Abstract. In a distributed system, one strategy for achieving mutual exclusion of groups of nodes without communication is to assign to each node a number of votes. Only a group with a majority of votes can execute the critical operations, and mutual exclusion is achieved because at any given time there is at most one such group. A second strategy, which appears to be similar to votes, is to define a priori a set of groups that intersect each other. Any group of nodes that finds itself in this set can perform the restricted operations. In this paper, both of these strategies are studied in detail and it is shown that they are not equivalent in general (although they are in some cases). In doing so, a number of other interesting properties are proved. These properties will be of use to a system designer who is selecting a vote assignment or a set of groups for a specific application.

This is the nineteenth edition of a (usually) quarterly column that covers new developments in the theory of NP-completeness. The presentation is modeled on that used by M. R. Garey and myself in our book ‘‘Computers and Intractability: A Guide to the Theory of NP-Completeness,’ ’ W. H. Freeman & Co., New York, 1979 (hereinafter referred to as ‘‘[G&J]’’; previous columns will be referred to by their dates). A background equivalent to that provided by [G&J] is assumed, and, when appropriate, cross-references will be given to that book and the list of problems (NP-complete and harder) presented there. Readers who have results they would like mentioned (NP-hardness, PSPACE-hardness, polynomial-time-solvability, etc.) or open problems they would like publicized, should

A protocol for transaction processing during partition failures is presented which guarantees mutual consistency between copies of data-items after repair is completed. The protocol is “optimistic ” in that transactions are processed without restrictions during failure; conflicts are then detected at repair time using a precedence graph, and are resolved by backing out transactions according to some backout strategy. The resulting database state then corresponds to a serial execution of some subset of transactions run during the failure. Results from simulation and probabilistic modeling show that the optimistic protocol is a reasonable alternative in many cases. Conditions under which the protocol performs well are noted, and suggestions are made as to how performance can be improved. In particular, a backout strategy is presented which takes into account individual transaction costs and attempts to minimize total backout cost. Although the problem of choosing transactions to minimize total backout cost is, in general, NP-complete, the backout strategy is efficient and produces very good results.

this document we will use the term transaction to mean IOT when there is no ambiguity. running committed resolution pending user invocation with partitioned file accesses without partitioned file accesses automatic or manual resolution validation fail validation succeed & reintegration first class transaction second class transaction Figure 1: A State Transition Diagram for IOT Execution 4. Why Isolation Only?

Disconnected operation is an important technique for providing mobile access to shared data in distributed file systems. However, data inconsistency resulting from partitioned sharing remains a serious concern. This paper presents a new mechanism called isolation-only transaction(IOT) that uses serializability constraints to automatically detect read/write conflicts. The IOT consistency model provides a set of options for automatic and manual conflict resolution. In addition, application specific knowledge can be incorporated to detect and resolve conflicts. To preserve upward Unix compatibility, the IOT mechanism is provided as an optional file system facility and its flexible interfaces allow any existing Unix application to be executed as an IOT. This paper describes high level system design and implementation and concludes with related work and current status. This research was supported by the Air Force Materiel Command (AFMC) and ARPA under contract number F196828-93-C0193. Addit...

With the advancement of wireless network and mobile computing, there is an increasing need to build a mobile file system that can perform efficiently and correctly for accessing online information. Previous system research on mobile file system is based on some experimental platforms. In this paper, we describe the design and implementation of a mobile file system on an open platform, the Linux kernel, and at the same time, our mobile file system is compatible to the popular NFS 2.0 protocol. In this paper, we formally define the file semantics of our mobile file system, which we called the NFS/M. We also specify the conditions of object conflict as well as our conflict resolution algorithms. NFS/M supports client side caching, data prefetching, file system service during the disconnected mode, data reintegration and conflict resolution on various file system objects. Since the NFS/M is based on an open platform, it serves as a basic building block for developing future mobile computin...

updates to an object must be treated as conflicting, and Optimistic replication is an important technique for merged manually by the user. Manual resolution is achieving high availability in distributed file systems. A undesirable because it reduces the overall usability of the key problem in optimistic replication is using semantic knowledge of objects to resolve concurrent updates from system. multiple partitions. In this paper, we describe how the Coda File System resolves partitioned updates to An extremely important object, with known semantics, in directories. The central result of our work is that logging of Unix file systems is a directory. We refer to the process of updates is a simple yet efficient and powerful technique for examining replicas of a directory, deducing the set of directory resolution in Unix file systems. Measurements partitioned updates and merging them using Unix from our implementation show that the time for resolution is typically less than 10% o...