  This paper presents BBCA-LEDGER, a Byzantine log replication technology for
partially synchronous networks enabling blocks to be broadcast in parallel,
such that each broadcast is finalized independently and instantaneously into an
individual slot in the log. Every finalized broadcast is eventually committed
to the total ordering, so that all network bandwidth has utility in
disseminating blocks. Finalizing log slots in parallel achieves both high
throughput and low latency. BBCA-LEDGER is composed of two principal protocols
that interweave together, a low-latency/high-throughput happy path, and a
high-throughput DAG-based fallback path. The happy path employs a novel
primitive called BBCA, a consistent broadcast enforcing unique slot numbering.
In steady state, BBCA ensures that a transaction can be committed with low
latency, in just 3 network steps. Under network partitions or faults, we
harness recent advances in BFT and build a fallback mechanism on a direct
acyclic graph (DAG) created by BBCA broadcasts. In this manner, BBCA-LEDGER
exhibits the throughput benefits of DAG-based BFT in face of gaps.
