  General solutions of state machine replication have to ensure that all
replicas apply the same commands in the same order, even in the presence of
failures. Such strict ordering incurs high synchronization costs caused by
distributed consensus or by the use of a leader.
  This paper presents a protocol for linearizable state machine replication of
conflict-free replicated data types (CRDTs) that neither requires consensus nor
a leader. By leveraging the properties of state-based CRDTs - in particular,
the monotonic growth of a join semilattice - synchronization overhead is
greatly reduced. As a result, updates only need a single round trip and modify
the state 'in-place' without the need for a log. Furthermore, the message size
overhead for coordination consists of a single counter per message. For
queries, we guarantee finite writes termination. We show in an experimental
evaluation that more than 99 % of queries can be handled in one to three round
trips under highly concurrent accesses.
  Our protocol achieves high throughput without auxiliary processes such as
command log management or leader election. Thus, it is well suited for
practical scenarios that need linearizable access to CRDT data on a
fine-granular scale.
