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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.apache.cassandra.concurrent;

import org.apache.cassandra.service.ClientWarn;
import org.apache.cassandra.tracing.Tracing;

public interface ExecutorLocal
{
    ExecutorLocal[] all = { Tracing.instance, ClientWarn.instance };

    /**
     * This is called when scheduling the task, and also before calling {@link #set(Object)} when running on a
     * executor thread.
     *
     * @return The thread-local value that we want to copy across executor boundaries; may be null if not set.
     */
    T get();

    /**
     * Before a task has been run, this will be called with the value from the thread that scheduled the task, and after
     * the task is finished, the value that was previously retrieved from this thread is restored.
     *
     * @param value Value to use for the executor local state; may be null.
     */
    void set(T value);
}