Staged Multi-step UTXO Workflows via Recursive Invariants
Stateless UTXO-style execution validates transactions using local and referenced data, enabling parallel validation and predictable serialized-size/weight accounting. Multi-step workflows thread state across outputs, and a prepared next-step transaction may become stale if another valid spend confirms first. Explicit state threading therefore shifts consistency maintenance, off-chain tracking, and transaction rebuilding to the protocol boundary, increasing coordination cost and latency. Recursive invariants (RIs), our proposed transaction-level logic and toolchain, address this gap by expressing workflow rules as transaction-level predicates over a transaction's inputs and indexed successor positions referenced by the RI. An accepted transaction realizing such a successor position re-checks the predecessor's RI one step later, carrying the workflow rule forward without shared mutable application state or executable output logic. Thus, multi-step protocol rules preserve validation-time locality and admit explicit cost accounting, while cross-transaction guarantees arise from repeated one-step checking. Not all successor clauses are checkable at validation time, so our small statically typed domain-specific language (DSL) uses three-valued semantics (true, false, unknown) to defer future-dependent obligations until checkable. Co-designed with this DSL, our framework formalizes UTXO validation and ledger extension, identifies the validation-time-evaluable one-step fragment, and proves the deduction system sound w.r.t. the three-valued semantics. We give validation and ledger-extension algorithms for this model. We implement a prototype RI interpreter and benchmarking toolchain for six workloads. Six practice-motivated case studies exhibit roughly linear cumulative validation-cost proxy growth and illustrate staged workflow constraints without preconstructing each successor.