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CHAPTER 7. EPOCHS REVISED 135 path) and pre-allocation for all other epochs (slow path). We will now consider each of these three algorithms. 7.4.1 Multi-path Paxos using allocator One of the key limitations of allocating exclusive epochs using an allocator (§7.1) is that the liveness of the system now depends on the availability of the allocator, a single participant. This can be addressed by a hybrid approach consisting of using an allocator only for emin (fast path) and pre-allocation for all other epochs (slow path)1415. The fast path proposer algorithm begins with a message exchange with the allocator. If the proposer is allocated emin then it can bypass phase one and propose its candidate value in phase two of emin. If the fast path is unsuccessful, either because the allocator is unavailable or another proposer has already been allocated emin, then the proposer executes Paxos as usual16, this is referred to as the slow path17. Algorithm 30: Phase zero of Multi-path Paxos with an allocator /* Start of Phase 0 */ 1 2 3 4 5 6 7 8 9 10 send generate-next() to allocator switch do case allocate( emin) received e ← emin,v ← γ goto phase two case timeout or no-allocate() received e ← min(E) E ← E \ {e} goto phase one ... Algorithm 30 gives an example of phase zero, the epoch selection phase. If epoch emin is allocated to the proposer then it proceeds to phase two. Otherwise, if emin has already been allocated or the allocator does not respond, the proposer uses one of its pre-allocated epochs. The allocator could be implemented as a simple boolean flag to indicate whether emin has been allocated. 14Equally, epochs by phase one voting could be used for all other epochs instead of pre-allocation. 15Note that we could extend this approach to the allocator for the first n epochs instead of just emin. 16Except that emin cannot be pre-allocated. 17In practice, the proposer may choose whether to first try the fast path or proceed directly to the slow path.PDF Image | Distributed consensus
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