All the supposed "simpler" consensus protocols I've seen just hide all the complexity somewhere (like raft with its leader election). Where does streamlet hide its complexity?
That is, we're now solving consensus for a synchronous system.
So for example the Paxos assumption:
>Messages are sent asynchronously and may take arbitrarily long to deliver.
Does no longer hold AFAIK.
Edit: Good article, they even say this:
>Partially Synchronous Network: The network can at times be unreliable, or adversarial; the protocol must not lose consistency in this case. However, when network conditions are good, i.e., when honest players can communicate with each other within 1 second, the protocol must make progress.
The only assumption is that clocks are synchronized, which is much, much weaker. (We can actually remove even this assumption. But it's easier to think about this way.)
i.e. I think it's monday at the same time you think it's monday, or I think it's epoch 1 at the same time you think it's epoch 1. or my quartz crystal oscillates at the same rate as your quartz crystal. Importantly, a message sent during epoch 1 might arrive in epoch 3, or never at all.
Paxos is also in the partially synchronous setting - it requires some network synchrony in order to terminate, just like streamlet (For truly asynchronous protocols, there's a whole, complicated literature).
As far as I've understood it Paxos is fully asynchronous, but you can either guarantee correctness or termination and never both for those systems. Paxos decided on correctness. Am I wrong in this?
Thus, for Paxos to make progress, the network needs to eventually "heal" and deliver messages in a timely manner -- that's what we mean by partial synchrony. Streamlet, likewise, tolerates arbitrary network delays (in the sense that no two players will agree on different things even if Comcast maliciously scheduled every message), but also requires partial synchrony to make progress (i.e. once in a while Comcast will deliver messages in timely fashion and in order, like you expect).
(Partial synchrony is arguably the most practical of the theoretical synchrony models for consensus. There are other models, i.e. "periods of synchrony", but they all end up capturing the same idea).
FLP, however, is a fairly weak statement. It only rules out termination for deterministic protocols. We can in fact design randomized asynchronous protocols that make progress (with good probability), regardless of the ordering in which messages are delivered. I.e. as long as Comcast eventually delivers every message -- and letting it choose delays arbitrarily -- a randomized protocol could make progress. These are the "true" asynchronous protocols. (Paxos is not one of them)
I guess the original asynchronous consensus protocol is Ben-Or's protocol (which takes 2^n time in expectation IIRC, where n is the number of players). We can reduce the running time using some (fairly sophisticated) cryptography -- the most famous is the Canetti-Rabin protocol, and Cachin and many others have worked on it since. Most of the complexity (for polynomial time protocols) is in building a "global common coin" -- i.e. a global coin that everyone agrees on the result of; once you have that we can use Mostefaoui's protocol for instance. Building this coin is incredibly complicated (much more so than Paxos) unless we assume some strong cryptographic trusted setup (i.e. a nice threshold signature scheme). How practical any of these schemes are is an open question.
Raft on the other hand... I've implemented versions of both paxos and raft, and by golly was the latter more difficult to get right.
Could you clarify what you mean? "Paxos" as defined in the literature (and as presented in this article) is a single-shot consensus algorithm. Raft is a leader-based replicated state machine. They are not comparable at all and it's not surprising if you find Raft more difficult.
However, in practice, when people say "Paxos" they usually mean some variant of MultiPaxos and "Raft vs Paxos" is typically interpreted as "Raft vs MultiPaxos". It's also well-established that MultiPaxos and Raft have roughly the same complexity.
You're right, I guess we generally talk about state machine replication instead of one-shot systems.
From the link; "The protocol runs sequentially in synchronized ‘epochs’ that are 2 seconds long. Every player starts in epoch 0 at the same time...."
Safety in particular doesn't require any timing assumption.
(Note that messages can still be delayed arbitrarily in both protocols)
Consensus protocols are either Paxos, Paxos with unnecesary additional cruft, or broken.