diff --git a/include/ircd/ctx/concurrent.h b/include/ircd/ctx/concurrent.h
index dce244430..55d1cb6a1 100644
--- a/include/ircd/ctx/concurrent.h
+++ b/include/ircd/ctx/concurrent.h
@@ -9,192 +9,80 @@
 // full license for this software is available in the LICENSE file.
 
 #pragma once
-#define HAVE_IRCD_CTX_CONCURRENT_H
+#define HAVE_IRCD_CTX_CONCURRENT
 
 namespace ircd::ctx
 {
-	template<class arg> class concurrent;
+	template<class value> struct concurrent;
 }
 
-template<class arg>
+template<class value>
 struct ircd::ctx::concurrent
 {
-	using closure = std::function<void (arg &)>;
+	using closure = std::function<void (value)>;
 
-	pool *p {nullptr};
-	vector_view<arg> a;
-	std::vector<bool> b;
+	pool &p;
 	closure c;
 	dock d;
+	size_t snd {0};
+	size_t rcv {0};
+	size_t fin {0};
 	std::exception_ptr eptr;
-	uint64_t snd {0};           // sends to pool
-	uint64_t rcv {0};           // receives by worker
-	uint64_t fin {0};           // finished by worker
 
-	bool done() const;
-	bool avail() const;
-	void wait_done();
-	void wait_avail();
-	void rethrow_any_exception();
-	void receiver(const size_t pos) noexcept;
-	void sender(const size_t pos) noexcept;
+	template<class V> void operator()(V&&);
+	void wait();
 
-  public:
-	size_t nextpos() const;
-
-	void operator()(const arg &a);
-
-	concurrent(pool &, const vector_view<arg> &, closure);
-	concurrent(concurrent &&) = delete;
-	concurrent(const concurrent &) = delete;
+	concurrent(pool &, closure);
 	~concurrent() noexcept;
 };
 
-template<class arg>
-ircd::ctx::concurrent<arg>::concurrent(pool &p,
-                                       const vector_view<arg> &a,
-                                       closure c)
-:p{&p}
-,a{a}
-,b(this->a.size(), false)
-,c{std::move(c)}
-{
-	p.min(this->a.size());
-}
+template<class value>
+ircd::ctx::concurrent<value>::concurrent(pool &p,
+                                         closure c)
 
-template<class arg>
-ircd::ctx::concurrent<arg>::~concurrent()
+:p{p}
+,c{std::move(c)}
+{}
+
+template<class value>
+ircd::ctx::concurrent<value>::~concurrent()
 noexcept
 {
 	const uninterruptible::nothrow ui;
-	wait_done();
+	this->wait();
 }
 
-template<class arg>
+template<class value>
 void
-ircd::ctx::concurrent<arg>::operator()(const arg &a)
+ircd::ctx::concurrent<value>::wait()
 {
-	const uninterruptible ui;
-	rethrow_any_exception();
-	assert(avail());
-	const auto nextpos(this->nextpos());
-	assert(nextpos < b.size());
-	this->a.at(nextpos) = a;
-	assert(this->b.at(nextpos) == false);
-	this->b.at(nextpos) = true;
-	sender(nextpos);
-	wait_avail();
-}
-
-template<class arg>
-size_t
-ircd::ctx::concurrent<arg>::nextpos()
-const
-{
-	const auto it
+	d.wait([this]
 	{
-		std::find(begin(b), end(b), false)
-	};
-
-	return std::distance(begin(b), it);
+		return snd == fin;
+	});
 }
 
-template<class arg>
+template<class value>
+template<class V>
 void
-ircd::ctx::concurrent<arg>::sender(const size_t pos)
-noexcept
+ircd::ctx::concurrent<value>::operator()(V&& v)
 {
-	assert(pos < b.size());
-	auto &p(*this->p);
-	auto func
-	{
-		std::bind(&concurrent::receiver, this, pos) //TODO: alloc
-	};
-
 	++snd;
-	assert(snd > rcv);
-	if(likely(p.size()))
-		p(std::move(func));
-	else
-		func();
-}
-
-template<class arg>
-void
-ircd::ctx::concurrent<arg>::receiver(const size_t pos)
-noexcept
-{
-	++rcv;
-	assert(snd >= rcv);
-	if(!this->eptr) try
+	p([this, v(std::move(v))]
 	{
-		c(this->a.at(pos));
-	}
-	catch(...)
-	{
-		this->eptr = std::current_exception();
-	}
+		++rcv; try
+		{
+			c(std::move(v));
+		}
+		catch(...)
+		{
+			eptr = std::current_exception();
+		}
 
-	assert(pos < b.size());
-	assert(this->b.at(pos) == true);
-	this->b.at(pos) = false;
-	assert(rcv > fin);
-	++fin;
-	d.notify_one();
-}
-
-template<class arg>
-void
-ircd::ctx::concurrent<arg>::rethrow_any_exception()
-{
-	if(likely(!this->eptr))
-		return;
-
-	wait_done();
-	const auto eptr(this->eptr);
-	this->eptr = {};
-	std::rethrow_exception(eptr);
-}
-
-template<class arg>
-void
-ircd::ctx::concurrent<arg>::wait_avail()
-{
-	d.wait([this]
-	{
-		return avail();
+		++fin;
+		d.notify_all();
 	});
-}
 
-template<class arg>
-void
-ircd::ctx::concurrent<arg>::wait_done()
-{
-	d.wait([this]
-	{
-		return done();
-	});
-}
-
-template<class arg>
-bool
-ircd::ctx::concurrent<arg>::avail()
-const
-{
-	assert(snd >= rcv);
-	assert(rcv >= fin);
-	assert(snd - rcv <= a.size());
-	assert(snd - fin <= a.size());
-	return snd - fin < a.size() && nextpos() < a.size();
-}
-
-template<class arg>
-bool
-ircd::ctx::concurrent<arg>::done()
-const
-{
-	assert(snd >= rcv);
-	assert(rcv >= fin);
-	assert(snd - rcv <= a.size());
-	return snd - fin == 0 && nextpos() == 0;
+	if(eptr)
+		std::rethrow_exception(eptr);
 }
diff --git a/modules/client/sync/presence.cc b/modules/client/sync/presence.cc
index 3f5a64c3e..137ed8464 100644
--- a/modules/client/sync/presence.cc
+++ b/modules/client/sync/presence.cc
@@ -124,39 +124,32 @@ ircd::m::sync::presence_polylog(data &data)
 	}};
 
 	// Setup for concurrentization.
-	static const size_t fibers(64); //TODO: conf
-	using buffer = std::array<char[m::id::MAX_SIZE+1], fibers>;
-	const auto buf(std::make_unique<buffer>());
-	std::array<string_view, fibers> q;
-	ctx::concurrent<string_view> concurrent
+	static const size_t fibers(64);
+	sync::pool.min(fibers);
+	ctx::concurrent<std::string> concurrent
 	{
-		m::sync::pool, q, [&data, &append_event]
-		(const m::user::id user_id)
+		sync::pool, [&data, &append_event](std::string user_id)
 		{
 			const event::idx event_idx
 			{
-				m::presence::get(std::nothrow, user_id)
+				m::presence::get(std::nothrow, m::user::id{user_id})
 			};
 
-			if(apropos(data, event_idx))
-				m::get(std::nothrow, event_idx, "content", append_event);
+			if(!apropos(data, event_idx))
+				return;
+
+			m::get(std::nothrow, event_idx, "content", append_event);
 		}
 	};
 
 	// Iterate all of the users visible to our user in joined rooms.
 	const m::user::mitsein mitsein{data.user};
-	mitsein.for_each("join", [&concurrent, &q, &buf]
+	mitsein.for_each("join", [&concurrent]
 	(const m::user &user)
 	{
-		// Manual copy of the user_id string to the buffer and assignment
-		// of q at the next position. concurrent.snd is the position in q
-		// which ctx::concurrent wants us to store the next data at. The
-		// concurrent() call doesn't return (blocks this context) until there's
-		// a next position available; propagating flow-control for the iter.
-		const auto pos(concurrent.nextpos());
-		concurrent(strlcpy(buf->at(pos), user.user_id));
+		concurrent(std::string(user.user_id));
 	});
 
-	concurrent.wait_done();
+	concurrent.wait();
 	return ret;
 }
diff --git a/modules/client/sync/rooms/state.cc b/modules/client/sync/rooms/state.cc
index fafeae9c4..5be8b9a45 100644
--- a/modules/client/sync/rooms/state.cc
+++ b/modules/client/sync/rooms/state.cc
@@ -197,50 +197,44 @@ ircd::m::sync::room_state_polylog_events(data &data)
 	if(data.phased && data.range.first == 0)
 		return room_state_phased_events(data);
 
-	const m::room &room{*data.room};
-	const m::room::state state{room};
+	bool ret{false};
+	ctx::mutex mutex;
 	json::stack::array array
 	{
 		*data.out, "events"
 	};
 
-	ctx::mutex mutex;
-	std::array<event::idx, 64> md;
-	std::vector<event::fetch> event(md.size() * 3);
-	for(auto &fetch : event)
-		fetch = event::fetch{_default_fopts};
-
-	size_t _i(0);
-	bool ret{false};
-	const event::closure_idx each_idx{[&data, &array, &mutex, &ret, &event, &_i]
-	(const m::event::idx event_idx)
-	{
-		const size_t i{_i++ % event.size()};
-		if(unlikely(!seek(event.at(i), event_idx, std::nothrow)))
-		{
-			assert(data.room);
-			log::error
-			{
-				log, "Failed to fetch event idx:%lu in room %s state.",
-				event_idx,
-				string_view{data.room->room_id}
-			};
-
-			return;
-		}
-
-		const std::lock_guard lock{mutex};
-		room_state_append(data, array, event.at(i), event_idx);
-		ret = true;
-	}};
-
+	sync::pool.min(64); //TODO: XXX
 	ctx::concurrent<event::idx> concurrent
 	{
-		m::sync::pool, md, each_idx
+		sync::pool, [&](const event::idx &event_idx)
+		{
+			const m::event::fetch event
+			{
+				event_idx, std::nothrow, _default_fopts
+			};
+
+			if(unlikely(!event.valid))
+			{
+				log::error
+				{
+					log, "Failed to fetch event idx:%lu in room %s state.",
+					event_idx,
+					string_view{data.room->room_id},
+				};
+
+				return;
+			}
+
+			const std::lock_guard lock{mutex};
+			room_state_append(data, array, event, event_idx);
+			ret |= true;
+		}
 	};
 
-	state.for_each([&data, &concurrent, &each_idx]
-	(const m::event::idx &event_idx)
+	const room::state state{*data.room};
+	state.for_each([&data, &concurrent]
+	(const event::idx &event_idx)
 	{
 		if(!apropos(data, event_idx))
 			return;
@@ -248,7 +242,7 @@ ircd::m::sync::room_state_polylog_events(data &data)
 		concurrent(event_idx);
 	});
 
-	concurrent.wait_done();
+	concurrent.wait();
 	return ret;
 }