It is often useful to allow low resourced clients to participate in a Solana cluster. Be this participation economic or contract execution, verification that a client's activity has been accepted by the network is typically expensive. This proposal lays out a mechanism for such clients to confirm that their actions have been committed to the ledger state with minimal resource expenditure and third-party trust.
A Naive Approach
Validators store the signatures of recently confirmed transactions for a short period of time to ensure that they are not processed more than once. Validators provide a JSON RPC endpoint, which clients can use to query the cluster if a transaction has been recently processed. Validators also provide a PubSub notification, whereby a client registers to be notified when a given signature is observed by the validator. While these two mechanisms allow a client to verify a payment, they are not a proof and rely on completely trusting a validator.
We will describe a way to minimize this trust using Merkle Proofs to anchor the validator's response in the ledger, allowing the client to confirm on their own that a sufficient number of their preferred validators have confirmed a transaction. Requiring multiple validator attestations further reduces trust in the validator, as it increases both the technical and economic difficulty of compromising several other network participants.
A 'light client' is a cluster participant that does not itself run a validator. This light client would provide a level of security greater than trusting a remote validator, without requiring the light client to spend a lot of resources verifying the ledger.
Rather than providing transaction signatures directly to a light client, the validator instead generates a Merkle Proof from the transaction of interest to the root of a Merkle Tree of all transactions in the including block. This Merkle Root is stored in a ledger entry which is voted on by validators, providing it consensus legitimacy. The additional level of security for a light client depends on an initial canonical set of validators the light client considers to be the stakeholders of the cluster. As that set is changed, the client can update its internal set of known validators with receipts. This may become challenging with a large number of delegated stakes.
Validators themselves may want to use light client APIs for performance reasons. For example, during the initial launch of a validator, the validator may use a cluster provided checkpoint of the state and verify it with a receipt.
A receipt is a minimal proof that; a transaction has been included in a block, that the block has been voted on by the client's preferred set of validators and that the votes have reached the desired confirmation depth.
Transaction Inclusion Proof
A transaction inclusion proof is a data structure that contains a Merkle Path from a transaction, through an Entry-Merkle to a Block-Merkle, which is included in a Bank-Hash with the required set of validator votes. A chain of PoH Entries containing subsequent validator votes, deriving from the Bank-Hash, is the proof of confirmation. Clients can examine this ledger data and compute finality using Solana's fork selection rules.
An Entry-Merkle is a Merkle Root including all transactions in a given entry, sorted by signature.
A Block-Merkle is the Merkle Root of all the Entry-Merkles sequenced in the block.
A Bank-Hash is the hash of the concatenation of the Block-Merkle and Accounts-Hash
An Accounts-Hash is the hash of the concatentation of the state hashes of each account modified during the current slot.
Transaction status is necessary for the receipt because the state receipt is constructed for the block. Two transactions over the same state can appear in the block, and therefore, there is no way to infer from just the state whether a transaction that is committed to the ledger has succeeded or failed in modifying the intended state. It may not be necessary to encode the full status code, but a single status bit to indicate the transaction's success.
Account State Verification
An account's state (balance or other data) can be verified by submitting a transaction with a TBD Instruction to the cluster. The client can then use a Transaction Inclusion Proof to verify whether the cluster agrees that the acount has reached the expected state.
Leaders should coalesce the validator votes by stake weight into a single entry. This will reduce the number of entries necessary to create a receipt.
Chain of Entries
A receipt has a PoH link from the payment or state Merkle Path root to a list of consecutive validation votes.
It contains the following:
- Transaction -> Entry-Merkle -> Block-Merkle -> Bank-Hash
And a vector of PoH entries:
- Validator vote entries
- Light entries
The light entries are reconstructed from Entries and simply show the entry Merkle Root that was mixed in to the PoH hash, instead of the full transaction set.
Clients do not need the starting vote state. The fork selection algorithm is defined such that only votes that appear after the transaction provide finality for the transaction, and finality is independent of the starting state.
A light client that is aware of the supermajority set validators can verify a receipt by following the Merkle Path to the PoH chain. The Block-Merkle is the Merkle Root and will appear in votes included in an Entry. The light client can simulate fork selection for the consecutive votes and verify that the receipt is confirmed at the desired lockout threshold.
Synthetic state should be computed into the Bank-Hash along with the bank generated state.
- Epoch validator accounts and their stakes and weights.
- Computed fee rates
These values should have an entry in the Bank-Hash. They should live under known accounts, and therefore have an index into the hash concatenation.