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26. Stateless transaction observation & construction

· 4 min read
Sebastian Nagel

Status​

Proposed

Context​

  • ADR 18 merged both headState and chainState into one single state in the Hydra node, giving the chain layer a way to fetch and update the chainState when observing a chain event.

  • ADR 23 outlined the need for a local chain state in the chain layer again to correctly handle correct observation of multiple relevant transactions and the resulting chainState updates.

  • The ChainStateType tx for our "actual" Cardano chain layer is currently:

    data ChainStateAt = ChainStateAt
      { chainState :: ChainState
      , recordedAt :: Maybe ChainPoint
      }

    data ChainState
      = Idle
      | Initial InitialState
      | Open OpenState
      | Closed ClosedState

    where InitialState, OpenState and ClosedState hold elaborate information about the currently tracked Hydra head.

  • We face difficulties to provide sufficient user feedback when an initTx was observed but (for example) keys do not match our expectation.

    • Core problem is, that observeInit is required to take a decision whether it wants to "adopt" the Head by returning an InitialState or not.
    • This makes it impossible to provide user feedback through the HeadLogic and API layers.

  • We want to build a Hydra head explorer, which should be able to keep track and discover Hydra heads and their state changes even when the heads were initialized before starting the explorer.

Decision​

  • We supersede ADR 18 with the current ADR.

Changes internal to Direct chain layer​

  • Introduce a ResolvedTx type that has its inputs resolved. Where a normal Tx will only contain TxIn information of its inputs, a ResolvedTx also includes the TxOut for each input.

  • Change ChainSyncHandler signature to onRollForward :: BlockHeader -> [ResolvedTx] -> m ()

  • Change observing function signature to observeSomeTx :: ChainContext -> ResolvedTx -> Maybe (OnChainTx Tx). Notably there is no ChainState involved.

  • Do not guard observation by HeadId in the chain layer and instead do it in the HeadLogic layer.

  • Define a SpendableUTxO type that is a UTxO with potentially needed datums included.

    • TBD: instead we could decide to use inline datums and rely on UTxO containing them

  • Change transaction creation functions initialize, commit, abort, collect, close, contest and fanout in Hydra.Direct.Chain.State to take SpendableUTxO and HeadId/HeadParameters as needed.

  • Extend IsChainState type class to enforce that it can be updated by concurrent transactions update :: ChainStateType tx -> [tx] -> ChainStateType tx.

    • While this is not strictly needed "outside" of the chain layer, it will have us not fall into the same pit again.

  • Change ChainStateAt to only hold a spendableUTxO and the recordedAt.

  • Update the LocalChainState in onRollForward by using update and pushing a new ChainStateAt generically.

TBD:

  • Impact on generators

Chain interface changes​

  • Add HeadId and HeadParameters to PostChainTx.

  • Add HeadId to all OnChainTx constructors.

  • Extend OnInitTx with observed chain participants.

    • TBD: How are cardano verification keys generically represented in HeadLogic?

  • Extend OnContestTx with new deadline and a list of contesters.

  • Move off-chain checks for what makes a "proper head" to HeadLogic

TBD:

  • Merge HeadSeed and HeadId? How to abstract?

Consequences​

  • All logic is kept in the logic layer and no protocol decisions (i.e. whether to adopt or ignore a head initialization) are taken in the chain layer.

    • The HeadLogic gets informed of any proper initTx and can log that it is ignored and for what reason.

  • The transaction observation and construction functions can be moved into a dedicated package that is cardano-specific, but not requires special state knowledge of the "direct chain following" and can be re-used as a library.

  • All transaction observation functions used by observeSomeTx will need to be able to identify a Hydra Head transaction from only the ResolvedTx and the ChainContext

  • Any Chain Tx implementation wanting to re-use existing transaction observation functions must be able to resolve transaction inputs (against some ledger state) and produce ResolvedTx.

    • A chain-following implementation (as Hydra.Chain.Direct) can keep previous transactions around.
    • A chain indexer on "interesting" protocol addresses can be used to efficiently query most inputs.

  • We can get rid of the Hydra.Chain.Direct.State glue code altogether.

  • While this does not directly supersede ADR23, it paves the way to remove LocalChainState again as the ChainStateAt is now combinable from multiple transactions (see update above) and we can keep the state (again) only in the HeadState aggregate. Note that this would shift the rollback handling back into the logic layer.