Sui assumes the typical blockchain transaction is a user-to-user transfer or asset manipulation and optimizes for that scenario. As a result, Sui distinguishes between two types of assets:(i) owned objects that can be modified only by their specific owner and hence should never be under contention, and (ii) shared objects that have no specific owners and can be modified by more than one user. This distinction allows for a design that achieves very low latency by foregoing consensus for simple transactions involving only owned objects.
In Sui, consensus is only required when the transaction involves shared objects. For this, Sui employs the Narwhal DAG-based mempool and an efficient Byzantine Fault Tolerant (BFT) consensus via Bullshark. When shared objects are involved, the Sui validators play a similar role to validators in other blockchains and totally order transactions accessing shared objects.
Because Sui focuses on managing specific objects rather than a single aggregation of states, it also reports on them in a unique way: (i) every object in Sui has a unique version number, and (ii) every new version is created from a transaction that may involve several dependencies, themselves versioned objects.
As a consequence, a Sui validator – or any other entity with a copy of the state – can exhibit a causal history of an object, showing its history since genesis. Sui explicitly makes the bet that in many cases, the ordering of that causal history with the causal history of another object is irrelevant; and in the few cases where this information is relevant, Sui makes this relationship explicit in the data.
Sui guarantees transaction processing obeys eventual consistency in the classic sense. This breaks down into two parts:
Eventual delivery - if one honest validator processes a transaction, all other honest validators will eventually do the same.
Convergence - two validators that have seen the same set of transactions share the same view of the system (reach the same state).
But contrary to a typical blockchain, Sui does not stop the flow of transactions in order to witness the convergence until the end of an epoch.
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