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Chapter 7 Epochs revised In this chapter, we consider the alternatives to requiring pre-allocated unique epochs, as specified in the earlier description of Classic Paxos (Chapter 2). Thus far, we have relied on the fact that the proposers will not dispatch propose(e,v) for the same epoch e and different values v. This could be achieved by a priori allocation of epochs between the proposers, thus each proposer may use only a disjoint subset of epochs and requiring each proposer to use each epoch only once. We have also shown that this can be achieved by voting for epochs during phase one of the proposer algorithm (introduced in §3.9, generalised in §4.3.4). However, the need to allocate epochs to proposers limits what we can achieve with single- valued consensus. In particular, it is desirable for any proposer to be able to decide a value with only one round trip in the best case. Classic Paxos allows any proposer to decide a value in two round trips, though one round trip can be executed prior to knowledge of the value. Paxos revision B enabled the proposer which is able to use the minimum epoch to skip phase one due to the lack of a phase one quorum intersection requirement. However, at most one of the proposers is able to utilise this. This chapter explores how to overcome this limitation by removing the requirement to pre-allocate or vote for unique epochs, thus providing proposers with more flexibility over the epochs they use. The three approaches discussed are: • Using an allocator to dynamically allocate epochs (§7.1). • Pre-allocation of epochs based on the value to be proposed in phase two (§7.2). • Allowing different values to be proposed with the same epoch but requiring phase two intersection and strengthened intersection requirements across phases (§7.3). These approaches, in addition to the original techniques (unique epochs by pre-allocation and voting) can be combined on a per-epoch basis to create a hybrid algorithm (§7.4). We will now consider each approach in more detail. 113PDF Image | Distributed consensus
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