Extending APScheduler

This document is meant to explain how to develop your custom triggers and data stores.

Custom triggers

The built-in triggers cover the needs of the majority of all users, particularly so when combined using AndTrigger and OrTrigger. However, some users may need specialized scheduling logic. This can be accomplished by creating your own custom trigger class.

To implement your scheduling logic, create a new class that inherits from the Trigger interface class:

from __future__ import annotations

from apscheduler.abc import Trigger

class MyCustomTrigger(Trigger):
    def next() -> datetime | None:
        ... # Your custom logic here

    def __getstate__():
        ... # Return the serializable state here

    def __setstate__(state):
        ... # Restore the state from the return value of __getstate__()

Requirements and constraints for trigger classes:

  • next() must always either return a timezone aware datetime object or None if a new run time cannot be calculated

  • next() must never return the same datetime twice and never one that is earlier than the previously returned one

  • __setstate__() must accept the return value of __getstate__() and restore the trigger to the functionally same state as the original

Triggers are stateful objects. The next() method is where you determine the next run time based on the current state of the trigger. The trigger’s internal state needs to be updated before returning to ensure that the trigger won’t return the same datetime on the next call. The trigger code does not need to be thread-safe.

Custom data stores

If you want to make use of some external service to store the scheduler data, and it’s not covered by a built-in data store implementation, you may want to create a custom data store class. It should be noted that custom data stores are substantially harder to implement than custom triggers.

Data store classes have the following design requirements:

  • Must publish the appropriate events to an event broker

  • Code must be thread safe (synchronous API) or task safe (asynchronous API)

The data store class needs to inherit from either DataStore or AsyncDataStore, depending on whether you want to implement the store using synchronous or asynchronous APIs:

from apscheduler.abc import DataStore, EventBroker

class MyCustomDataStore(Datastore):
    def start(self, event_broker: EventBroker) -> None:
        ...  # Save the event broker in a member attribute and initialize the store

    def stop(self, *, force: bool = False) -> None:
        ...  # Shut down the store

    # See the interface class for the rest of the abstract methods

Handling temporary failures

If you plan to make the data store implementation public, it is strongly recommended that you make an effort to ensure that the implementation can tolerate the loss of connectivity to the backing store. The Tenacity library is used for this purpose by the built-in stores to retry operations in case of a disconnection. If you use it to retry operations when exceptions are raised, it is important to only do that in cases of temporary errors, like connectivity loss, and not in cases like authentication failure, missing database and so forth. See the built-in data store implementations and Tenacity documentation for more information on how to pick the exceptions on which to retry the operations.