Sunday, September 1, 2013

Level and Edge Triggered IO

There are "two models for readiness notification for a file descriptor.

  • Level-triggered notification: A file descriptor is considered to be ready if it is possible to perform an I/O system call without blocking.
  • Edge-triggered notification: Notification is provided if there is an I/O activity (e.g., new input) on a file descriptor since it was last monitored." [1]
(As a mnemonic, think of the 'E' in edge-triggered also standing for Event).

It's interesting to note that the two are isomorphic. If you were to have edge-triggered notifications and then kept a collection of them until used, you'd have level-triggered notification. Similarly, if you observed changes in the collection of level-triggered notifications, you'd have edge-triggered notifications. [2]

Anyway, in Java, "the Selector API requires a level-triggered polling mechanism. Edge-triggered interfaces require a much tighter coupling with the I/O methods." [3]

This edge-triggering can be seen in the standard read methods of Java I/O classes that tell you if data was received if the return value is greater than zero. 

[1] The Linux Programming Interface, Michael Kerrisk

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