The consensus algorithm in Tezos is based on the proof-of-stake mechanism. Proof-of-stake means that participants in the consensus algorithm are chosen in function of their stake (the amount of tokens a participant has). The same mechanism is used in the Tezos governance.
If one does not have enough stake to participate on its own or does not want to set up the needed infrastructure, (s)he can use delegation. Therefore, in Tezos, it is the delegates that may participate in consensus. However, at each level, not all delegates necessarily participate, and their participation weight may differ. The selection of the delegates’ participation rights at a level is done by running a PRNG (pseudo-random number generator). The PRNG’s seeds are obtained from random data that are regularly produced and stored on the blockchain. Thus, the procedure is deterministic in that delegates’ rights are uniquely determined from the seed; and it is random, in that its seed (and hence its results) cannot be predicted too much in advance.
A delegate is any implicit account registered as such by emitting a delegate registration operation.
Any account (implicit or originated) can specify a delegate
through a delegation operation.
Any account can change or revoke its delegate at any time. However, the change
only becomes effective after
PRESERVED_CYCLES + 2 cycles.
PRESERVED_CYCLES is a
A delegate participates in consensus and in governance with a weight
proportional with their delegated stake, which includes the balances
of all the accounts that delegate to it, and also the balance of the
delegate itself. To participate in consensus or in governance, a
delegate needs to have at least a minimal stake, which is given by the
TOKENS_PER_ROLL protocol constant.
Delegates place security deposits that may be forfeited in case they do not follow (some particular rules of) the protocol. Security deposits are deduced from the delegates’ own balance.
Active and passive delegates¶
A delegate can be marked as either active or passive. A passive delegate cannot participate in the consensus algorithm.
A delegate is marked as active at its registration.
A delegate becomes passive at the end of cycle
n when it has
failed to participate in the consensus algorithm in
PRESERVED_CYCLES + 1 cycles. That is, in cycles
n - PRESERVED_CYCLES.
Delegates’ rights selection¶
Tezos being proof-of-stake, the delegates’ rights are selected at random based on their stake. In what follows we detail the selection mechanism used in Tezos.
To each cycle is associated a random number called the seed. This seed is used within its cycle to generate pseudo-random values in the protocol, in particular for selecting delegates to participate in consensus.
For more information on randomness generation, see randomness-generation.
Before turning to the rights selection mechanism, we first introduce a new
terminology, stake snapshot, to denote the stake distribution for a given block,
as stored in the context.
Stake snapshots are taken (and stored) every
More precisely, a snapshot is taken at a level if and only if its cycle
BLOCKS_PER_STAKE_SNAPSHOT - 1.
Therefore, at the end of a cycle there are
BLOCKS_PER_STAKE_SNAPSHOT stored snapshots.
At the end of cycle
n-1-PRESERVED_CYCLES, the snapshot for cycle
n is randomly selected from the snapshots stored in cycle
n-1-PRESERVED_CYCLES. The selection is done through a very simple
PRNG having as seed the random seed for
Only the stake of active delegates with the minimal stake of
TOKENS_PER_ROLL is snapshot.
Delegates’ rights to participate are determined using the alias method, more precisely using Vose’s algorithm (see also this more pedagogic description; the algorithm is the last one listed there). This algorithm samples from a discrete probability distribution, which is given by the stakes in a particular stake snapshot: the probability to sample a particular delegate is its stake in the snapshot over the total stake in that snapshot.
Concretely, the delegates’ rights at a given level are expressed in terms of the (quantity of) slots that the delegate owns at that level. This quantity represents the delegate’s weight in consensus. We note that, in the long run (that is, on average over many levels), the number of slots is proportional to its stake. The owner of a slot is obtained by sampling using the algorithm mentioned above. More precisely, given a level and a slot (which is just a non-negative integer), the mentioned algorithm is invoked to assign a delegate to the given slot. Its input is the probability distribution given by the stake snapshot for the cycle to which the level belongs. And whenever the algorithm needs to draw a random value, this is obtained using a simple procedure which has as its initial state: the level, the random seed for the cycle to which the level belongs, and the slot.