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​Develop a ​Trigger

Here is how you can develop a Trigger.

The Trigger example below will create an execution randomly:

java
@SuperBuilder
@ToString
@EqualsAndHashCode
@Getter
@NoArgsConstructor
public class Trigger extends AbstractTrigger implements PollingTriggerInterface, TriggerOutput<Trigger.Random> {
    @Builder.Default
    private final Duration interval = Duration.ofSeconds(60);

    protected Double min = 0.5;

    @Override
    public Optional<Execution> evaluate(ConditionContext conditionContext, TriggerContext context) {
        RunContext runContext = conditionContext.getRunContext();
        Logger logger = conditionContext.getRunContext().logger();
        double random = Math.random();

        if (random < this.min) {
            return Optional.empty();
        }

        Execution execution = Execution.builder()
            .id(runContext.getTriggerExecutionId())
            .namespace(context.getNamespace())
            .flowId(context.getFlowId())
            .flowRevision(context.getFlowRevision())
            .state(new State())
            .trigger(ExecutionTrigger.of(
                this,
                Trigger.Random.builder().random(random).build()
            ))
            .build();

        return Optional.of(execution);
    }

    @Builder
    @Getter
    public class Random implements io.kestra.core.models.tasks.Output {
        private Double random;
    }
}

You need to extend PollingTriggerInterface and implement the Optional<Execution> evaluate(ConditionContext conditionContext, TriggerContext context) method.

You can have any properties you want, like for any task (validation, documentation, ...), and everything is working the same way.

The evaluate method will receive these arguments:

  • ConditionContext conditionContext: a ConditionContext which includes various properties such as the RunContext in order to render your properties.
  • TriggerContext context: to have the context of this call (flow, execution, trigger, date, ...).

In this method, you add any logic you want: connect to a database, connect to remote file systems, ... You don't have to take care of resources, Kestra will run this method in its own thread.

This method must return an Optional<Execution> with:

  • Optional.empty(): if the condition is not validated.
  • Optional.of(execution): with the execution created if the condition is validated.

You have to provide a Output for any output needed (result of query, result of file system listing, ...) that will be available for the flow tasks within the {{ trigger.* }} variables.

Documentation

Remember to document your triggers. For this, we provide a set of annotations explained in the Document each plugin section.