Improving Data Consistency for Mobile File Access Using Isolation-Only Transactions

Qi Lu
May 1996


Disconnected operation based on optimistic replication has been demonstrated as an effective technique enabling mobile computers to access shared data in distributed file systems. To guard against inconsistencies resulted from partitioned data sharing, past research has focused on detecting and resolving write/write conflicts. However, experience shows that undetected read/write conflicts pose a subtle but serious threat to data integrity in mobile file access. Solving this problem is critical for the future success of mobile computing.

This dissertation shows that isolation-only transaction (IOT), an upward compatible trans-action mechanism for the Unix File System, is a viable solution to this problem. The central idea of the IOT model is imposing serializability-based isolation requirements on partitioned transaction executions. Transactions executed on a disconnected client stay in a tentative state until the client regains connection to relevant servers. They are committed to the servers as soon as they pass consistency validation. Invalidated transactions are automatically or manu-ally resolved to ensure global consistency. Powerful resolution mechanisms such as automatic transaction re-execution and application specific resolver invocation can transparently resolve conflicts for many common Unix applications. In addition, a concise conflict representation scheme enables application semantics to be smoothly integrated for only conflict resolution and consistency validation. The practical usability of IOT is further enhanced by a flexible interactive interface, full compatibility with existing Unix applications, and the ability to retain overall file system scalability, security and transparency.

A working IOT implementation in the Coda file system has been developed and used in experiments in software development and document processing applications.  Quantitative evaluation based on controlled experiments and trace-driven simulations establish that the IOT model is scalable and incurs modest performance and resource overhead.

The main contributions of this thesis research are the following: the design of an isolation-only transaction model specialized for improving mobile file consistency while preserving upward compatibility with existing Unix applications; the development of a working IOT implementation in the Coda file system; experimentation and evaluation demonstrating the feasibility and practicality of the IOT model.