The chapter describes how forks naturally occur as a consequence of leader rotation.
Nodes take turns being leader and generating the PoH that encodes state changes. The cluster can tolerate loss of connection to any leader by synthesizing what the leader would have generated had it been connected but not ingesting any state changes. The possible number of forks is thereby limited to a "there/not-there" skip list of forks that may arise on leader rotation slot boundaries. At any given slot, only a single leader's transactions will be accepted.
Transactions are ingested by the current leader.
Leader filters valid transactions.
Leader executes valid transactions updating its state.
Leader packages transactions into entries based off its current PoH slot.
Leader transmits the entries to validator nodes (in signed shreds) 1. The PoH stream includes ticks; empty entries that indicate liveness of
the leader and the passage of time on the cluster.
A leader's stream begins with the tick entries necessary complete the PoH
back to the leaders most recently observed prior leader slot.
Validators retransmit entries to peers in their set and to further
Validators validate the transactions and execute them on their state.
Validators compute the hash of the state.
At specific times, i.e. specific PoH tick counts, validators transmit votes
to the leader.
Votes are signatures of the hash of the computed state at that PoH tick
Votes are also propagated via gossip
Leader executes the votes as any other transaction and broadcasts them to
Validators observe their votes and all the votes from the cluster.
Forks can arise at PoH tick counts that correspond to a vote. The next leader may not have observed the last vote slot and may start their slot with generated virtual PoH entries. These empty ticks are generated by all nodes in the cluster at a cluster-configured rate for hashes/per/tick
There are only two possible versions of the PoH during a voting slot: PoH with
T ticks and entries generated by the current leader, or PoH with just ticks. The "just ticks" version of the PoH can be thought of as a virtual ledger, one that all nodes in the cluster can derive from the last tick in the previous slot.
Validators can ignore forks at other points (e.g. from the wrong leader), or slash the leader responsible for the fork.
Validators vote based on a greedy choice to maximize their reward described in Tower BFT.
The diagram below represents a validator's view of the PoH stream with possible forks over time. L1, L2, etc. are leader slots, and
Es represent entries from that leader during that leader's slot. The
xs represent ticks only, and time flows downwards in the diagram.
Note that an
E appearing on 2 forks at the same slot is a slashable condition, so a validator observing
E3' can slash L3 and safely choose
x for that slot. Once a validator commits to a forks, other forks can be discarded below that tick count. For any slot, validators need only consider a single "has entries" chain or a "ticks only" chain to be proposed by a leader. But multiple virtual entries may overlap as they link back to the a previous slot.
It's useful to consider leader rotation over PoH tick count as time division of the job of encoding state for the cluster. The following table presents the above tree of forks as a time-divided ledger.
ticks since prev
Note that only data from leader L3 will be accepted during leader slot L3. Data from L3 may include "catchup" ticks back to a slot other than L2 if L3 did not observe L2's data. L4 and L5's transmissions include the "ticks to prev" PoH entries.
This arrangement of the network data streams permits nodes to save exactly this to the ledger for replay, restart, and checkpoints.
When a new leader begins a slot, it must first transmit any PoH (ticks) required to link the new slot with the most recently observed and voted slot. The fork the leader proposes would link the current slot to a previous fork that the leader has voted on with virtual ticks.