Version Headers for Flexible Synchronization and Conflict Resolution

We propose a set of metadata fields to be included in object versions to enable a form of optimistic replication in which synchronization is performed by copying versions from site to site. To enable determining whether one version has been derived from another, we include in the metadata an explicit list of predecessor version IDs, utilizing range encoding to achieve a compact representation. We allow conflicting versions to be copied from site to site, creating a possibility that multiple sites will attempt to resolve the conflict. The paper defines a scheme for consistently arbitrating among different resolutions of the same conflict, and for avoiding run-away cascades involving conflicts among conflict resolutions. The proposed architecture enables the use of a variety of synchronization protocols and conflict resolution algorithms in the same distributed system. We are using it in a research prototype of a replicated XML database running on desktop computers and Symbian OS mobile phones.