Offline mode

[ch1bo]

Why

• SundaeLabs has submitted a Catalyst Proposal to be able to use a hydra-node and a Hydra head as a wrapper over a (possibly customised) Cardano ledger
• Enabling this feature makes it easier to implement various applications that need to compute or validate transactions operating on a well-known universe, eg. as opposed to arbitrary transactions over Cardano network, in a fast and safe way while staying Cardano compatible and using the same tools as in the rest of the ecosystem

What

• Users should be able to run a hydra-node without needing a cardano-node or even a network connection, initialise the Hydra ledger with some UTxO, send and process transactions using the ledger rules, and retrieve the result as part of the Closing and Fanout logic
• Out-of-scope: While it could be interesting to keep the off-chain part of the protocol and allow to form an "offline" Hydra Head connecting hydra-nodes without the mediation of a layer 1, this is left for future work and contributions.

How

• Materialise the decision in an ADR
• Provide a --offline command-line flag to enable this special mode of operation
• Implement a "mock" Chain interface that automatically drives the Head state machine and responds to state changed and Effects emitted by the HeadLogic
• Implement a "mock" Network interface that does not connect to anything and ignores messages. There won't be any callback anyway because network messages emitted by the HeadLogic as an Effect are reinjected immediately as Events within the core reaction loop
• Provide end-to-end testing for offline mode

See

• Initial offline mode ADR #1012
• Offline mode implementation #1118
• Refactor offline-mode #1222