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18. Single state in Hydra.Node.

· 4 min read

Status

Proposed

Context

  • Currently the hydra-node maintains two pieces of state during the life-cycle of a Hydra Head:
    1. A HeadState tx provided by the HydraHead tx m handle interface and part of the Hydra.Node module. It provides the basis for the main hydra-node business logic in Hydra.Node.processNextEvent and Hydra.HeadLogic.updateCreation, Usage
    2. SomeOnChainHeadState is kept in the Hydra.Chain.Direct to keep track of the latest known head state, including notable transaction outputs and information how to spend it (e.g. scripts and datums) Code, Usage 1, Usage 2, Usage 3 (There are other unrelated things kept in memory like the event history in the API server or a peer map in the network heartbeat component.)
  • The interface between the Hydra.Node and a Hydra.Chain component consists of
    • constructing certain Head protocol transactions given a description of it (PostChainTx tx):
      postTx :: MonadThrow m => PostChainTx tx -> m ()
    • a callback function when the Hydra.Chain component observed a new Head protocol transaction described by OnChainTx tx:
      type ChainCallback tx m = OnChainTx tx -> m ()
  • Given by the usage sites above, the Hydra.Chain.Direct module requires additional info to do both, construct protocol transactions with postTx as well as observe potential OnChainTx (here). Hence we see that, operation of the Hydra.Chain.Direct component (and likely any implementing the interface fully) is inherently stateful.
  • We are looking at upcoming features to handle rollbacks and dealing with persisting the head state.
    • Both could benefit from the idea, that the HeadState is just a result of pure Event processing (a.k.a event sourcing).
    • Right now the HeadState kept in Hydra.Node alone, is not enough to fully describe the state of the hydra-node. Hence it would not be enough to just persist all the Events and replaying them to achieve persistence, nor resetting to some previous HeadState in the presence of a rollback.

Decision

  • We define and keep a "blackbox" ChainState tx in the HeadState tx
    • It shall not be introspectable to the business logic in HeadLogic
    • It shall contain chain-specific information about the current Hydra Head, which will naturally need to evolve once we have multiple Heads in our feature scope
    • For example:
    data HeadState tx
    = IdleState
    | InitialState
    { chainState :: ChainState tx
    -- ...
    }
    | OpenState
    { chainState :: ChainState tx
    -- ...
    }
    | ClosedState
    { chainState :: ChainState tx
    -- ...
    }
  • We provide the latest ChainState tx to postTx:
    postTx :: ChainState tx -> PostChainTx tx -> m ()
  • We change the callback interface of Chain to
    type ChainCallback tx m = (ChainState tx -> Maybe (OnChainTx tx, ChainState tx)) -> m ()
    with the meaning, that invoking the callback indicates receival of a potential Hydra transaction which is Maybe observing a relevant OnChainTx tx paired with a (potentially updated) ChainState tx.
  • We also decide to extend OnChainEvent and OnChainEffect with a ChainState tx and threading it through the Hydra.HeadLogic.

Consequences

  • We need to change the construction of Chain handles and the call sites of postTx
  • We need to extract the state handling (similar to the event queue) out of the HydraNode handle and shuffle the main of hydra-node a bit to be able to provide the latest ChainState to the chain callback as a continuation.
  • We need to make the ChainState already serializable (ToJSON, FromJSON) as it will be part of the HeadState.
  • We can drop the TVar of keeping OnChainHeadState in the Hydra.Chain.Direct module.
  • We might be able to simplify the ChainState tx to be just a UTxOType tx later.

Alternative

  • We could extend PostChainTx and OnChainTx with ChainState and keep the signatures:
postTx :: MonadThrow m => PostChainTx tx -> m ()
type ChainCallback tx m = (ChainState tx -> Maybe (OnChainTx tx) -> m ()
  • Consequences:
    • We need to change the interface between Hydra.Chain.Direct.Tx and Hydra.Chain.Direct.State to something else than OnChainTx as the former would not have a suitable ChainState to construct, e.g. observeInitTx would return Maybe InitObervation. This may be an orthogonally desired change though.
    • Traces (e.g. ToPost) and errors (e.g. PostTxError) would automatically include the full ChainState, which might be helpful but also possible big.