ACP-181 ProposerVM Epochs #181
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| Epochs have a constant duration $D$, such that for any given epoch $E_n$, the difference between its start and end times equals this duration: | ||
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| T_{end}^n - T_{start}^n = D | ||
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| $D$ is hardcoded into the ProposerVM source code, and may only be changed by a required network upgrade. |
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What if, instead of having a set epoch duration, where edge case 2.
You would lose being able to keep track of the numbering of the epochs, but I don't think that's important anyway. In this case we should always be able to check the timestamp of the start of the epoch, because we already have to know PChainEpochHeight.
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If there's a long gap between the epoch's end time and the block that seals it, you'd still have the same situation of PChainEpochHeight and PChainHeight skew.
Predictable epochs make implementing the current implementation very straightforward. I initially decided against including pseudocode of this in the ACP itself, but think it would go a long way to explaining why the definition is what it is. I'll add that.
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I'm pretty sure it would eliminate the skew and actually simplify the current implementation. I'll write out an example...
D = 100
Epoch 0 Tstart = 0 Tend = 100 PChainEpochHeight = 0
Block 1 time = 0
Block 2 time = 57
Block 3 time = 99
Block 4 time = 103 SEALS EPOCH 0
Epoch 1 Tstart = 103 Tend = 203 PChainEpochHeight = 4
Block 5 time = 107
Block 6 time = 202
Block 7 time = 215 SEALS EPOCH 1
Epoch 2 Tstart = 215 Tend = 315 PChainEpochHeight = 7
Block 8 time = 400 SEALS EPOCH 2
Epoch 3 Tstart = 400 Tend = 500 PChainEpochHeight = 8
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All we need to check is if the timestamp of the parent is greater than D + timestamp of the PChainEpochHeight of the parent. If so, we advance the epoch.
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The edge case occurs if there's a long gap between the last block within the epoch bounds and the block that seals the epoch. Modifying your example:
Epoch 0 Tstart = 0 Tend = 100 PChainEpochHeight = 0
Block 1 time = 0
Block 2 time = 57
Block 3 time = 99
Block 4 time = 500 SEALS EPOCH 0
Block 4 will use the same PChainEpochHeight as block 1, even though many epoch durations have elapsed.
Proposes enabling epochs in the ProposerVM to provide more stable views of the P-Chain to inner VMs
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