action-pipe.md

+This is a proposal of new Knot feature: a system of (named) pipes, as a substitute for direct calling of user scripts from knotd process.
+
+The user might want to call a script when an event occurs. First motivation for this is publishing CDS/CDNSKEY records - the user shall react with updating DS record at the parent zone.
+
+The configuration might look like follows:
+```
+event:
+  - fifo: /my/scripts/knot_hok_ds.fifo
+    action: publish_ds
+```
+and the user script like this:
+```
+cat /my/scripts/knot_hok_ds.fifo | while read x; do
+  perform_action "$x"
+done
+```
+
+Some notes to this solution:
+ - this avoids `system()` or `popen()` calls by knotd which are known to be very evil
+ - this avoids active waiting at the script side (would be harmful if many zones)
+ - `O_NONBLOCK` has to be used at knotd side. If no script is listening on the configured fifo, knot just logs a warning
+ - just one script can listen at a fifo. However, it is possible to configure more fifos for any set of possible event actions
+
+Some suggestions:
+ - the emitted action request consists of a single line (terminated with `\n`), with fields separated by colon `:`, fields are: zone, action type, data ... do we need a timestamp?? can colon conflict with zone name or data???
+