DeQing Chen

Integrating remote invocation and distributed shared state

Summary form only given. Most distributed applications require, at least conceptually, some sort of shared state: information that is nonstatic but mostly read, and needed at more than one site. At the same time, RPC-based systems such as Sun RPC, Java RMl, CORBA, and .NET have become the de facto standards by which distributed applications communicate. As a result, shared state tends to be implemented either through the redundant transmission of deep-copy RPC parameters or through ad-hoc, application-specific caching and coherence protocols. The former option can waste large amounts of bandwidth; the latter significantly complicates program design and maintenance. To overcome these problems, we propose a distributed middleware system that works seamlessly with RPC-based systems, providing them with a global, persistent store that can be accessed using ordinary reads and writes. Relaxed coherence models and aggressive protocol optimizations reduce the bandwidth required to maintain shared state. Integrated support for transactions allows a chain of RPC calls to update shared state atomically. We focus on the implementation challenges involved in combining RPC with shared state and transactions. In particular, we describe a transaction metadata table that allows processes inside a transaction to share data invisible to other processes and to exchange data modifications efficiently. Using microbenchmarks and a large-scale datamining application, we demonstrate how the integration of RPC, transactions, and shared state facilitates the rapid development of robust, maintainable code.

Multi-level shared state for distributed systems

As a result of advances in processor and network speeds, more and more applications can productively be spread across geographically distributed machines. In this paper we present a transparent system for memory sharing, InterWeave, developed with such applications in mind. InterWeave can accommodate hardware coherence and consistency within multiprocessors (level-1 sharing), software distributed shared memory (S-DSM) within tightly coupled clusters (level-2 sharing), and version-based coherence and consistency across the Internet (level-3 sharing). InterWeave allows processes written in multiple languages, running on heterogeneous machines, to share arbitrary typed data structures as if they resided in local memory. Application-specific knowledge of minimal coherence requirements is used to minimize communication. Consistency information is maintained in a manner that allows scaling to large amounts of shared data. In C, operations on shared data, including pointers, take precisely the same form as operations on non-shared data. We demonstrate the ease of use and efficiency of the system through an evaluation of several applications. In particular, we demonstrate that InterWeaves support for sharing at higher (more distributed) levels does not reduce the performance of sharing at lower (more tightly coupled) levels.

Efficient distributed shared state for heterogeneous machine architectures

InterWeave is a distributed middleware system that supports the sharing of strongly typed, pointer-rich data structures across heterogeneous platforms. As a complement to RPC-based systems such as CORBA, .NET and Java RMI, InterWeave allows processes to access shared data using ordinary reads and writes. Experience indicates that InterWeave-style sharing facilitates the rapid development of distributed applications, and enhances performance through transparent caching of state. In this paper we focus on the aspects of InterWeave specifically designed to accommodate heterogeneous machine architectures. Beyond the traditional challenges of message-passing in heterogeneous systems, InterWeave (1) identifies and tracks data changes in the face of relaxed coherence models, (2) employs a wire format that captures not only data but also diffs in a machine and language-independent form, and (3) swizzles pointers to maintain long-lived (cross-call) address transparency. To support these operations, InterWeave maintains an extensive set Of metadata structures, and employs a variety of performance optimizations. Experimental results show that InterWeave achieves performance comparable to that of RPC parameter passing when transmitting previously uncached data. When updating data that have already been cached, InterWeaves use of platform-independent diffs allows it to significantly outperform the straightforward use of RPC.

Distributed shared state

Increasingly, Internet-level distributed systems are oriented as much toward information access as they are toward computation. From computer-supported collaborative work to peer-to-peer computing, e-commerce, and multi-player games-even web caching and Internet chat-applications devote a significant fraction of their code to maintaining shared state: information that has dynamic content but relatively static structure, and that is needed at multiple sites. We argue that tools to automatically manage shared state have the potential to dramatically simplify the construction of distributed applications and, in important cases, to improve their performance as well. We discuss the characteristics that such tools must possess, placing them in the context of past work on distributed file systems, distributed object systems, and software distributed shared memory. We present the InterWeave system as a prototype implementation, and discuss its strengths and limitations.