Shared register emulations on top of message- passing systems provide an illusion of a simpler shared memory system which can make the task of a system designer easier. Numerous shared register applications have a considerably high read to write ratio. Thus having algorithms that make reads more efficient than writes is a fair trade-off.
Typically such algorithms for reads and writes are asymmetric and sacrifice the stringent consistency condition atomicity as it is impossible to have fast reads for multi-writer atomicity. Safety is a consistency condition has has gathered interest from both the systems and theory community as it is weaker than atomicity yet provides strong enough guarantees like “strong consistency” or read-my-write consistency. One requirement that is assumed by many researchers is that of the reliable broadcast (RB) primitive, which ensures the all or none property during a broadcast. One drawback is that such a primitive takes 1.5 rounds to complete.
This paper implements an efficient multi-writer multi-reader safe register without using a reliable broadcast primitive. More- over, we provide fast reads or one-shot reads – our read operation can be completed in one round of client-to-server communication. Of course, this comes with the price of requiring more servers when compared to prior solutions assuming reliable broadcast. However, we show that this increased number of servers is indeed necessary as we prove a tight bound on the number of servers required to implement Byzantine-fault tolerant safe registers in a system without reliable broadcast.
We extend our results to data stored using erasure coding as well. We present an emulation of single-writer multi-reader safe register based on MDS code. The usage of MDS code reduces storage cost and communication cost. On the negative side, we also show that to use MDS code and achieve one-shot read at the same time, we need even more servers.
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SNOW Revisited: Understanding When IdealREAD Transactions Are Possible
Abstract—READ transactions that read data distributed across servers dominate the workloads of real-world distributed storage systems. The SNOW Theorem [13] stated that ideal READ transactions that have optimal latency and the strongest guarantees—i.e., “SNOW” READ transactions—are impossible in one specific setting that requires three or more clients: at least two readers and one writer. However, it left many open questions.We close all of these open questions with new impossibility results and new algorithms. First, we prove rigorously the result from [13] saying that it is impossible to have a READ transactions system that satisfies SNOW properties with three or more clients.The insight we gained from this proof led to teasing out the implicit assumptions that are required to state the results and also, resolving the open question regarding the possibility of SNOW with two clients. We show that it is possible to design an algorithm, where SNOW is possible in a multi-writer, single-reader (MWSR) setting when a client can send messages to other clients; on the other hand, we prove it is impossible to implement SNOW in a multi-writer, single-reader (MWSR) setting–which is more general than the two-client setting–when client-to-client communication is disallowed. We also correct the previous claim in [13] that incorrectly identified one existing system, Eiger [12], as supporting the strongest guarantees (SW)and whose read-only transactions had bounded latency. Thus,there were no previous algorithms that provided the strongest guarantees and had bounded latency. Finally, we introduce the first two algorithms to provide the strongest guarantees with bounded latency
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- Award ID(s):
- 2003830
- NSF-PAR ID:
- 10249610
- Date Published:
- Journal Name:
- 35th IEEE International Parallel and Distributed Processing Symposium (IPDPS)
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
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- The DOI is not currently available.
