A Trace Semantics for Long-Running Transactions

Abstract

Abstract. A long-running transaction is an interactive component of a distributed system which must be executed as if it were a single atomic action. In principle, it should not be interrupted or fail in the middle, and it must not be interleaved with other atomic actions of other concurrently executing components of the system. In practice, the illusion of atomicity for a long-running transaction is achieved with the aid of compensation actions supplied by the original programmer: because the transaction is interactive, familiar automatic techniques of check-pointing and roll-back are no longer adequate. This paper constructs a model of long-running transactions within the framework of the CSP process algebra, showing how the compensations are orchestrated to achieve the illusion of atomicity. It introduces a method for declaring that a process is a transaction, and for declaring a compensation for it in case it needs to be rolled back after it has committed. The familiar operator of sequential composition is redefined to ensure that all necessary compensations will be called in the right order if a later failure makes this necessary. The techniques are designed to work well in a highly concurrent and distributed setting. In addition we define an angelic choice operation, implemented by speculative execution of alternatives; its judicious use can improve responsiveness of a system in the face of the unpredictable latencies of remote communication. Many of the familiar properties of process algebra are preserved by these new definitions, on reasonable assumptions of the correctness and independence of the programmer-declared compensations.

Comparing Two Approaches to Compensable Flow Composition

“…We have compared two recent formal approaches to the modelling of compensable flow composition, that have been proposed independently in [5,7]. For the sequential case we have shown that the two frameworks essentially coincide by providing fully abstract encodings.…”

“…Several proposals have recently appeared in the literature focused on the formalisation of compensable processes using process calculi. They can be roughly divided into two types: (i) compensable flow composition [6,5,7] closer to the spirit of orchestration languages like BPEL4WS, where suitable process algebras are designed from the scratch to describe the possible flow of control among services; and (ii) interaction based compensations [1,4,9,11], as suitable extensions of well-known name passing calculi, like the π-calculus and join-calculus, for describing transactional choreographies, where each service describes its possible interactions, and the actual composition takes place dynamically, i.e. when services interact.…”

“…Nevertheless, we are not aimed at designing a new language but at comparing two main proposals, namely compensating CSP (cCSP) [7] and Sagas calculi [5]. Apart from stylistic differences (e.g., the trace semantics of cCSP and the big step SOS semantics of Sagas calculi), this comparison highlights the fact that such proposals account for different compensation policies when handling concurrent processes.…”

“…We start by recalling in § 2 the syntax and semantics of cCSP and Sagas from [5,7]. Then, we outline the conceptual and stylistic similarities and differences between the two approaches in § 3.…”

“…In this section we summarise the basics of Compensating CSP (cCSP) proposed in [7] and of Sagas calculi from [5]. We focus on simplified versions by leaving out several features present in both proposals, like exception handling and nesting.…”