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The aim of this Request for Comment (RFC) is to describe the key elements of the Tari second layer, also known as the Digital Assets Network (DAN).
The Tari DAN is based on a sharded BFT consensus mechanism called Cerberus.
One particular note is that Tari has chosen Hotstuff as the base BFT consensus algorithm over pBFT mentioned in the paper.
The core idea of Cerberus is that instead of dividing work up between validator nodes according to the contracts they are managing (as per Tari DANv1, Polkadot, Avalanche, etc.), Cerberus distributes nodes evenly over a set of shard addresses. Any time an instruction modifies the state of a contract, it will affect one or more shard addresses, and only those nodes that are responsible for covering those addresses will reach consensus on the correct state changes.
This means that nodes have to be prepared to execute instructions on any contract in the network. This does create a data synchronisation burden, but the added benefit of a highly scalable, decentralised DAN significantly outweighs this trade-off.
There are several components on both the DAN and base layer that interoperate to collectively enable scalable smart contracts on Tari.
These components include:
- Tari base layer - Enforces Tari monetary policy and plays the role of global registrar.
- Templates - Reusable smart contract components.
- Contracts - Self-contained pieces of code that describe the behaviour of a smart contract. They are compiled and executed in the Tari VM.
- Validator Nodes - VNs validate smart contracts and earn fees for doing so.
- Cerberus consensus engine - highly scalable, high-speed sharded BFT consensus engine.
- Tari Virtual Machine - Runs smart contracts in a secure sandbox.
- Thaums - the unit of magic that fuels the Tari DAN.
The remainder of this document describes these elements in a little more detail and how they relate to each other.
Obviously, the most important role of the Tari base layer is to issue and secure the base Tari token. As it relates to the DAN, the base layer also serves as an immutable global registry for several key pieces of data:
- It maintains the register of all validator nodes.
- It provides the only means of minting more [Thaum]s into the DAN economy.
- It maintains the register of all DAN contract templates.
The base layer also forces the DAN to make progress in the case of a Byzantine stoppage.
Templates are parameterised smart contracts. Templates are intended to be well-tested, secure, reusable components for building and running smart contracts on the DAN.
For example, an NFT template would allow a user to populate a few fields, such as name, number of tokens, media locations, and then launch a new NFT series without having to write any actual code.
Templates are stored and managed on the base layer. You can think of the Tari base layer as a type of git for smart contracts. Templates will also have version control features and a smooth upgrade path for existing contracts.
Tari smart contracts are the meat of the DAN ecosystem. Usually a smart contract will be comprised of one or more Tari templates, glue code, and initialisation code.
The contracts are always executed in the Tari Virtual machines. The input and output of every contract instruction is validated by validator nodes that reach consensus using the Cerberus consensus engine.
Validator Nodes (VNs) execute and reach consensus on DAN contract instructions. VNs must register on the base layer and lock up funds (the registration deposit) in order to participate in the DAN.
With this in place, every base node has an up-to-the-minute list of all active validator nodes and their metadata. The registration deposit also serves as a Sybil prevention mechanism.
Validator nodes are the bridge between the consensus layer and the Tari Virtual Machine (TVM).
VNs are required to re-register periodically as a proof-of-liveness mechanism.
Validator nodes must be able to
- interpret the instructions they receive from clients, identifying the contract code that the instruction refers,
- retrieve and deserialize the relevant input state,
- compute the output state that result from applying the contract logic to the input state, and
- reach consensus with its peers.
Steps 1 - 3 are carried out in the Tari Virtual Machine (TVM). Step 4 is achieved by communicating with peers via the DAN consensus layer.
Thaums exist at the Validator node level, and VNs earn fees, in Thaums, for each instruction that it aids in getting finalised.
The Cerberus BFT consensus algorithm runs on the consensus layer. This layer is completely ignorant of the semantics of DAN smart contracts.
This layer only cares that:
- only VNs that have registered on the base layer are participating in consensus.
- VNs are self-organising into VN committees and are carrying out the rules of Cerberus correctly.
In particular, the consensus layer has no idea whether an instruction's output is correct. If two-thirds (plus one) of the committee agree on the results, then consensus has been reached and the consensus layer is happy.
For example, if consensus decides that 2 + 2 = 5, then for the purposes of this contract, that is the case.
The TVM is a WASM-based virtual machine designed to run Tari contracts. Contracts are composed of one or more Tari templates, glue code and a state schema. Tari Labs provides a Rust implementation for TVM contracts, but in principle, other languages could implement the specification as well.
The TVM is able to
- load a contract.
- provide a list of methods that the contract exposes.
- Execute calls on the contract.
- Persist and restore the state of the contract.
Thaums is a unit of magic. Thaums are used to power the DAN's economic engine. Thaums are created with a one-way perpetual peg as described in RFC-0320. Briefly, Tari is burnt to create Thaums, which are used to pay for the execution of instructions on the DAN. A portion of instruction fees are burnt with every instruction to provide a constant source of demand for Thaums in the DAN.
There is no peg out back to the base layer for Thaums. The reason for this is explained in RFC-0320. Thaum holders wishing to convert back to Tari will be able to perform a submarine swap with a Tari seller. We also anticipate that exchanges will list the Thaum-Tari pair to enable easy conversion of Thaums back to Tari.
|1 Nov 2022||High-level overview||CjS77|
|26 Oct 2022||First outline||SWvHeerden|