The Escrow transactional method

Abstract

A method is presented for permitting record updates by long-lived transactions without forbidding simultaneous access by other users to records modified. Earlier methods presented separately by Gawlick and Reuter are comparable but concentrate on “hot-spot” situations, where even short transactions cannot lock frequently accessed fields without causing bottlenecks. The Escrow Method offered here is designed to support nonblocking record updates by transactions that are “long lived” and thus require long periods to complete. Recoverability of intermediate results prior to commit thus becomes a design goal, so that updates as of a given time can be guaranteed against memory or media failure while still retaining the prerogative to abort. This guarantee basically completes phase one of a two-phase commit, and several advantages result: (1) As with Gawlick's and Reuter's methods, high-concurrency items in the database will not act as a bottleneck; (2) transaction commit of different updates can be performed asynchronously, allowing natural distributed transactions; indeed, distributed transactions in the presence of delayed messages or occasional line disconnection become feasible in a way that we argue will tie up minimal resources for the purpose intended; and (3) it becomes natural to allow for human interaction in the middle of a transaction without loss of concurrent access or any special difficulty for the application programmer. The Escrow Method, like Gawlick's Fast Path and Reuter's Method, requires the database system to be an “expert” about the type of transactional updates performed, most commonly updates involving incremental changes to aggregate quantities. However, the Escrow Method is extendable to other types of updates.