Abstract This paper presents Atomic RMI, a distributed transactional mem-ory framework that supports the control flow model of execution. Atomic RMI extends Java RMI with distributed transactions that can run on many Java Virtual Machines located on different network nodes. Our system employs SVA, a fully-pessimistic concurrency control algorithm that provides exclu-sive access to shared objects and supports rollback and fault tolerance. SVA is capable of achieving a relatively high level of parallelism by interweaving trans-actions that access the same objects and by making transactions that do not share objects independent of one another. It also allows any operations within transactions, including irrevocable ones, like system calls, and provides an un-obtrusive API. Our evaluation shows that in most cases Atomic RMI performs better than fine grained mutual-exclusion and read/write locking mechanisms. Atomic RMI also performs better than an optimistic transactional memory in environments with high contention and a high ratio of write operations, while being competitive otherwise. Keywords concurrency control · distributed systems · software transactional memory 1

Abstract This paper presents Atomic RMI, a distributed transactional mem-ory framework that supports the control flow model of execution. Atomic RMI extends Java RMI with distributed transactions that can run on many Java Virtual Machines located on different network nodes. Our system employs SVA, a fully-pessimistic concurrency control algorithm that provides exclu-sive access to shared objects and supports rollback and fault tolerance. SVA is capable of achieving a relatively high level of parallelism by interweaving trans-actions that access the same objects and by making transactions that do not share objects independent of one another. It also allows any operations within transactions, including irrevocable ones, like system calls, and provides an un-obtrusive API. Our evaluation shows that in most cases Atomic RMI performs better than fine grained mutual-exclusion and read/write locking mechanisms. Atomic RMI also performs better than an optimistic transactional memory in environments with high contention and a high ratio of write operations, while being competitive otherwise. Keywords concurrency control · distributed systems · software transactional memory 1

Abstract Given the limitations imposed on distributed systems that are necessary to maintain strong consistency guarantees there is a growing interest in relaxed consistency models. Such models are often sufficient for particular applications, but allow more freedom to improve scalability and availability. Eventual consistency is a particularly useful approach, where the correct state spreads throughout the system over time, so that at any point any element of the system may be inconsistent, but all elements will eventually converge upon a consistent state. On the other hand relaxing properties may be unacceptable in the general case: a slightly stale shopping cart is one thing, but inconsistent payment processing is quite another. In this paper we try to balance strong and eventual consistency by proposing a general-purpose pessimistic distributed transactional memory that allows eventually consistent transactions to run alongside consistent ones. While the former maintain read-isolation (i.e., read from a consistent snapshot), they do not interfere with the latter's safety properties. The relaxed-consistency transactions are later followed by their consistent counterpart so that the user view and global state eventually agree. Our contribution is to show that we can significantly relax synchronization (to the point of eliminating it completely from eventually consistent transactions) while retaining useful properties, but without imposing additional constraints about system architecture or data operations, common to other relaxed consistency approaches. All this, without affecting those transactions that execute in consistent mode.