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c onsistency without borders

c onsistency without borders. Peter Alvaro , Peter Bailis , Neil Conway, Joseph M. Hellerstein UC Berkeley. The transaction concept. DEBIT_CREDIT: BEGIN_TRANSACTION; GET MESSAGE; EXTRACT ACCOUT_NUMBER, DELTA, TELLER, BRANCH

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c onsistency without borders

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  1. consistency without borders Peter Alvaro, Peter Bailis, Neil Conway, Joseph M. Hellerstein UC Berkeley

  2. The transaction concept DEBIT_CREDIT: BEGIN_TRANSACTION; GET MESSAGE; EXTRACT ACCOUT_NUMBER, DELTA, TELLER, BRANCH FROM MESSAGE; FIND ACCOUNT(ACCOUT_NUMBER) IN DATA BASE; IF NOT_FOUND | ACCOUNT_BALANCE + DELTA < 0 THEN PUT NEGATIVE RESPONSE; ELSE DO; ACCOUNT_BALANCE = ACCOUNT_BALANCE + DELTA; POST HISTORY RECORD ON ACCOUNT (DELTA); CASH_DRAWER(TELLER) = CASH_DRAWER(TELLER) + DELTA; BRANCH_BALANCE(BRANCH) = BRANCH_BALANCE(BRANCH) + DELTA; PUT MESSAGE ('NEW BALANCE =' ACCOUNT_BALANCE); END; COMMIT;

  3. An application-level contract Transactions Application Write Read Opaque store

  4. Pervasive distribution asynchrony CAP partial failure

  5. Research on consistency (translation) Assert: balance > 0 Application causal? PRAM? delta? fork/join? red/blue? release? SC? Consistency models Write Read Opaque store R1(X=1) R2(X=1) W1(X=2) W2(X=0) W1(X=1) W1(Y=2) R2(Y=2) R2(X=0)

  6. Meanwhile, in industry… (prayer) Assert: balance > 0 Custom solutions Application Write Read Opaque store

  7. Distributed consistency: staying relevant • Is this an important problem? • Is academia disconnected from reality? • OK, what now? Goal: help programmers write correct applications. Today: some promising approaches

  8. Case study: a graph

  9. Partitioned, for scalability

  10. Replicated, for availability

  11. Problem: deadlock detection Task: Identify strongly-connected components Waits-for graph

  12. Problem: garbage collection Task: Identify nodes not reachable from Root. Root Refers-to graph

  13. Correctness Deadlock detection • Safety:No false positives- • Liveness:Identify all deadlocks Garbage collection • Safety:Never GC live memory! • Liveness: GC all orphaned memory Root Partition

  14. Consistency at the extremes Custom solutions? Efficient Correct Linearizable key-value store?

  15. Consistency across the stack

  16. Object-level consistency Capture semanticsof data structures that • allow greater concurrency • maintain guarantees (e.g. convergence)

  17. Object-level consistency Reordering Batching Retry/duplication Commutativity Associativity Idempotence Tolerant to Insert Read Read Insert Convergent data structure (e.g., Set CRDT)

  18. Object-level consistency GC Assert: No live nodes are reclaimed Application ? ? Convergent data structures Assert: Graph replicas converge

  19. Flow-level consistency

  20. Flow-level consistency Capture semantics of datain motion • Asynchronous dataflow model • component properties  system-wide guarantees

  21. Flow-level consistency Order-insensitivity (confluence) output set = f(input set) { } = { }

  22. Flow-level consistency Confluence is compositional output set = f g(input set)

  23. Graph queries as dataflow Confluent Coordinate here

  24. Language-level consistency DSLs for distributed programming? • Capture consistency concerns in the type system

  25. Language-level consistency CALM Theorem: Monotonic  confluent Conservative, syntactic test for confluence

  26. Language-level consistency Deadlock detector Garbage collector nonmonotonic

  27. Where we’ve been;where we’re headed correct reusable efficient intuitive

  28. Remember • Consistency is an application-level property • Correctness and performance are compatible • Meet programmers on their home turf • Build bridges!

  29. Queries?

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