This paper presents the abstraction of lazy consensus and argues for its use as an effective component for building distributed agreement protocols in practical asynchronous systems where processes and links can crash and recover. Lazy consensus looks like consensus, is equivalent to consensus, but is not consensus. The specification of lazy consensus has an on-demand and a re-entrant flavors that makes its use very efficient, especially in terms of forced logs, which are known to be major sources of overhead in distributed systems. We illustrate the use of lazy consensus as a building block to develop efficient atomic broadcast and atomic commitment protocols: two central abstractions in our DACE middleware environment.
