Don’t believe everything you learn

A minute always has 60 seconds, right? Wrong! Ok, but a country always has a capital city, right? Wrong again!

Come and join us for a lighthearted talk about some of the common falsehoods that developers believe which cause issues in real, production systems. We’ll cover the classics like timezones and names, as well as some new contenders such as geography and software versions.

Transcript

1. THINGS THAT WE
   BELIEVE ARE TRUE…
   

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2. THINGS THAT WE
   BELIEVE ARE TRUE…
   But they’re not
   

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3. Falsehoods
   programmers believe
   about X
   

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4. “Constraints placed upon a
   complex system by a
   mixture of ignorance, apathy,
   business requirements, or
   other legal constraints”
   

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5. Hello!
   I’M MICHAEL
   I’ve made lots of mistakes in the past
   You can find me at @mheap
   

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6. TIME
   

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7. The number of days in
   every month starts with a
   3, so either 30 or 31?
   February has 28
   

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8. Each month has the same
   number of days every
   year though?
   February has 29 in Leap Years
   

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9. But we can calculate the
   leap years - they're every
   4 years?
   Yes... unless its divisible by 100
   

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10. But that rules holds all the
    time right?
    Yes… unless you’re Sweden
    

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11. But we’re agreed,
    February is 28 or 29
    days?
    Yes… unless you’re Sweden
    

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12. But we’re agreed,
    February is 28 or 29
    days?
    Or the Soviet Union
    

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13. If we ignore Sweden, Leap Years are
    easy.
    Every 4 years unless it divides by 100
    …unless it doesn’t divide by 400.
    1900 wasn’t a leap year, neither is 2100
    2000 is.
    

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14. It's a good job that only small
    companies and independent
    programmers have issues with
    leap years though…. right?
    https://azure.microsoft.com/en-us/blog/
    summary-of-windows-azure-service-
    disruption-on-feb-29th-2012/
    

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15. TIME
    

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16. A month starts and ends
    in the same year
    Unless you’re an ancient
    Roman
    

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17. A month starts and ends
    in the same year
    Or modern Ethiopian
    

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18. Time always goes
    forwards
    Unless you’re in Shanghai
    1927-12-31 23:54:08 happened twice
    

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19. MODERN TIME
    

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20. The day before Saturday
    is always Friday.
    Unless you’re in Samoa in 2011
    

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21. The offsets between two
    time zones will remain
    constant.
    10 changes in 2016
    2 so far this year
    

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22. If they do change there’ll
    be plenty of notice
    North Korea gave 8 days' notice of a 30
    minute change in their time zone in 2015
    

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23. When they do change,
    they change by an
    integer number of hours
    UTC+04:30 Afghanistan
    UTC+12:45 New Zealand (Chatham Islands)
    

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24. Ok… so 15 minutes then?
    That’s safe?
    UTC-00:25:21 - Dublin Mean
    Time
    

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25. Audience
    Participation
    

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26. When are there more or
    less than 24 hours in a
    day?
    Daylight savings time!
    Means 23 or 25 hours in a day
    

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27. Is the time 23:59:60
    invalid?
    When leap seconds are inserted,
    a minute will be 61 seconds long.
    

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28. NAMES
    Michael
    

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29. A child will have the same
    surname as either their
    mother or father (or both)
    Not in Iceland
    

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30. People have at least two
    names
    Teller (of Penn and Teller fame)
    Akihito (Emperor of Japan)
    

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31. However many names they
    have... People’s names do
    not contain numbers
    John5… but that’s a stage name
    

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32. However many names they
    have... People’s names do
    not contain numbers
    How about K8lyn? (Kaitlyn)
    Ok.. it was actually KVIIIlyn
    

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33. For every rule you come up
    with there are guaranteed to
    be hundreds of exceptions
    

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34. For every rule you come up
    with there are guaranteed to
    be hundreds of exceptions
    No spaces
    van Buren
    

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35. For every rule you come up
    with there are guaranteed to
    be hundreds of exceptions
    Only letters
    O’Reilly
    

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36. For every rule you come up
    with there are guaranteed to
    be hundreds of exceptions
    One capital letter
    McDonald
    

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37. Audience
    Participation
    Michael
    

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38. “A name must contain one
    or more alphanumeric
    characters”
    

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40. I can safely assume that
    this dictionary of bad words
    contains no names in it.
    

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41. I can safely assume that
    this dictionary of bad words
    contains no names in it.
    Ever been to Middlesex?
    

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42. I can safely assume that
    this dictionary of bad words
    contains no names in it.
    Or eaten Shitake
    mushrooms?
    

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43. I can safely assume that
    this dictionary of bad words
    contains no names in it.
    Been named Craig
    Cockburn?
    

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44. I can safely assume that
    this dictionary of bad words
    contains no names in it.
    Or Linda Callahan?
    

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45. What about less obvious
    ones?
    Super Bowl XXX?
    

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46. Planning permission
    emails bouncing
    They talked about
    erection of a structure
    

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47. The sad story of
    Christopher Null
    if lastname = null then… well, then try
    again with a lastname that isn’t “null.”
    

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48. Addresses
    

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49. When there is a building
    number, it will be all-
    numeric
    1A Egmont Road, Middlesbrough, TS4 2HT
    

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50. No buildings are
    numbered zero
    0 Egmont Road, Middlesbrough, TS4 2HT
    

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51. If the addresses on the left of
    the road are even, the
    addresses on the right must be
    odd
    You can have even numbers on both sides.. but
    that’s not the interesting part
    

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52. Well, at least you can
    omit leading zeros
    101 Alma St, Palo Alto - where apartments 1 and 001
    were on different floors.
    

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53. A street name won't
    include a number
    8 Seven Gardens Burgh, Woodbridge, IP13 6SU
    

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54. OK, but at the very least
    you wouldn't name a
    town Street
    Actually there's a town called Street in Somerset,
    UK.
    

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55. You can't have two towns
    or cities with the same
    name in the same country
    The UK has three towns called Newport.
    

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56. OK, but those cities won't
    have duplicate street
    names
    10 High Street, Newport, PO30 1SS
    10 High Street, Newport, NP20 1FQ
    10 High Street, Newport, TF10 7AN
    

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57. OK, but you don't get
    multiple postcodes per
    building
    DVLA Swansea:
    V5Cs are processed at SA99 1BA,
    Driving licences at SA99 1AB
    

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58. But street names will be
    reasonably short - certainly
    less than 50 characters
    Aleja Alije Izetbegovića Prvig
    Predsjednika Predsjedništva Republika
    Bosna i Hercegovina
    

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59. Addresses are a fixed
    point in space
    BF1 4FB
    

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60. Concrete buildings are a
    fixed point in space
    In Zürich, a 6200 ton building was
    moved by 60 meters to make way for
    railway tracks
    

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61. Audience
    Participation
    

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62. Do all countries have a
    capital city?
    Switzerland does not. The
    government is currently in Bern,
    but the city is not the capital.
    

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63. Commerce
    

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64. A product has a price
    Products sold on auction site do not
    yet have a price. The moment the
    price is known is actually the moment
    the item will not be on sale anymore.
    

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65. Except for auctioned
    items, products have one
    price
    Price with tax, price without tax,
    sale price, list price, minimum
    price
    

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66. Currencies symbols
    uniquely identify a
    currency.
    The peso and dollar sign $ is used
    my many countries: USA, Cananda,
    Australia, Brunei, Namibia
    

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67. Currencies have a
    unicode symbol that
    represents them
    The Swiss franc does not, and
    until 2010, neither did the India
    rupee
    

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68. Audience
    Participation
    

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69. Which currency does not
    have a three letter
    identifier?
    The Canadian dollar:
    CA$
    

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70. Computers
    

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71. “
    The network is reliable.
    Latency is zero.
    Bandw
    idth is infinite.
    The network is secure.
    T
    opology doesn't change.
    There is one administrator.
    T
    ransport cost is zero.
    The network is homogeneous.
    

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72. “
    (?:(?:\r\n)?[ \t])*(?:(?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:
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    [ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\
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73. In Summary
    Computers are hard
    Humans make assumptions
    Things are never as simple as
    they seem.
    Ask questions, find out more. It’ll
    make life easier in the long run
    

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74. Thanks!
    I’M MICHAEL
    You can find me at @mheap
    

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