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From ShawnWalker <...@git.apache.org>
Subject [GitHub] accumulo pull request: ACCUMULO-4229 BatchWriter Locator cache out...
Date Tue, 26 Apr 2016 16:30:35 GMT
Github user ShawnWalker commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/96#discussion_r61118538
  
    --- Diff: core/src/main/java/org/apache/accumulo/core/client/impl/SyncingTabletLocator.java
---
    @@ -0,0 +1,118 @@
    +/*
    + * 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.accumulo.core.client.impl;
    +
    +import java.util.Collection;
    +import java.util.List;
    +import java.util.Map;
    +
    +import org.apache.accumulo.core.client.AccumuloException;
    +import org.apache.accumulo.core.client.AccumuloSecurityException;
    +import org.apache.accumulo.core.client.Instance;
    +import org.apache.accumulo.core.client.TableNotFoundException;
    +import org.apache.accumulo.core.data.KeyExtent;
    +import org.apache.accumulo.core.data.Mutation;
    +import org.apache.accumulo.core.data.Range;
    +import org.apache.accumulo.core.security.Credentials;
    +import org.apache.hadoop.io.Text;
    +
    +/**
    + * Syncs itself with the static collection of TabletLocators, so that when the server
clears it, it will automatically get the most up-to-date version. Caching
    + * TabletLocators locally is safe when using SyncingTabletLocator.
    + */
    +public class SyncingTabletLocator extends TabletLocator {
    +  /*
    +   * Implementation: the invalidateCache calls do not need to call syncLocator because
it is okay if an outdated locator is invalidated. In this case, on the
    +   * next call to a meaningful function like binRanges, the new locator will replace
the current one and be used instead. The new locator is fresh; it has no
    +   * cache at all when created.
    +   */
    +
    +  /**
    +   * Used to tell SyncingTabletLocator how to fetch a new locator when the current one
is outdated. For example,
    +   * <code>TabletLocator.getLocator(instance, tableId);</code>
    +   */
    +  public interface GetLocatorFunction {
    +    TabletLocator getLocator();
    +  }
    +
    +  private volatile TabletLocator locator;
    +  private final GetLocatorFunction getLocatorFunction;
    +
    +  public SyncingTabletLocator(GetLocatorFunction getLocatorFunction) {
    +    this.getLocatorFunction = getLocatorFunction;
    +    this.locator = getLocatorFunction.getLocator();
    +  }
    +
    +  public SyncingTabletLocator(final Instance instance, final Text tableId) {
    +    this(new GetLocatorFunction() {
    +      @Override
    +      public TabletLocator getLocator() {
    +        return TabletLocator.getLocator(instance, tableId);
    +      }
    +    });
    +  }
    +
    +  private void syncLocator() {
    +    if (!locator.isValid())
    --- End diff --
    
    This looks like a double-checked locking pattern.  DCL is often incorrect, at least in
theory (see e.g. http://www.javaworld.com/article/2074979/java-concurrency/double-checked-locking--clever--but-broken.html)
due to possible out-of-order execution or out-of-order memory consistency.
    
    Seeing as potential inconsistency here is unlikely to do anything other than cause us
to have to retry (again?), I suspect we could do away with the synchronized block.


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