Watcher Reference

A JSON schema describes these files and you can configure some editors to validate the YAML file based on that. The command choria machine validate can validate a machine.yaml against this schema.

Common watcher properties

All watchers share a common set of properties detailed below, watcher specific properties are always in the properties key:

Property Required Description
name yes A unique name for the watcher
type yes A known type of watcher like file or exec
state_match no A list of state names where this watcher is valid for
fail_transition If set this event fires on failure
success_transition If set this event fires on success
interval Runs the watcher every interval, valid intervals are of the form 1s, 1m, 1h
announce_interval Announce the current state of the watcher regularly, valid intervals are of the form 1s, 1m, 1h
properties yes Watcher specific settings

File watcher

The file watcher observes a specific file for changes and presence. Today only a basic mtime check is done, in time other dimensions like hashes or even inotify based observation will be supported.

Properties

Property Required Description
path yes The path to the file to watch relative to the watcher manifest directory
gather_initial_state Gathers the initial file mode, stats etc for regular announces but only perform first watch after interval

Behavior

A file watcher will at interval times do a mtime check on the file.

If the file is missing a fail_transition event fires and an announcement is made.

If the file has changed since the previous run a success_transition event fires and an announcement is made. This means the first check would set the initial state after which changes are detected. You could by setting gather_initial_state have the system gather initial file state on startup so the first regular watch would detect a change.

If the file have not changed nothing is published on every check, however a regular state announce can be done by setting announce_interval.

Exec watcher

The exec watcher supports running shell commands, it has a very basic exit code based interface, output from the commands is not significant.

Properties

Property Required Description
command yes The command to run relative to the watcher manifest directory
timeout How long the command is allowed to run, 10s default
suppress_success_announce Do not publish a state JSON document after every run, useful for frequently run items. Still publish on error. Still support regular publish via announce_interval
environment A list of custom environment variables to set in the form VAR=val

Behavior

An exec watcher will at interval times run the command specified with a few machine specific environment variables set in addition to any set using environment. Since version 0.11.1 when the interval is not set or set to 0 the the command will run only on transitions.

Variable Description
MACHINE_WATCHER_NAME The name of the watcher being run
MACHINE_NAME The name of the machine being run
PATH Includes the machine directory as last entry

The command is run with current directory set to the directory where the machine.yaml is, when the command exits 0 a success_transition fires, when it exits !0 a fail_transition fires. Both cases publish an event announcing the execution.

Scheduler watcher

The scheduler watcher flips between success and fail states based on a set of schedules specified in a crontab like format. Use it to enter and exit a state on a schedule and combine it with an exec watcher to run commands on a schedule.

This feature is available since Choria Server 0.11.1

Properties

Property Required Description
duration yes How long the scheduler should be in the success state once triggered
schedules yes A list of crontab like schedules based on which the success transitions will fire

Behavior

The schedules specified is a list of times when the scheduler will be in success transition, at the end of the trigger time + duration it will fire a fail transition. The fields are like crontab(5), supports ranges, special characters and predefined schedules like @daily, see robfib/cron section CRON Expression Format for what we’d understand. We do not support the seconds field.

 - name: scheduler
   type: schedule
   fail_transition: stop
   success_transition: start
   state_match: [switched_on, switched_off]
   interval: 1h
   properties:
     duration: 1h
     schedules:
       - "0 8 * * *"
       - "0 12 * * *"
       - "0 17 * * *"
       - "0 20 * * SAT,SUN"

The scheduler above will switch on daily at 8am, 12pm and 5pm but also at 8pm on Saturdays and Sundays. It will stay on for a hour.

If the machine transitions into a eligable state_match while a schedule is started it will immediately fire the success_transition. If Choria starts up in the middle of a scheduled period it will be ignored and the next schedule will trigger. Overlapping schedules is supported.