A theory of relaxed atomicity (extended abstract)

Supporting atomicity of multi-site transactions in a distributed transaction management system is equated with long-duration delays, blocking, and loss of autonomy of the individual sites. The two-phase Commit protocol embodies these problems. The focus of this paper is on an alternative notion of relaxed atomicity where these difficult ies are alleviated. Relaxed atomicity is characterized by an asynchronous process of recovering from uncoordinated local decisions as to whether to commit or abort a multi-site transaction, and finally coercing a unanimous outcome. The consequent atomicity notion is weaker than the standard all-or-nothing atomicity, as transactions with discrepant commit decisions are recovered semantically rather than physically. A formal model that unifies the two dual methods of semantic recovery, namely compensation and retry, is constructed. Due to the asynchrony introduced to the commit procedure, non-atomic executions of t ransactions occur, and it is required to isolate them from other transactions until they are semantically recovered. The required isolation property is defined, and a protocol that satisfies this property is presented.