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Showing posts from October, 2015

HPTS trip report (days 0 and 1)

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Last week, from Sunday to Tuesday night, I attended the 16th International Workshop on High Performance Transaction Systems (HPTS). HPTS is an unconventional workshop. "Every two years, HPTS brings together a lively and opinionated group of participants to discuss and debate the pressing topics that affect today's systems and their design and implementation, especially where scalability is concerned. The workshop includes position paper presentations, panels, moderated discussions, and significant time for casual interaction. The only publications are slide decks by presenters who choose to post them." HPTS is by invitation only and keeps it under 100 participants. The workshop brings together experts from both industry and academia so they mix and interact. Looking at the program committee, you can see names of entrepreneurs venture capitalists (David Cheriton, Sequoia Capital), large web companies (Google, Facebook, Salesforce, Cloudera), and academics. HPTS is legacy …

Consensus in the wild

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The consensus problem has been studied in the theory of distributed systems literature extensively. Consensus is a fundamental problem in distributed systems. It states that n nodes agree on the same decision eventually. Consistency part of the specification says that no two nodes decide differently. Termination states that all nodes eventually decide. And NonTriviality says that the decision cannot be static (you need to decide a value among inputs/proposals to the system, you can't keep deciding 0 discarding the inputs/proposals). This is not a hard problem if you have reliable and bounded-delay channels and processes, but becomes impossible in the absence of either. And with even temporary violation of reliability and timing/synchronicity assumptions, a consensus system can easily spawn multiple corner-cases where consistency or termination is violated. E.g., 2-phase commit is blocking (this violates termination), and 3-phase commit is unproven and has many corner cases involvi…

Analysis of Bounds on Hybrid Vector Clocks

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This work is in collaboration with Sorrachai Yingchareonthawornchai and Sandeep Kulkarni at the Michigan State University and is currently under submission.


Practice of distributed systems employs loosely synchronized clocks, mostly using NTP. Unfortunately, perfect synchronization is unachievable due to messaging with uncertain latency, clock skew, and failures. These sync errors lead to anomalies. For example, a send event at Branch1 may be assigned a timestamp greater than the corresponding receive event at Branch2, because Branch1's clock is slightly ahead of Branch2's clock. This leads to /inconsistent state snapshots/ because, at time T=12:00, a money transfer is recorded as received at Branch2, whereas it is not recorded as sent at Branch1.

Theory of distributed systems shrugs and doesn't even try. Theory abstracts away from the physical clock and uses logical clocks for ordering events. These are basically counters, as in Lamport's clocks and vector clocks. The…

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