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. Delegates’ rights to participate are determined by a follow-the-coin strategy. This procedure is random, in that its result cannot be predicted too much in advance. The randomness is obtained from information already found on the blockchain. Thus, the procedure is also deterministic: delegates’ rights are uniquely determined from the random element.
A delegate is any implicit account registered as such by emitting a delegate registration operation.
Any accounts (implicit or originated) can specify a delegate through a delegation operation.
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.
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. Therefore delegates may be subject to over-delegation.
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 for cycle
n when they
fail to participate in the consensus algorithm in
PRESERVED_CYCLES 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 theory, it would be possible to give each token a serial number and track the specific tokens assigned to specific delegates. However, it would be too demanding of nodes to track assignments at such a granular level. Instead, Tezos works with sets of tokens which are called rolls.
A roll holds
TOKENS_PER_ROLL tokens. When tokens are moved, or a delegate for an
account is changed, the rolls change delegate according to the following
Each delegate has a stack of roll identifiers plus some “change” which is always
an amount smaller than
TOKENS_PER_ROLL. When tokens are moved from one
delegate to the other, first, the change is used. If it is not enough, rolls
need to be “broken” which means that they move from the delegate stack to a
global, unallocated, roll stack. This is done until the amount is covered, and
some change possibly remains.
Then, the other delegate is credited. First, the amount is added to the
“change”. If it becomes greater than
TOKENS_PER_ROLL, then rolls are
unstacked from the global unallocated roll stack onto the delegate stack. If the
global stack is empty, a fresh roll is created.
This preserves the property that if the delegate is changed through several transactions, the roll assignment is preserved, even if each operation moves less than a full roll.
The advantage of tracking tokens in this way is that a delegate creating a malicious fork cannot easily change the specific rolls assigned to them, even if they control the underlying tokens and shuffle them around.
n is associated with a random seed. The random seed for cycle
n is a 256-bit number generated at the very end of cycle
nonces to which delegates commit during cycle
n-2. One block out of every
BLOCKS_PER_COMMITMENT can contain a commitment. A commitment is the hash of
a nonce. The commitment is generated by the block proposer and is included in
the block header.
The committed nonce must be revealed by the original block proposer during cycle
n-1 under penalty of forfeiting the rewards and fees of the block that
included the commitment. The associated security deposit is not forfeited.
A nonce revelation is an operation, and multiple nonce revelations can thus be
included in a block. A reward
SEED_NONCE_REVELATION_TIP is given for
including a revelation. Revelations are free operations which do not compete
with transactions for block space. Up to
wallet activations and denunciations can be contained in any given block.
The seed for cycle
n is obtained as follows: the seed of cycle
hashed with a constant and then with each nonce revealed in cycle
To return to the rights selection mechanism, we first introduce a new
terminology, roll snapshot, to denote the stored (in the
context) distribution of rolls for a given block. Roll
snapshots are taken (and stored) every
The delegates’ rights at a given level and for a particular role in the protocol are expressed in terms of slots that the delegate receives for that role. The slot owner is obtained by running a PRNG (pseudo-random number generator) with the following input:
the role (a string)
the slot (a non-negative integer)
Let n be the cycle the level belongs to.
The seed of the PRNG is the random seed associated with cycle
The PRNG first selects a snapshot from cycle
n-PRESERVED_CYCLES-2 and then it selects a roll in the selected snapshot.
The slot owner is then the roll owner.
Protocols are parameterized by several parameters called protocol constants, which may vary from one protocol to another or from one network to another (for instance, test networks move faster).
An example of a parameter is the number of tez constituting a roll. This number is given by the constant named
The list of protocol constants can be found in the API of the Constants module.
In particular, the protocol constants related to the proof-of-stake mechanism are detailed below.