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The core of Neo4j Bolt Protocol, this contains the state machine for Bolt sessions.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.bolt.fsm;

import org.neo4j.bolt.fsm.error.NoSuchStateException;
import org.neo4j.bolt.fsm.error.StateMachineException;
import org.neo4j.bolt.fsm.state.State;
import org.neo4j.bolt.fsm.state.StateReference;
import org.neo4j.bolt.protocol.common.connector.connection.Connection;
import org.neo4j.bolt.protocol.common.connector.connection.ConnectionHandle;
import org.neo4j.bolt.protocol.common.fsm.response.ResponseHandler;
import org.neo4j.bolt.protocol.common.message.request.RequestMessage;
import org.neo4j.dbms.admissioncontrol.AdmissionControlToken;

/**
 * Represents a {@link Connection connection}-bound state machine instance which follows the state
 * transitions configured within a parent {@link StateMachineConfiguration configuration}.
 *
 * @see StateMachineConfiguration
 */
public interface StateMachine {

    /**
     * Retrieves the connection for which this context manages state machine state.
     *
     * @return a connection.
     */
    ConnectionHandle connection();

    /**
     * Retrieves the current configured default state.
     *
     * @return a state reference.
     */
    StateReference defaultState();

    /**
     * Alters the default state to which this instance shall return when reset.
     *
     * @param state a state reference.
     * @throws NoSuchStateException when no state with the given reference exists within the state
     *                              machine configuration.
     */
    void defaultState(StateReference state) throws NoSuchStateException;

    /**
     * Identifies whether this state machine has failed in a recoverable fashion and will ignore all
     * further requests until it has been reset.
     *
     * @return true if failed, false otherwise.
     */
    boolean hasFailed();

    /**
     * Identifies whether this state machine has been interrupted and will ignore all further requests
     * until it has been reset.
     * 
     * This method is thread-safe.
     *
     * @return true if interrupted, false otherwise.
     */
    boolean isInterrupted();

    /**
     * Interrupts the execution of the current message processed by this state machine if any.
     * 

     * Note: Currently the executing thread is not directly interrupted by invoking this
     * function. The state machine will merely be placed within an interrupted state in which it
     * cannot handle messages.
     * 
     * This method is thread-safe.
     */
    void interrupt();

    /**
     * Reverts this state machine back to its default state and clears any remaining errors.
     */
    void reset();

    /**
     * Validates whether the state machine remains within a state in which it is capable of
     * performing operations.
     */
    boolean validate();

    /**
     * Processes a request within the scope of a state machine instance and advances its state
     * accordingly.
     *
     * @param message a request message.
     * @param handler a response handler.
     * @throws StateMachineException when the state machine fails to transition and is incapable of
     *                               recovering from this condition.
     * @see State#process(Context, RequestMessage, ResponseHandler)
     */
    default void process(RequestMessage message, ResponseHandler handler) throws StateMachineException {
        process(message, handler, null);
    }

    /**
     * Processes a request within the scope of a state machine instance and advances its state
     * accordingly.
     *
     * @param message a request message.
     * @param handler a response handler.
     * @param admissionControlToken a token to await handling message to ensure server health.
     * @throws StateMachineException when the state machine fails to transition and is incapable of
     *                               recovering from this condition.
     * @see State#process(Context, RequestMessage, ResponseHandler)
     */
    void process(RequestMessage message, ResponseHandler handler, AdmissionControlToken admissionControlToken)
            throws StateMachineException;
}