Espresso DA

Architecture

ConsensusAn agreement on the latest and correct state of a blockchain. Unlike L1 blockchains which coordinate participating nodes with consensus rules, rollups rely on L1s for reaching consensus by checking the state of the rollup smart contract deployed thereon.

Espresso uses the HotShot consensus protocol, a communication-efficient proof-of-stake system that is Byzantine Fault Tolerant (BFT). The validatorIn the context of L2s, a Validator is an actor that validates the correctness of state transitions. For optimistic rollups this corresponds to challengers, and for ZK rollups this corresponds to the onchain verifier set is permissionlessAnyone willing should be able to join and leave the network at any time, without causing significant disturbance to the network or being detrimental to the party in question. No single entity should have the power to allowlist or blocklist participants.: anyone can stake ESP tokens and the top 100 validators by total stake form the active consensus set, which is dynamically adjusted at epoch boundaries. Built on HotStuff-2, it achieves linear communication complexity using a pacemaker module to synchronize views and ensures safety and livenessLiveness refers to the ability of a system to respond to requests and to process them in a timely manner. In the context of L2s, it refers to the ability of settling transactions, proofs and state roots to the base layer. as long as over two-thirds of the stake is controlled by honest nodes. Although validators are required to stake ESP to participate, there is currently no slashing mechanism in place for misbehaving nodes.

HotShot operates in a view-by-view manner, where each view designates a leader and an external builder. During a view, the consensus proposerIn the context of L2s, the actor that proposes a claimed state root on L1. The term is also used in the context of Ethereum to refer to the actor that proposes a new block. finalizes a blockAn ordered list of transactions and chain-related metadata that gets bundled together and published to the L1/DA layer. Nodes execute the transactions contained within blocks to change the rollup chain’s state. Protocol rules dictate what constitutes a valid block, and invalid blocks are skipped over. with a certificate of availability by utilizing Espresso DA for data availabilityThe property of a rollup's data being reachable by any node retrieving the data that were rolled up and executed to reach the proposed state. Data availability (DA), specifically decoupling it from the rollup nodes themselves, is one of the preeminent factors which allows a rollup to scale securely. A rollup is faced with a decision of what to use as a DA layer to guarantee that any node can retrieve this data--permissionlessly under any circumstance. For this reason, using Ethereum for DA currently provides the strongest security guarantees. If data is stored somewhere other than a permissionless L1, then the project is not a rollup, but rather a validium or an optimium..

Data Availability Certificate

Once the proposer sends data to HotShot nodeA software client that participates in the network. operators, they initiate Espresso DA’s three layers of data availability:

• VID Layer: Disperses erasure-coded data to all nodes. VID layer nodes only store chunks of the data.
• DA Committee Layer: Uploads the data and commitment to a small DA committee. Every node in the committee stores the full data.
• CDN Layer: Uploads the full data to a content delivery networkA constellation of nodes (peers) that communicate via a peer-to-peer protocol, for example, in propagating transactions and blocks to other nodes. (CDN).

Once nodes receive and store the data, they return votes to the proposer. DAVotes are votes from committee nodes storing the full data, while QuorumVotes are votes from nodes storing erasure-coded shares of the data. A DA certificate consists of two components, the retrievability certificate and the optimistic DAC certificate:

• Retrievability Certificate: Formed when the DA leader collects 2/3 + 1 QuorumVotes.
• Optimistic DAC Certificate: Formed when the DA leader gathers 2/3 + 1 DAVotes from the DA committee. Currently, the committee size is 21 members, so the threshold is 15 signatures.

Once the DAC is formed, the DA leader stops broadcasting data to the nodes.

L2s Data Availability

The life cycle of L2Layer 2 (L2) is a category of technical solutions aimed to scale the base layer in a trust minimized way. This category includes solutions like rollups as well as state channels and plasma. Other solutions are able to scale further, but with the introduction of additional trust assumptions, which are therefore not trust minimized. Sometimes the term Layer 2 is used to refer to include these solutions too, like validiums and optimiums, but to distinguish between trust minimized and non trust minimized solutions they are often referred to as "light" L2s, opposed to "strong" L2s like rollups. transactions begins with users submitting transactions to the Espresso DA mempool through an RPC endpoint, or directly to the block builder private mempool, including a namespace ID to indicate the target L2 rollupA blockchain that inherits consensus and data availability from another blockchain called L1. Rollups enable trust minimized bridges with the base layer via proof systems, either optimistic or zero-knowledge. A rollup without a bridge, or without considering the bridge, is called a sovereign rollup.. A DA leader collects and disperses these transactions across Espresso DA’s layers to form a DA certificate. The leader then broadcasts a proposal with a vector commitment for the transactions to the HotShot consensus layer. The finalization of the block commitment in HotShot establishes data availability for the corresponding transactions. After block finalization in HotShot, the relayer propagates the commitment and quorum certificates to the L1Layer 1 (L1) is a blockchain that is self-reliant on its validator set for its security and consensus properties. Ethereum is an example of a layer 1. Blockchains started receiving the moniker of layer 1 once layer 2 became a meaningful area of development. Light ClientSometimes labelled interchangeably as a “node”, they are tasked with processing transactions and managing the blockchains's state. They run the computations for each transaction according to the rollup's virtual machine and protocol rules. If comparing to Ethereum clients, these would be execution clients such as Geth, as opposed to consensus clients. contract, which verifies the certificate and the HotShot state SNARKShort for "succinct non-interactive argument of knowledge", a SNARK is a widely used type of zero-knowledge proof that is short and fast to verify. Different kinds of SNARKs are usually systematized by proof size, verification time, and type of setup. The most famous SNARKs are Groth16, PLONK/Marlin, Bulletproofs, and STARKs. proof via the verifyProof function.

Users can retrieve data by querying any of Espresso DA’s layers, though the VID layer is slower due to the reconstruction of erasure-coded shares. L2s can also use a verifyInclusion function on an L1 light client smart contract to confirm a blobThe data that a rollup publishes to its L1/data availability (DA) layer. They consist of the L2 transactions that are rolled up, along with some metadata. Blobs are introduced as a new transaction type within Ethereum with EIP-4844, and has rollup scaling specifically in mind. Blobs persist on Ethereum’s Beacon Chain ephemerally.’s inclusion in the Espresso DA HotShot chain.

No DA bridgeSystem that verifies that data has been made available. It takes the form of a smart contract verifying a consensus or, if the data is verified directly by either downloading the full data or sampling, of an enshrined bridge. is selected. Without a DA bridge, Ethereum has no proof of data availabilityThe property of a rollup's data being reachable by any node retrieving the data that were rolled up and executed to reach the proposed state. Data availability (DA), specifically decoupling it from the rollup nodes themselves, is one of the preeminent factors which allows a rollup to scale securely. A rollup is faced with a decision of what to use as a DA layer to guarantee that any node can retrieve this data--permissionlessly under any circumstance. For this reason, using Ethereum for DA currently provides the strongest security guarantees. If data is stored somewhere other than a permissionless L1, then the project is not a rollup, but rather a validium or an optimium. for this project.