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From: keith-turner <git@git.apache.org>
To: dev@accumulo.apache.org
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References: <git-pr-96-accumulo@git.apache.org>
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Subject: [GitHub] accumulo pull request: ACCUMULO-4229 BatchWriter Locator
 cache out...
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Message-Id: <20160429181156.269D4DFE04@git1-us-west.apache.org>
Date: Fri, 29 Apr 2016 18:11:56 +0000 (UTC)

Github user keith-turner commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/96#discussion_r61621905
  
    --- 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 --
    
    Two style comments, feel free to ignore.  Just things I thought of while looking at the code.
    
    Could use an atomic reference instead of synchronized, but thats just a matter of style.  The synchronized code does seem correct.
    
    Unrelated I would have  a getter function that does the freshness check, this way the locator volatile only needs to be read once.  Current code reads the locator volatile in `syncLocator()` and then immediately after `syncLocator()` it reads the volatile again.  I doubt 2 round trips to main memory instead of three makes a performance difference in the big scheme of things, its another stylistic comment.
    
    ```java
    private TabletLocator getLocator() {
        TabletLocator localLocator = locator.get(); // in the normal case only read locator volatile once, if all internal code calls getLocator()
        if(!localLocator.isValid()){
           TabletLocator nl = getNewLocator();
           if(locator.compareAndSet(localLocator, nl)){
              localLocator = nl;
           } else {
              localLocator = locator.get();
           }
        }
        
       return localLocator;
    }
    ```


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