I think BFS will become an “obvious” choice, if we think about Tezos operations in terms of ACID transactions. Tezos already provides isolation (each new operation and its spawned operations are executed in isolation from the other operations in the pool) and durability (once a block is baked, the operation result cannot be lost).
Tezos solves atomicity for the following cases:
- Individual contract call (individual transaction) atomically updates state of the single contract
- A contract can spawn a list of operations to invoke other contracts. Such a list can be considered a composite transaction (composed of a sequence of individual transactions). BFS order guarantees that the composite transaction will not be interleaved with the operations from other composite transactions.
However, each operation can depend on the state of exactly one contract. What if we want to express calculation that still updates the state of the called contract, but depends on data from other contracts on the chain? Currently, the only available option is continuation passing style (CPS) entry points; Read-Only Calls are also proposed.
But those techniques introduce their own problems that violate composite transaction consistency. There is a detailed analysis and some proposed solutions here.
Monitors are an interesting feature, but it looks like they do not address multi-contract interactions.
