Scheduling

Use Cases

Use a Schedule to

• Execute a flow in fixed or flexible time intervals (minutes, hours, days, weeks, months, etc...)
• Execute a flow on particular days of a calendar
• Execute a flow in arbitrary complex intervals
• Permanently polling a third party system

Concept

Cloudomation separates

• the logic which ensures a flow is executed repeatedly (the Scheduler)
• the configuration how the flow should be scheduled (the Setting)
• the logic of the flow itself (the Flow)
• the resource which combines all this together (the Schedule)

Using this separation it is possible to use the same scheduler logic (e.g. schedule something daily) for different flows with varying settings (e.g. scheduled at different hours)

Scheduler

A scheduler contains the script which implements logic to wait until the next iteration and start the flow.

A Scheduler script should start the scheduled flow only once and then end. Cloudomation will take care to start the next iteration. In case of an error in the Scheduler script Cloudomation will wait one minute and start another iteration.

import flow_api
def handler(system: flow_api.System, this: flow_api.Execution):
    """A simple scheduler script which starts the flow every minute"""
    # The execution receives the IDs of the referenced setting, flow,
    # and schedule resources.
    # In this example we'll only use the flow_id
    inputs = this.get('input_value')
    flow = system.flow(inputs['flow_id'], by='id')
    # wait one minute
    this.sleep(60)
    # start the flow
    this.flow(flow.get('name'))
    return this.success('all done')

Setting

The associated setting contains configuration options for the scheduler. Different schedulers can support different options. E.g. the number of minutes between two iterations, or the time at which to start the flow every day.

time: '09:00'
timezone: 'Europe/Vienna'
days_of_week: [1, 2, 3, 4, 5]

Flow

The flow contains the script which is repeatedly executed.

A Scheduler might pass context information to the flow execution. Which inputs are passed (if any) is up to the Scheduler script. The information passed can be used to have different behaviour in different iterations or for logging/audit purposes.

Here are some examples:

• The iteration counter
• A flag if the current day is a public holiday
• Email of the operator on shift

Schedule

The schedule links one scheduler with one setting and one flow. The schedule can be disabled or enabled.

Once the schedule is enabled, Cloudomation will immediately start an execution of the scheduler script.

When a schedule is disabled, all currently running executions of the scheduler script are cancelled by Cloudomation.

Complex schedulers

A Scheduler script uses the same syntax as a regular flow script. This means that a Scheduler can use arbitrary complex logic, connect to third party systems, and start child executions. Some examples:

• Connect to a calendar to check the next appointment time
• Check a weather API when some flow should only run after rain
• Analyse the status of other executions
• Check an email inbox

There are two recommendations to keep in mind when designing a Scheduler:

1. Separation of concerns

   The Scheduler should only contain logic needed to do the scheduling. The referenced Flow instead contains the logic which is scheduled.

2. Reusability

   A Scheduler should be designed so it can be used for different flows.

Considering those points the examples above should rather be placed in the Flow which is schedlued, than in the Scheduler.

Example schedulers

At the time of writing Cloudomation provides two example schedulers.

Schedule recurring

Schedule a flow to run in recurring intervals

Configuration options

• max_iterations: int = None

  if set, the schedule will disable itself once the number of iterations is reached

• interval_seconds: int = 60

  how many seconds to wait between iterations

• timezone: str = 'Europe/Vienna'

  the timezone to use for timestamps

• input_value: dict = {}

  additional inputs passed to the flow

Schedule daily

Schedule a flow to run at the same time every day

Configuration options

• max_iterations: int = None

  if set, the schedule will disable itself once the number of iterations is reached

• time: str = '08:15'

  the time at which the flow is started daily

• timezone: str = 'Europe/Vienna'

  the timezone to use for timestamps

• input_value: dict = {}

  additional inputs passed to the flow

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