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Failure or (The Unexpected Virtue of Functional Programming) @markhibberd

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Act I Working Software

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“Why do we continue in this miserable condition” - George Orwell, Animal Farm

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Reliability

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Correctness Reliability

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Correctness Reliability (the correct answer)

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Correctness Correctness (the correct answer)

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Correctness Reliability Correctness (the correct answer)

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Correctness Reliability (whenever i need it) (the correct answer) Correctness

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“Several of them would have protested if they could have found the right arguments.” - George Orwell, Animal Farm

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Act II Post Functional

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Data

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Decisions

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Outcomes

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Measurement

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λx.f x

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120+ code bases Pure, Typed FP Haskell, Scala & Stuff

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Stats and Reliability

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bad things can happen…

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P(failure) = 0.1

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P(failure) = 0.1

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redundancy

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redundancy

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P(individual failure) = 0.1

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P(system failure) = 0.1^10

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are failures really independent?

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P(mutually assured destruction) = 1

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redundancy

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but if one goes…

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they all do

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P(individual failure) = 0.1

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P(individual success) = 1 - 0.1 = 0.9

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P(all successes) = 0.9^10

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P(system failure) = 1 - 0.9^10

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P(system failure) = 1 - 0.9^10 = 0.65

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P(system failure) = 1 - 0.9^10 = 0.65

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Correctness Reliability (whenever i need it) (the correct answer) Correctness

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Correctness Reliability (produce the decisions by X o’clock using the last vetted dataset) (the best set of measurable decisions for today) Correctness

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Separation of Data and Computation

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If we can achieve reliable data, reliable computation should be pretty straightforward

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Can you restart your system at any point?

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Could you turn your long running daemon into a cron job?

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Reliable Data

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If you have untangled your computation from your data, someone has probably solved your data storage requirements

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But… Failure is never clean. One of the most difficult challenges is ensuring that we only have known good states, failure must not corrupt.

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Do you know the provenance of each piece of data in your system?

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If you detected a failure, would you be able to identify the downstream effects?

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Are there multiple paths to build a dataset? Could we rebuild from an alternate source if we needed to?

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Fail Hard or Monitor

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“fault isolation advocates that the process software be fail-fast, it should either function correctly or it should detect the fault, signal failure and stop operating” - Jim Gray, Why Do Computers Stop and What Can Be Done About It?

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But… often it is the sorta close, kinda reasonable, inputs that will hurt

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garbage in, garbage out 9134

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garbage in, garbage out 9134 42

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Fail Fast & Hard, otherwise Monitor Heavily

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monitor data in context 9134 4 3 3

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monitor data in context 9134 4 3 3

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Reliable Sub-Systems

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P(failure) = 0.1

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P(failure) = 0.01

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P(failure) = 0.001

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P(failure) = $$$

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P(failure) = sleep

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“All animals are equal, but some animals are more equal than others.” - George Orwell, Animal Farm

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überblock

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0x00bab10c

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0x00bab10c

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0x00bab10c

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0x00bab10c

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Ditto Blocks

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More Important, More Replication

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*bonus*

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*bonus* Built in data verification & self healing

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*bonus* Each block maintains integrity of children

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*bonus* Merkle Tree hash(b1, b2) hash(g1, g2) hash(g3, g4) hash(data)

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“ZFS has been subjected to over a million forced, violent crashes without losing data integrity or leaking a single block.” - Bonwick & Moore, ZFS The Last Word in File Systems

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Isolation End-to-End

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code

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build & test

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fail

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fail

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fail

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Almost everything that happens after a build undermines the isolation we have worked hard to achieve

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If I can’t run multiple versions of the same code in parallel, one programming error can bring everything down

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remember these?

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If I can run multiple versions of my code, but only one version of my infrastructure…

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Act III Building Systems

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“construct reliable systems from unreliable parts … from the knowledge that any component in the system might fail” - Holzman & Joshi, Reliable Software Systems Design

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the library worst library ever…

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the library worst library ever…

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the library worst library ever…

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the library P(failure) = 0.8

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the library P(failure) = 0.8 No Separation of Computation and Data

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the library P(failure) = 0.8

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the library P(failure) = 0.8 Crashes Corrupt The Data Store

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the library P(failure) = 0.8 proxy

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the library P(failure) = 0.8 proxy journal Reliable data storage

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the library P(failure) = 0.8 proxy journal On failure replay journal

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the library P(failure) = 0.8 proxy journal We have isolated failures

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the library P(failure) = 0.8^n proxy journal the library

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the library P(failure) = 0.8^2 = 0.64 proxy journal the library

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the library P(failure) = 0.8^10 = 0.10 proxy journal the library

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the library P(failure) = 0.8^20 = 0.01 proxy journal the library

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“A beach house isn’t just real estate. It’s a state of mind.” - Douglas Adams, Mostly Harmless

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