__event_refs__cmp;
 	extern const db::descriptor events__event_refs;
 
+	// event horizon
+	extern conf::item<size_t> events__event_horizon__block__size;
+	extern conf::item<size_t> events__event_horizon__meta_block__size;
+	extern conf::item<size_t> events__event_horizon__cache__size;
+	extern conf::item<size_t> events__event_horizon__cache_comp__size;
+	extern const db::prefix_transform events__event_horizon__pfx;
+	extern const db::descriptor events__event_horizon;
+
 	// events sender
 	extern conf::item<size_t> events__event_sender__block__size;
 	extern conf::item<size_t> events__event_sender__meta_block__size;
diff --git a/include/ircd/m/dbs/util.h b/include/ircd/m/dbs/util.h
index 618500f01..4e080495b 100644
--- a/include/ircd/m/dbs/util.h
+++ b/include/ircd/m/dbs/util.h
@@ -20,6 +20,10 @@ namespace ircd::m::dbs
 	std::tuple<ref, event::idx> event_refs_key(const string_view &amalgam);
 	string_view reflect(const ref &);
 
+	constexpr size_t EVENT_HORIZON_KEY_MAX_SIZE {id::MAX_SIZE + 1 + 8};
+	string_view event_horizon_key(const mutable_buffer &out, const id::event &, const event::idx &);
+	std::tuple<event::idx> event_horizon_key(const string_view &amalgam);
+
 	constexpr size_t EVENT_SENDER_KEY_MAX_SIZE {id::MAX_SIZE + 1 + 8};
 	string_view event_sender_key(const mutable_buffer &out, const string_view &origin, const string_view &localpart = {}, const event::idx & = 0);
 	string_view event_sender_key(const mutable_buffer &out, const id::user &, const event::idx &);
diff --git a/ircd/m_dbs.cc b/ircd/m_dbs.cc
index dd82ad802..7117ea4b0 100644
--- a/ircd/m_dbs.cc
+++ b/ircd/m_dbs.cc
@@ -36,6 +36,11 @@ decltype(ircd::m::dbs::event_refs)
 ircd::m::dbs::event_refs
 {};
 
+/// Linkage for a reference to the event_horizon column.
+decltype(ircd::m::dbs::event_horizon)
+ircd::m::dbs::event_horizon
+{};
+
 /// Linkage for a reference to the event_sender column.
 decltype(ircd::m::dbs::event_sender)
 ircd::m::dbs::event_sender
@@ -150,6 +155,7 @@ ircd::m::dbs::init::init(std::string dbopts)
 	event_idx = db::column{*events, desc::events__event_idx.name};
 	event_json = db::column{*events, desc::events__event_json.name};
 	event_refs = db::index{*events, desc::events__event_refs.name};
+	event_horizon = db::index{*events, desc::events__event_horizon.name};
 	event_sender = db::index{*events, desc::events__event_sender.name};
 	event_type = db::index{*events, desc::events__event_type.name};
 	room_head = db::index{*events, desc::events__room_head.name};
@@ -432,9 +438,14 @@ ircd::m::dbs::_index_event_refs_prev(db::txn &txn,
 			m::index(prev_id, std::nothrow)  // query
 		};
 
-		if(!prev_idx)
+		if(opts.appendix.test(appendix::EVENT_HORIZON) && !prev_idx)
 		{
-			log::warning
+			_index_event_horizon(txn, event, opts, prev_id);
+			continue;
+		}
+		else if(!prev_idx)
+		{
+			log::derror
 			{
 				log, "No index found to ref %s PREV of %s",
 				string_view{prev_id},
@@ -487,6 +498,9 @@ ircd::m::dbs::_index_event_refs_auth(db::txn &txn,
 
 		if(unlikely(!auth_idx))
 		{
+			if(opts.appendix.test(appendix::EVENT_HORIZON))
+				_index_event_horizon(txn, event, opts, auth_id);
+
 			log::error
 			{
 				log, "No index found to ref %s AUTH of %s",
@@ -548,7 +562,10 @@ ircd::m::dbs::_index_event_refs_state(db::txn &txn,
 		state.get(std::nothrow, at<"type"_>(event), at<"state_key"_>(event)) // query
 	};
 
-	if(!prev_state_idx || prev_state_idx == opts.event_idx)
+	if(!prev_state_idx)
+		return;
+
+	if(prev_state_idx >= opts.event_idx)
 		return;
 
 	thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
@@ -614,7 +631,12 @@ ircd::m::dbs::_index_event_refs_m_receipt_m_read(db::txn &txn,
 		m::index(event_id, std::nothrow) // query
 	};
 
-	if(!event_idx)
+	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
+	{
+		_index_event_horizon(txn, event, opts, event_id);
+		return;
+	}
+	else if(!event_idx)
 	{
 		log::derror
 		{
@@ -622,6 +644,7 @@ ircd::m::dbs::_index_event_refs_m_receipt_m_read(db::txn &txn,
 			string_view{event_id},
 			json::get<"event_id"_>(event)
 		};
+
 		return;
 	}
 
@@ -705,9 +728,14 @@ ircd::m::dbs::_index_event_refs_m_relates_m_reply(db::txn &txn,
 		m::index(event_id, std::nothrow) // query
 	};
 
-	if(!event_idx)
+	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
 	{
-		log::dwarning
+		_index_event_horizon(txn, event, opts, event_id);
+		return;
+	}
+	else if(!event_idx)
+	{
+		log::derror
 		{
 			log, "Cannot index m.in_reply_to in %s; referenced %s not found.",
 			json::get<"event_id"_>(event),
@@ -758,8 +786,22 @@ ircd::m::dbs::_index_event_refs_m_room_redaction(db::txn &txn,
 		m::index(event_id, std::nothrow) // query
 	};
 
-	if(!event_idx)
+	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
+	{
+		_index_event_horizon(txn, event, opts, event_id);
 		return;
+	}
+	else if(!event_idx)
+	{
+		log::derror
+		{
+			log, "Cannot index m.room.redaction in %s; referenced %s not found.",
+			json::get<"event_id"_>(event),
+			string_view{event_id}
+		};
+
+		return;
+	}
 
 	thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
 	assert(opts.event_idx != 0 && event_idx != 0);
@@ -778,6 +820,28 @@ ircd::m::dbs::_index_event_refs_m_room_redaction(db::txn &txn,
 	};
 }
 
+void
+ircd::m::dbs::_index_event_horizon(db::txn &txn,
+                                   const event &event,
+                                   const write_opts &opts,
+                                   const m::event::id &unresolved_id)
+{
+	thread_local char buf[EVENT_HORIZON_KEY_MAX_SIZE];
+	assert(opts.event_idx != 0 && unresolved_id);
+	const string_view &key
+	{
+		event_horizon_key(buf, unresolved_id, opts.event_idx)
+	};
+
+	db::txn::append
+	{
+		txn, dbs::event_horizon,
+		{
+			opts.op, key, string_view{}
+		}
+	};
+}
+
 void
 ircd::m::dbs::_index_event_sender(db::txn &txn,
                                   const event &event,
@@ -1708,6 +1772,152 @@ ircd::m::dbs::desc::events__event_refs
 	size_t(events__event_refs__meta_block__size),
 };
 
+//
+// event_horizon
+//
+
+decltype(ircd::m::dbs::desc::events__event_horizon__block__size)
+ircd::m::dbs::desc::events__event_horizon__block__size
+{
+	{ "name",     "ircd.m.dbs.events._event_horizon.block.size" },
+	{ "default",  512L                                          },
+};
+
+decltype(ircd::m::dbs::desc::events__event_horizon__meta_block__size)
+ircd::m::dbs::desc::events__event_horizon__meta_block__size
+{
+	{ "name",     "ircd.m.dbs.events._event_horizon.meta_block.size" },
+	{ "default",  1024L                                              },
+};
+
+decltype(ircd::m::dbs::desc::events__event_horizon__cache__size)
+ircd::m::dbs::desc::events__event_horizon__cache__size
+{
+	{
+		{ "name",     "ircd.m.dbs.events._event_horizon.cache.size" },
+		{ "default",  long(16_MiB)                                  },
+	}, []
+	{
+		const size_t &value{events__event_horizon__cache__size};
+		db::capacity(db::cache(event_horizon), value);
+	}
+};
+
+decltype(ircd::m::dbs::desc::events__event_horizon__cache_comp__size)
+ircd::m::dbs::desc::events__event_horizon__cache_comp__size
+{
+	{
+		{ "name",     "ircd.m.dbs.events._event_horizon.cache_comp.size" },
+		{ "default",  long(0_MiB)                                        },
+	}, []
+	{
+		const size_t &value{events__event_horizon__cache_comp__size};
+		db::capacity(db::cache_compressed(event_horizon), value);
+	}
+};
+
+ircd::string_view
+ircd::m::dbs::event_horizon_key(const mutable_buffer &out,
+                                const event::id &event_id,
+                                const event::idx &event_idx)
+{
+	mutable_buffer buf(out);
+	consume(buf, copy(buf, event_id));
+	consume(buf, copy(buf, "\0"_sv));
+	consume(buf, copy(buf, byte_view<string_view>(event_idx)));
+	return
+	{
+		data(out), data(buf)
+	};
+}
+
+std::tuple<ircd::m::event::idx>
+ircd::m::dbs::event_horizon_key(const string_view &amalgam)
+{
+	return
+	{
+		byte_view<event::idx>(lstrip(amalgam, "\0"_sv))
+	};
+}
+
+const ircd::db::prefix_transform
+ircd::m::dbs::desc::events__event_horizon__pfx
+{
+	"_event_horizon",
+	[](const string_view &key)
+	{
+		return has(key, "\0"_sv);
+	},
+
+	[](const string_view &key)
+	{
+		assert(size(key) >= sizeof(event::idx));
+		return split(key, "\0"_sv).first;
+	}
+};
+
+const ircd::db::descriptor
+ircd::m::dbs::desc::events__event_horizon
+{
+	// name
+	"_event_horizon",
+
+	// explanation
+	R"(Unresolved references in the reverse reference graph of events.
+
+	event_id | event_idx => --
+
+	The first part of the key is an event_id which the server does not have.
+	The suffix of the key is the index number of an event which the server
+	does have and it contains a reference to event_id.
+
+	We use the information in this column to find all of the events which
+	have an unresolved reference to this event and complete the holes in the
+	event_refs graph which could not be completed without this event.
+
+	When a new event is written to the database the event_horizon column is
+	queried seeking the event's ID. Each entry in event_horizon is the index
+	of an event which we previously wrote to the database without knowing the
+	index of the event currently being written (an out-of-order write).
+
+	)",
+
+	// typing (key, value)
+	{
+		typeid(string_view), typeid(string_view)
+	},
+
+	// options
+	{},
+
+	// comparator
+	{},
+
+	// prefix transform
+	events__event_horizon__pfx,
+
+	// drop column
+	false,
+
+	// cache size
+	bool(events_cache_enable)? -1 : 0, //uses conf item
+
+	// cache size for compressed assets
+	bool(events_cache_comp_enable)? -1 : 0,
+
+	// bloom filter bits
+	0,
+
+	// expect queries hit
+	false,
+
+	// block size
+	size_t(events__event_horizon__block__size),
+
+	// meta_block size
+	size_t(events__event_horizon__meta_block__size),
+};
+
 //
 // event_sender
 //
@@ -4220,6 +4430,10 @@ ircd::m::dbs::desc::events
 	// Reverse mapping of the event reference graph.
 	events__event_refs,
 
+	// event_idx | event_idx
+	// Mapping of unresolved event refs.
+	events__event_horizon,
+
 	// origin | sender, event_idx
 	// Mapping of senders to event_idx's they are the sender of.
 	events__event_sender,