doc/sequencer.pseudo


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// Sequencer based on "Distributed Systems: Concepts and Design" 5e, Coulouris et
// al. (2012) pp. 654-655. Depends on reliable multicast, implemented here as the
// Trans protocol.

type dataMsg struct {
	id int
	data
}

type orderMsg struct {
	id int
	seq int
}

class groupMember {
	seq := 0
	holdback := new Queue[dataMsg]
	deliver := new Queue[dataMsg]
	rMulticast := new Trans

	multicast(m dataMsg) {
		rMulticast.send(m)
	}

	on rMulticast.recv(m dataMsg) {
		holdback.enqueue(m)
	}

	on rMulticast.recv(m orderMsg) {
		wait until m.id in holdback && seq == m.seq
		data := delete(m.id, holdback)
		deliver.enqueue(data)
		seq++
	}
}

// Sequencer is also a member of the multicast group.
class sequencer {
	seq := 0
	rMulticast := new  Trans

	on rMulticast.recv(m dataMsg) {
		order := orderMsg{m.id, seq}
		rMulticast.send(order)
		seq++
	}
}