Cassandra

History

1. What happens if there is a liveness break (i.e. an outage of validators) in a sharded network?

Etymology

State Model

Consensus

1. Proposal (e.g. transaction) initiated by client (e.g. wallet).
1. init function defines the initial state of the ledger within each shard involved in the transaction.
2. partition function determines which shards must be syncronized to maintain correct partial ordering.
3. State changes batched into a proposal (batch of state changes) and sent to relevant shard groups (validator + replicator sets).
2. Assessment stage:
1. Local Cerberus - Prepare:
1. Validators provision (lock) their relevant state.
2. Relevant shard groups gossip internally to determine well-formedness of the proposal (signatures, variable constraints, etc).
3. If a supermajority (2f+1) of validators (measured by stake) agree, each shard group adds a quorum signature to a state certificate with:
1. Proposal hash.
2. State inputs.
3. State outputs.
4. Merkle root hash.
5. Vote participants.
6. Validator stakes.
2. Local Cerberus - Pre-Commit:
1. map function collates state certificate into an executable ‘promise’.
2. Replicators call the reduce function to share relevant state inputs with every shard group identified by partition.
3. State certificate signed by shard group.
3. Local Cerberus - Commit:
1. apply function applies the new shard state within shard groups.
2. Shard groups vote on the new state.
3. Shard groups exchange state certificates within a default timeout window.
4. Event certificate formed as an aggregation of state certificates.
5. If one of the state certificates disagrees with the rest, the operation is aborted.
4. Emergent Cerberus:
1. Emergent ‘shard’ created by braided local Cerberus instances.
3. Commit stage:
1. Event certificate added to the consensus queue.
2. Event certificate committed to the ledger.

Further Reading

• Dan Hughes explaining the consensus process.

TPS Results