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Module containing all necessary infrastructure components to support Event Sourcing Command and Query models.

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/*
 * Copyright (c) 2010-2022. Axon Framework
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.axonframework.eventsourcing;

import org.axonframework.eventhandling.EventMessage;

import javax.annotation.Nonnull;

/**
 * Interface describing a mechanism that keeps track of an Aggregate's activity in order to trigger a snapshot.
 * The trigger is notified of events handled by the aggregate and the time at which initialization of "current state"
 * has been completed.
 * 
 * It is up to the Trigger implementation to decide when, how and where a snapshot is triggered.
 * 

 * SnapshotTrigger implementations do not need to be thread-safe.
 * 
 * It is highly recommended to make trigger instances {@code Serializable}. The trigger instance is attached to the
 * aggregate instance which it monitors, causing it to be cached and potentially serialized alongside it.
 *
 * @see SnapshotTriggerDefinition
 */
public interface SnapshotTrigger {

    /**
     * Invoked when an event is handled by an aggregate. While these messages usually extend {@link
     * org.axonframework.eventhandling.DomainEventMessage}, this is not guaranteed. The given message can either be a
     * historic Message from the Event Store, or one that has been just applied by the aggregate.
     *
     * @param msg The message handled by the aggregate
     */
    void eventHandled(@Nonnull EventMessage msg);

    /**
     * Invoked when the initialization of the aggregate based on passed events is completed. Any subsequent invocation
     * of {@link #eventHandled(EventMessage)} involves an event being applied on "current state".
     */
    void initializationFinished();
}