Daml and Canton: Privacy-aware smart contracts on interoperable ledgers by Andreas Lochbihler [2021]

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👉 #Daml is a Haskell-based smart contract programming language used to coordinate business workflows across trust boundaries. Daml’s semantics are defined over an abstract ledger, which provides a clear semantics for Daml’s authorization rules, double-spending protection, and privacy guarantees. In its simplest form, a ledger is represented as a list of commits, i.e., hierarchical transactions and their authorizers. This representation allows for easy reasoning about Daml smart contracts because the total order hides the intricacies of a distributed, Byzantine-fault tolerant system. It is also adequate for Daml running on a single blockchain, as it defines a total order on all transactions. Yet, for distributed ledgers to fully eliminate data silos, smart contracts must not be tied to a single blockchain, which would then just become another silo. Daml therefore runs on different blockchains such as #HyperledgerFabric, #Ethereum, and FISCO-BCOS as well as off-the-shelf databases. The underlying protocol Canton supports atomic transactions across all these Daml ledgers. This makes Daml ledgers sharded for higher throughput as well as interoperable to avoid data silos. Semantically, Canton creates a virtual shared ledger by merging the individual ledgers’ lists of commits. The virtual shared ledger is not totally ordered, to account for the fact that there is no global notion of time across ledgers. Still, transactions can use only contracts that have been created within earlier transactions. This ensures that causality is respected even though individual system users cannot see all dependencies due to the privacy rules. Canton tracks privacy-aware causality using vector clocks. To ensure that Daml and Canton achieve their clai