Berlin 2013 - Session - Abe Stanway

Transcript

1. @abestanway MOM! my algorithms SUCK
2. i know how to fix monitoring once and for all.
3. a real human physically staring at a single metric 24/7
4. that human will then alert a sleeping engineer when her metric does something weird
5. Boom. Perfect Monitoring™.
6. this works because humans are excellent visual pattern matchers* *there are, of course, many advanced statistical applications where signal cannot be determined from noise just by looking at the data.
7. can we teach software to be as good at simple anomaly detection as humans are?
8. let’s explore.
9. anomalies = not “normal”
10. humans can tell what “normal” is by just looking at a timeseries.
11. “if a datapoint is not within reasonable bounds, more or less, of what usually happens, it’s an anomaly” the human definition:
12. there are real statistics that describe what we mentally approximate
14. “what usually happens” the mean
15. “more or less” the standard deviation
16. “reasonable bounds” 3σ
17. so, in math speak, a metric is anomalous if the absolute value of latest datapoint is over three standard deviations above the mean
18. we have essentially derived statistical process control.
19. pioneered in the 1920s. heavily used in industrial engineering for quality control on assembly lines.
20. traditional control charts specification limits
21. grounded in exchangeability past = future
22. needs to be stationary
23. produced by independent random variables, with well- defined expected values
24. this allows for statistical inference
25. in other words, you need good lookin’ timeseries for this to work.
26. normal distribution: a more concise definition of good lookin’ μ 34.1% 13.6% 2.1% 34.1% 13.6% μ - σ 2.1%
27. if you’ve got a normal distribution, chances are you’ve got an exchangeable, stationary series produced by independent random variables
28. 99.7% fall under 3σ
29. μ 34.1% 13.6% 2.1% 34.1% 13.6% 2.1% μ - σ if your datapoint is in here, it’s an anomaly.
30. when only .3% lie above 3σ...
31. ...you get a high signal to noise ratio...
32. ...where “signal” indicates a fundmental state change, as opposed to a random, improbable variation.
33. a fundamental state change in the process means a different probability distribution function that describes the process
34. determining when probability distribution function shifts have occurred, as early as possible. anomaly detection:
35. μ 1
36. μ 1 a new PDF that describes a new process
37. drilling holes sawing boards forging steel
38. snapped drill bit teeth missing on table saw steel, like, melted
39. processes with well planned expected values that only suffer small, random deviances when working properly...
40. ...and massive “deviances”, aka, probability function shifts, when working improperly.
41. the bad news:
42. server infrastructures aren’t like assembly lines
43. systems are active participants in their own design
44. processes don’t have well defined expected values
45. they aren’t produced by genuinely independent random variables.
46. large variance does not necessarily indicate poor quality
47. they have seasonality
48. skewed distributions! less than 99.73% of all values lie within 3σ, so breaching 3σ is not necessarily bad 3σ possibly normal range
49. the dirty secret: using SPC-based algorithms results in lots and lots of false positives, and probably lots of false negatives as well
50. no way to retroactively find the false negatives short of combing with human eyes!
51. how do we combat this?* *warning! ideas!
52. we could always use custom fit models...
53. ...after all, as long as the *errors* from the model are normally distributed, we can use 3σ
54. Parameters are cool! a pretty decent forecast based on an artisanal handcrafted model
55. but fitting models is hard, even by hand.
56. possible to implement a class of ML algorithms that determine models based on distribution of errors, using Q-Q plots
57. Q-Q plots can also be used to determine if the PDF has changed, although hard to do with limited sample size
58. consenus: throw lots of different models at a series, hope it all shakes out.
59. [yes] [yes] [no] [no] [yes] [yes] = anomaly!
60. of course, if your models are all SPC-based, this doesn’t really get you anywhere
61. use exponentially weighted moving averages to adapt faster
62. fourier transforms to detect seasonality
63. second order anomalies: is the series “anomalously anomalous”?
64. ...this is all very hard.
65. so, we can either change what we expect of monitoring...
66. ...and treat it as a way of building noisy situational awareness, not absolute directives (alerts)...
67. ...or we can change what we expect out of engineering...
68. ...and construct strict specifications and expected values of all metrics.
69. neither are going to happen.
70. so we have to crack this algorithm nut.
71. ...ugh. @abestanway