flink-runtime/src/main/java/org/apache/flink/runtime/state/CheckpointStorageLocationReference.java

/*
 * 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.flink.runtime.state;

import org.apache.flink.util.StringUtils;

import java.io.ObjectStreamException;
import java.util.Arrays;

import static org.apache.flink.util.Preconditions.checkArgument;
import static org.apache.flink.util.Preconditions.checkNotNull;

/**
 * A reference to a storage location. This is a wrapper around an array of bytes that are subject to
 * interpretation by the state backend's storage locations (similar as a serializer needs to
 * interpret byte streams). There is special handling for a 'default location', which can be used as
 * an optimization by state backends, when no extra information is needed to determine where the
 * checkpoints should be stored (all information can be derived from the configuration and the
 * checkpoint id).
 *
 * <h3>Why is this simply a byte array?</h3>
 *
 * <p>The reference is represented via raw bytes, which are subject to interpretation by the state
 * backends. We did not add any more typing and serialization abstraction in between, because these
 * types need to serialize/deserialize fast in between network streams (byte buffers) and barriers.
 * We may ultimately add some more typing if we simply keep the byte buffers for the checkpoint
 * barriers and forward them, thus saving decoding and re-encoding these references repeatedly.
 */
public class CheckpointStorageLocationReference implements java.io.Serializable {

    private static final long serialVersionUID = 1L;

    /** The encoded location reference. null indicates the default location. */
    private final byte[] encodedReference;

    /**
     * Creates a new location reference.
     *
     * @param encodedReference The location reference, represented as bytes (non null)
     */
    public CheckpointStorageLocationReference(byte[] encodedReference) {
        checkNotNull(encodedReference);
        checkArgument(encodedReference.length > 0);

        this.encodedReference = encodedReference;
    }

    /** Private constructor for singleton only. */
    private CheckpointStorageLocationReference() {
        this.encodedReference = null;
    }

    // ------------------------------------------------------------------------

    /**
     * Gets the reference bytes.
     *
     * <p><b>Important:</b> For efficiency, this method does not make a defensive copy, so the
     * caller must not modify the bytes in the array.
     */
    public byte[] getReferenceBytes() {
        // return a non null object always
        return encodedReference != null ? encodedReference : new byte[0];
    }

    /** Returns true, if this object is the default reference. */
    public boolean isDefaultReference() {
        return encodedReference == null;
    }

    // ------------------------------------------------------------------------

    @Override
    public int hashCode() {
        return encodedReference == null ? 2059243550 : Arrays.hashCode(encodedReference);
    }

    @Override
    public boolean equals(Object obj) {
        return obj == this
                || obj != null
                        && obj.getClass() == CheckpointStorageLocationReference.class
                        && Arrays.equals(
                                encodedReference,
                                ((CheckpointStorageLocationReference) obj).encodedReference);
    }

    @Override
    public String toString() {
        return encodedReference == null
                ? "(default)"
                : StringUtils.byteToHexString(encodedReference, 0, encodedReference.length);
    }

    /** readResolve() preserves the singleton property of the default value. */
    protected final Object readResolve() throws ObjectStreamException {
        return encodedReference == null ? DEFAULT : this;
    }

    // ------------------------------------------------------------------------
    //  Default Location Reference
    // ------------------------------------------------------------------------

    /** The singleton object for the default reference. */
    private static final CheckpointStorageLocationReference DEFAULT =
            new CheckpointStorageLocationReference();

    public static CheckpointStorageLocationReference getDefault() {
        return DEFAULT;
    }
}