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Sagas: distributed transactions without locks in Erlang MARK ALLEN - ALERTLOGIC [email protected] @BYTEMEORG

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Sagas

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Types of Sagas §Backward Recovery §Forward Recovery §“Recovery Blocks” §Parallel Sagas

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Sagas in Erlang

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The Big Idea Fold over a list of closures…

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The Big Idea Fold over a list of closures… ...unless there’s an error.

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The Big Idea Fold over a list of closures… ...unless there’s an error; Then, fold over a list of closures.

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Flows Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5 BOOM

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Flows Forward 1 Forward 2 Forward 3 Rollback 1 Rollback 2 Rollback 3 BOOM

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Flows Forward 1 Forward 2 Forward 3 Rollback 1 Rollback 2 Rollback 3 BOOM

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Flows Forward 1 Forward 2 Forward 3 Rollback 1 Rollback 2 Rollback 3 BOOM

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Flows Forward 1 Forward 2 Forward 3 Rollback 1 Rollback 2 Rollback 3 BOOM

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Terminology §Operation §Step §Transaction

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Terminology Forward 1 Forward 2 Forward 3 Forward 4 Forward 5 Rollback 1 Rollback 2 Rollback 3 Rollback 4 Rollback 5

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Implicit Assumptions and Requirements

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Compensating Closures Cannot Abort

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Idempotent Requests

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What if?? Forward 1 Forward 2 Forward 3 Rollback 1 Rollback 2 Rollback 3 BOOM

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What if?? Forward 1 Forward 2 Forward 3 Forward 4 Rollback 1 Rollback 2 Rollback 3

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Futures in Erlang

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Why does Erlang need futures?!

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It doesn’t! Except…

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Asynchronous Saga Operations

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Implementation

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Semantics?

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Property Based Tests

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What is it? §Write one or more invariants §Test values are automatically generated and the invariant tested §Failures are shrunk to the smallest possible failure case §Modify code (or test case) and restart test §Tests are usually timeboxed §Extremely useful to build confidence in complex scenarios

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Academic history QuickCheck: A Lightweight Tool for Random Testing of Haskell Programs

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Implementations for §Go §C/C++ §Clojure §Scala §Haskell (of course) §… lots of others … §Erlang/Elixir (watch Thomas Arts talk at ElixirConf EU 2015)

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Erlang implementations §Quviq Commercial QuickCheck (http://www.quviq.com) §Free (but not open source) “QuickCheck mini” – doesn’t do statem §PropEr (https://github.com/manopapad/proper) §Triq (https://github.com/triqng/triq)

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Generators §All standard basic types: §integers, §floats, §atoms, §binaries, §strings, §lists §Lists of basic types §User defined generators using ?LET and ?SUCHTHAT

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Propositions §?FORALL(Variable, generator_function(), test_function(Variable))

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Resources §https://github.com/mrallen1/gisla §https://github.com/mrallen1/criswell §Sagas paper: http://www.cs.cornell.edu/andru/cs711/2002fa/reading/sagas.pdf §Caitie McCaffrey, Papers We Love on Sagas: https://youtu.be/7dc4Tl5ZHRg?t=27m16s §Caitie McCaffrey, Distributed Sagas: https://speakerdeck.com/caitiem20/distributed-sagas-a-protocol-for- coordinating-microservices §These slides: https://speakerdeck.com/mrallen1/sagas-distributed-transactions- without-locks