ISS is the first modular algorithm to make leader-driven total order broadcast scale in a robust way, published in March 2022. At its interface, ISS is a classic state machine replication (SMR) system that establishes a total order of client transactions with typical liveness and safety properties, applicable to any replicated service, such as resilient databases or a blockchain ordering layer. It is a further development and a successor of the Mir-BFT protocol (not to be confused with the Mir library used to implement ISS, see the description of Mir).
ISS achieves scalability without requiring a primary node to periodically decide on the protocol configuration. It multiplexes multiple instances of a leader-driven consensus protocol which operate concurrently and (almost) independently. We abstract away the logic of the used consensus protocol and only define an interface - that we call "Sequenced Broadcast" (SB) - that such a consensus protocol must use to interact with ISS.
ISS maintains a contiguous log of (batches of) client transactions at each node. Each position in the log corresponds to a unique sequence number and ISS agrees on the assignment of a unique request batch to each sequence number. Our goal is to introduce as much parallelism as possible in assigning batches to sequence numbers while avoiding request duplication, i.e., assigning the same request to more than one sequence number. To this end, ISS subdivides the log into non-overlapping segments. Each segment, representing a subset of the log's sequence numbers, corresponds to an independent consensus protocol instance that has its own leader and executes concurrently with other instances.
To prevent the leaders of two different segments from concurrently proposing the same request, and thus wasting resources, while also preventing malicious leaders from censoring (i.e., not proposing) certain transactions, we adopt and generalize the partitioning of the request space introduced by Mir-BFT. At any point in time, ISS assigns a different subset of client transactions (that we call a bucket) to each segment. ISS periodically changes this assignment, such that each request is guaranteed to eventually be assigned to a segment with a correct leader.
The figure below shows the high-level architecture of the ISS protocol.
