Major focus: coordination-free execution, or guaranteed response from every replica Availability Low latency Perfect horizontal scalability Benefits: OpTRM!
Serializability COORDINATION REQUIRED GUARANTEED SAFETY Eventual Consistency COORDINATION FREE NO SAFETY COORDINATION AVOIDANCE GUARANTEED SAFETY WITHOUT COORDINATION THESIS WORK: What is the coordination cost of a given safety guarantee? How do we achieve the minimum? “ACID” “NoSQL”
(Abridged) Related Work » Semantics-based concurrency control: esp. commutativity and CALM analysis, laws of order » Available storage systems: optimistic replication, causal memory, CRDTs, eventually consistent transactions » Distributed computing: CAP, FLP, NBAC, quorums » Here: focus on necessary coordination for common, modern data-intensive apps
Unavailable Sticky Available Highly Available Legend prevents lost update†, prevents write skew‡, requires recency guarantees⊕ Sticky Available Unavailable Highly Available [VLDB 2014]
CONSTRAINT: User IDs are unique OPERATION: Add users MERGE: Set union {{Stu,ID=1}, {Ann,ID=1}} Constraint violated! {} MERGE add {Stu,ID=1} add {Ann,ID=1} Key idea: Check if constraints can be violated by “merging” independent operations ICT: Invariant Confluence Test
Key idea: Check if constraints can be violated by “merging” independent operations CONSTRAINT: User IDs are positive OPERATION: Add users MERGE: Set union {{Stu,ID=1}, {Ann,ID=1}} Constraint holds! {} MERGE add {Stu,ID=1} add {Ann,ID=1} ICT: Invariant Confluence Test
Key idea: Check if constraints can be violated by “merging” independent operations OUR CONTRIBUTION: Generalizes classic partitioning-based indistinguishability arguments Theorem. A globally I-valid system can execute a set of transactions T with coordination-freedom, transactional availability, and convergence if and only if T are I-confluent with respect to I. [VLDB 2015] ICT ⟺ safe, coordination-free execution possible ICT: Invariant Confluence Test
Constraint Operation OK? Equality, Inequality Any Y Generate unique ID Any Y Specify unique ID Insert N > Increment Y > Decrement N < Decrement Y < Increment N Foreign Key Insert Y Foreign Key Delete Y* Secondary Indexing Any Y Materialized Views Any Y AUTO_INCREMENT Insert N [VLDB 2015] Typical database constraints and operations (SQL) Under set merge R A M P [SIGMOD 2014]
Unruly developers are fantastic inspiration • As applications have evolved, so have their database demands and desired semantics • Our opportunity: build systems that implement the semantics users want (not just what we want)
• Mounting evidence: many programmers don’t use transactions correctly (or at all!) • Need not despair: opportunity for new theory and systems ACIDRain: Concurrency-Related Attacks on Database-Backed Web Applications [SIGMOD17]
Michael R. Bernstein, Rick Branson, Mark Callaghan, Adrian Colyer, Sean Cribbs, Jonathan Ellis, Alex Feinberg, Andy Gross, Coda Hale, Colin Jones, Evan Jones, Kyle Kingsbury, Adam Marcus, Caitie McCaffrey, Christopher Meiklejohn, Mike Miller, Jeremiah Peschka, Mark Phillips, Henry Robinson, Mehul Shah, Xavier Shay, Justin Sheehy, Ines Sombra, Kelly Sommers, Sriram Srinivasan and a cast of unruly developers and renegades: Also many thanks to a host of phenomenal colleagues and collaborators Peter Alvaro, Neil Conway, Shivaram Venkataraman, Joey Gonzalez, Haoyuan Li, Zhao Zhang, Aaron Davidson, Mike Jordan
Eventual Consistency COORDINATION FREE NO SAFETY Atomic Visibility SIGMOD14 Database Constraints VLDB15, SIGMOD15 Model Prediction and Training CIDR15, TBA Weak Isolation HotOS13, VLDB14 Causality SOCC12, SIGMOD13 COORDINATION AVOIDANCE GUARANTEED SAFETY WITHOUT COORDINATION MORE APP SEMANTICS MORE SAFETY PBS VLDB12, VLDBJ14, SIGMOD13, CACM14 COORDINATION FREE Joint work with Ali Ghodsi, Joe Hellerstein, Ion Stoica, Mike Franklin, Michael Jordan, Alan Fekete, Dan Crankshaw, Shivaram Venkataraman, Neil Conway, Peter Alvaro, Aaron Davidson, Joey Gonzalez, Kyle Kingsbury, Haoyuan Li, and Zhao Zhang
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