how-sui-works.md

title
How Sui Works

This document is written for engineers, developers, and technical readers knowledgeable about the blockchain. It does not assume deep programming language or distributed systems expertise. See the Sui white paper for a much deeper explanation of how Sui works. See How Sui Differs from Other Blockchains for a high-level overview of the differences between Sui and other blockchain systems.

Overview

The Sui blockchain operates at a speed and scale previously thought unattainable. Sui assumes most blockchain transactions touch non-overlapping states, meaning that transactions can run in parallel. Sui optimizes for single-writer objects, allowing a design that forgoes consensus for simple transactions.

Instead of the traditional blockchain’s fire-and-forget broadcast, Sui's design enables a requestor or a proxy to proactively talk to validators to bring a transaction to finality. This results in near instant finality for simple transactions.

With this low latency, Sui makes it easy to incorporate transactions into games and other settings that need completion in real time. Sui also supports smart contracts written in Move, a language designed for blockchains with strong inherent security and a more understandable programming model.

In a world where the cost of bandwidth is diminishing steadily, we are creating an ecosystem of services that will find it easy, fun, and perhaps profitable to ensure transaction voting on behalf of users.

Components

Become familiar with these key Sui concepts:

• Objects - Sui has programmable objects created and managed by Move packages (a.k.a. smart contracts). Move packages themselves are also objects. Thus, Sui objects can be partitioned into two categories: mutable data values and immutable packages.
• Transactions - All updates to the Sui ledger happen via a transaction. This section describes the transaction types supported by Sui and explains how their execution changes the ledger.
• Validators - The Sui network is operated by a set of independent validators, each running its own instance of the Sui software on a separate machine (or a sharded cluster of machines operated by the same entity).

Architecture

Sui is a distributed ledger that stores a collection of programmable objects, each with a globally unique ID. Every object is owned by a single address, and each address can own an arbitrary number of objects.

The ledger is updated via a transaction sent by a particular address. A transaction can create, destroy, and write objects, as well as transfer them to other addresses.

Structurally, a transaction contains a set of input object references and a pointer to a Move code object that already exists in the ledger. Executing a transaction produces updates to the input objects and (if applicable) a set of freshly created objects along with their owners. A transaction whose sender is address A can accept objects owned by A, shared objects, and objects owned by other objects in the first two groups as input.

flowchart LR
    CC(CLI Client) --> ClientService
    RC(Rest Client) --> ClientService
    RPCC(RPC Client) --> ClientService
    ClientService --> AuthorityAggregator
    AuthorityAggregator --> AC1[AuthorityClient] & AC2[AuthorityClient]
    subgraph Authority1
      AS[AuthorityState]
    end
    subgraph Authority2
      AS2[AuthorityState]
    end
    AC1 <==>|Network TCP| Authority1
    AC2 <==>|Network TCP| Authority2

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Sui validators agree on and execute transactions in parallel with high throughput using Byzantine Consistent Broadcast.

System overview

This section is written for a technical audience wishing to gain more insight about how Sui achieves its main performance and security objectives.

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 (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 forgoing consensus for simple transactions involving only owned objects.

Sui mitigates a major hindrance to blockchain growth: head-of-line blocking. Blockchain nodes maintain an accumulator that represents the state of the entire blockchain, such as the latest certified transactions. Nodes participate in a consensus protocol to add an update to that state reflecting the transaction’s modification to blocks (add, remove, mutate). That consensus protocol leads to an agreement on the state of the blockchain before the increment, the validity and suitability of the state update itself, and the state of the blockchain after the increment. On a periodic basis, these increments are collected in the accumulator.

In Sui, this consensus protocol is required only when the transaction involves shared objects. For this, Sui offers the Narwhal and Bullshark DAG-based mempool and efficient Byzantine Fault Tolerant (BFT) consensus. When shared objects are involved, the Sui validators play the role of more active validators in other blockchains to totally order the transaction with respect to other transactions accessing shared objects.

Because Sui focuses on managing specific objects rather than a single aggregation of state, 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 classical sense. This breaks down in 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 blockchain, Sui does not stop the flow of transactions in order to witness the convergence.

Simple transactions

Many transactions do not have complex interdependencies with other, arbitrary parts of the blockchain state. Often financial users just want to send an asset to a recipient, and the only data required to gauge whether this simple transaction is admissible is a fresh view of the sender's address. This observation allows Sui to forgo consensus and instead use simpler algorithms based on Byzantine Consistent Broadcast. See our list of potential single-writer apps for examples of real-world simple transactions.

These protocols are based on the FastPay design that comes with peer-reviewed security guarantees. In a nutshell, Sui takes the approach of taking a lock (or "stopping the world") only for the relevant piece of data rather than the whole chain. In this case, the only information needed is the sender address, which can then send only one transaction at a time.

Sui further expands this approach to more involved transactions that may explicitly depend on multiple elements under their sender's control, using Move’s object model and leveraging Move's strong ownership model. By requiring that dependencies be explicit, Sui applies a multi-lane approach to transaction validation, making sure those independent transaction flows can progress without impediment from the others.

Sui validates transactions individually, rather than batching them into traditional blocks. The key advantage of this approach is low latency; each successful transaction quickly obtains a certificate of finality that proves to anyone the transaction will be processed by the Sui network.

The process of submitting a Sui transaction is thus a bit more involved than in traditional blockchains. Whereas a usual blockchain can accept a bunch of transactions from the same author in a fire-and-forget mode, Sui transaction submission follows these steps:

1. The sender broadcasts a transaction to all Sui validators.
2. Each Sui validator replies with an individual vote for this transaction. Each vote has a certain weight based on the stake owned by the validator.
3. The sender collects a Byzantine-resistant-majority of these votes into a certificate and broadcasts that back to all Sui validators. This settles the transaction, ensuring finality that the transaction will not be dropped (revoked).
4. Optionally, the sender collects a certificate detailing the effects of the transaction.

While those steps demand more of the sender, performing them efficiently can still yield a cryptographic proof of finality with minimum latency. Aside from crafting the original transaction itself, the session management for a transaction does not require access to any private keys and can be delegated to a third party. Sui takes advantage of this observation to provide Sui Gateway services.

Complex contracts

Complex smart contracts may benefit from shared objects where more than one user can mutate those objects (following smart contract specific rules). In this case, Sui totally orders all transactions involving shared objects using a consensus protocol. Sui uses a novel peer-reviewed consensus protocol based on Narwhal. This is state-of-the-art in terms of both performance and robustness.

The Narwhal mempool offers a high-throughput data availability engine and a scaled architecture, splitting the disk I/O and networking requirements across several workers. And Bullshark offers a zero-message overhead consensus algorithm, leveraging graph traversals.

Transactions involving shared objects also contain at least one owned object to pay for gas fees. It is thus essential to carefully compose the protocol dealing with owned objects with the protocol sequencing the transaction to guarantee Sui’s security properties. When shared objects are involved, transaction submission follows these steps:

1. The sender broadcasts a transaction to all Sui validators.
2. Each Sui validator replies with an individual vote for this transaction. Each vote has a certain weight based on the stake owned by the validator.
3. The sender collects a Byzantine-resistant-majority of these votes into a certificate and broadcasts it back to all Sui validators. This time however, the certificate is sequenced through Byzantine Agreement.
4. Once the transaction has been successfully sequenced, the user broadcasts again the certificate to the validators to settle the transaction.

Scalability

As mentioned, Sui does not impose a total order on the transactions containing only owned objects. Instead, transactions are causally ordered. If a transaction T1 produces an output object O1 used as input objects in a transaction T2, a validator must execute T1 before it executes T2. Note that T2 does not need to use these objects directly for a causal relationship to exist, e.g., T1 might produce output objects which are then used by T3, and T2 might use T3's output objects. However, transactions with no causal relationship can be processed by Sui validators in any order. This insight allows Sui to massively parallelize execution, and shard it across multiple machines.

Sui employs the state-of-the-art Narwhal consensus protocol to totally order transactions involving shared objects. The consensus sub-system also scales in the sense that it can sequence more transactions by adding more machines per validator.

Smart contract programming

Sui smart contracts are written in the Move language. Move is safe and expressive, and its type system and data model naturally support the parallel agreement/execution strategies that make Sui scalable. Move is an open source programming language for building smart contracts originally developed at Meta for the Diem blockchain. The language is platform-agnostic, and in addition to being adopted by Sui, it has been gaining popularity on other platforms (e.g., 0L, StarCoin).

Find a more thorough explanation of Move’s features in:

• the Move Programming Language book
• Sui-specific Move instructions and differences on this site
• the Sui whitepaper and its formal description of Move in the context of Sui