Edge vs. Level triggered logic

A brief explanation of "edge triggered" and "level triggered" logic.

Transcript

1. Google Cloud Platform
   Kubernetes: Edge vs. Level
   Triggered Logic
   Tim Hockin
   Principal Software Engineer
   @thockin
   

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2. Google Cloud Platform
   Once upon a time there was hardware
   When the hardware needed attention, it would
   signal the CPU == “interrupt”
   Interrupts used be a literal wire from the device
   to the CPU
   

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3. Google Cloud Platform
   Device CPU
   interrupt line
   

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4. Google Cloud Platform
   The device can change the voltage on the wire
   The CPU can detect changes in voltage
   

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5. Google Cloud Platform
   low voltage
   high voltage
   edge edge
   

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6. Google Cloud Platform
   Simple: the device “pulses” the interrupt line
   The CPU reacts
   This is “edge triggered”
   

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7. Google Cloud Platform
   Device CPU
   

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8. Google Cloud Platform
   Problem: what happens if the CPU doesn’t detect
   a pulse?
   The interrupt doesn’t get serviced!
   

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9. Google Cloud Platform
   low voltage
   high voltage
   level level
   

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10. Google Cloud Platform
    The device can change the voltage and keep it
    there until the interrupt is serviced
    The CPU can’t miss edges
    The device can “stack” interrupt reasons - leave
    the voltage until all reasons are ACK’ed
    This is “level triggered”
    

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11. Google Cloud Platform
    Device CPU
    

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12. Google Cloud Platform
    What does this mean for Kubernetes?
    State is more useful than events
    Level-driven software is about asserting state
    Clients can check and re-check state at any time
    This is the heart of Kubernetes’ controller model
    

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