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diff --git a/src/server/Task.hpp b/src/server/Task.hpp
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+/*
+  This file is part of Ingen.
+  Copyright 2007-2017 David Robillard <http://drobilla.net/>
+
+  Ingen is free software: you can redistribute it and/or modify it under the
+  terms of the GNU Affero General Public License as published by the Free
+  Software Foundation, either version 3 of the License, or any later version.
+
+  Ingen is distributed in the hope that it will be useful, but WITHOUT ANY
+  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+  A PARTICULAR PURPOSE.  See the GNU Affero General Public License for details.
+
+  You should have received a copy of the GNU Affero General Public License
+  along with Ingen.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef INGEN_ENGINE_TASK_HPP
+#define INGEN_ENGINE_TASK_HPP
+
+#include <atomic>
+#include <cassert>
+#include <deque>
+#include <functional>
+#include <memory>
+#include <string>
+#include <utility>
+
+namespace ingen {
+namespace server {
+
+class BlockImpl;
+class RunContext;
+
+class Task {
+public:
+	enum class Mode {
+		SINGLE,      ///< Single block to run
+		SEQUENTIAL,  ///< Elements must be run sequentially in order
+		PARALLEL     ///< Elements may be run in any order in parallel
+	};
+
+	Task(Mode mode, BlockImpl* block = nullptr)
+		: _block(block)
+		, _mode(mode)
+		, _done_end(0)
+		, _next(0)
+		, _done(false)
+	{
+		assert(!(mode == Mode::SINGLE && !block));
+	}
+
+	Task(Task&& task)
+		: _children(std::move(task._children))
+		, _block(task._block)
+		, _mode(task._mode)
+		, _done_end(task._done_end)
+		, _next(task._next.load())
+		, _done(task._done.load())
+	{}
+
+	Task& operator=(Task&& task)
+	{
+		_children = std::move(task._children);
+		_block    = task._block;
+		_mode     = task._mode;
+		_done_end = task._done_end;
+		_next     = task._next.load();
+		_done     = task._done.load();
+		return *this;
+	}
+
+	/** Run task in the given context. */
+	void run(RunContext& context);
+
+	/** Pretty print task to the given stream (recursively). */
+	void dump(std::function<void (const std::string&)> sink, unsigned indent, bool first) const;
+
+	/** Return true iff this is an empty task. */
+	bool empty() const { return _mode != Mode::SINGLE && _children.empty(); }
+
+	/** Simplify task expression. */
+	static std::unique_ptr<Task> simplify(std::unique_ptr<Task>&& task);
+
+	/** Steal a child task from this task (succeeds for PARALLEL only). */
+	Task* steal(RunContext& context);
+
+	/** Prepend a child to this task. */
+	void push_front(Task&& task) {
+		_children.emplace_front(std::unique_ptr<Task>(new Task(std::move(task))));
+	}
+
+	Mode       mode()  const { return _mode; }
+	BlockImpl* block() const { return _block; }
+	bool       done()  const { return _done; }
+
+	void set_done(bool done) { _done = done; }
+
+private:
+	typedef std::deque<std::unique_ptr<Task>> Children;
+
+	Task(const Task&) = delete;
+	Task& operator=(const Task&) = delete;
+
+	Task* get_task(RunContext& context);
+
+	void append(std::unique_ptr<Task>&& t) {
+		_children.emplace_back(std::move(t));
+	}
+
+	Children              _children;  ///< Vector of child tasks
+	BlockImpl*            _block;     ///< Used for SINGLE only
+	Mode                  _mode;      ///< Execution mode
+	unsigned              _done_end;  ///< Index of rightmost done sub-task
+	std::atomic<unsigned> _next;      ///< Index of next sub-task
+	std::atomic<bool>     _done;      ///< Completion phase
+};
+
+} // namespace server
+} // namespace ingen
+
+#endif // INGEN_ENGINE_TASK_HPP