The Scientist & The Engineer

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THE SCIENTIST ANDREW GODWIN // @andrewgodwin THE ENGINEER AND

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Hi, I’m Andrew Godwin

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I call myself an "engineer"

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But I studied Computer Science

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I knew programming going in. But this was different.

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“ Edsger Dijkstra Computer science is no more about computers than astronomy is about telescopes.

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Computer Science is idealistic

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"Turing-complete"

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We first want to prove it is possible

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async def sleep_sort(numbers): async def sleep_add(number): await asyncio.sleep(number) result.append(number) result = [] await asyncio.wait([ sleep_add(number) for number in numbers ]) return result

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We can prove that programs are correct

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def is_two(number): return number == 2

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>>> import __builtin__ >>> __builtin__.True = False >>> True False >>> True == False True (Thankfully, only works in Python 2!)

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"Communicating Sequential Processes", C. A. R Hoare

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contains :: (Ord a) => (Tree a) -> a -> Bool contains Nil _ = False contains (Node t1 v t2) x | x == v = True | x < v = contains t1 x | x > v = contains t2 x

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A perfect world, of perfect programs

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“ Benjamin Brewster In theory there is no difference between theory and practice. In practice there is.

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Software Engineering takes shortcuts

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When is it safe to take a risk?

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No content

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Software is faster and cheaper to change

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The real world is a nasty place

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How often does a cosmic ray affect RAM? 36 hours (for 16GB; "SEU at Ground Level", Eugene Normand) How long can an unpowered SSD keep data? 3 weeks - 1 year (Enterprise at 40ºC, Client at 30ºC; Intel/JEDEC) Does quantum tunneling affect CPUs? Continuously!

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Always design for failure

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How do we reason about software?

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Input Process Output

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async def disable_alarm(): ... async def open_door(): ... await asyncio.wait( [disable_alarm(), open_door()] )

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Input Rendering Output Validation Reporting Storage Processing Logging Cache

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Django has 250,000 lines of code

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An Airbus A380 has 4,000,000 parts

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A new car has 100,000,000 lines of code

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How are we supposed to handle this?

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Abstract, verify and forget

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Abstract Define a contract - types, behaviour, exceptions Verify Write tests to keep the contract valid Forget Work with the contract, not the fine details

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You have to learn to forget

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Engineering is communication

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“ Grace Hopper A ship in port is safe, but that's not what ships are built for.

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Build to expect growth

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Build to expect failure

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Everyone can build near-perfect software

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Know when not to!

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Python lets you do both.

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Types Start without types, progressively add with mypy Async Make it work synchronously first. Add async later. Speed Write slow, understandable code. Test it. Then improve it.