Github user dhutchis commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/96#discussion_r61015577
  
    --- Diff: 
core/src/main/java/org/apache/accumulo/core/client/impl/SyncingTabletLocator.java
 ---
    @@ -0,0 +1,108 @@
    +/*
    + * 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.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syncLocator because it is okay if an outdated locator is invalidated.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 {
    --- End diff --
    
    The two places that a `SyncingTabletLocator` are called are
    
    1. `ConditionalWriterImpl`
    2. `TimeoutTabletLocator`
    
    Indeed, both lookup a tablet via an instance and table id.
    I put in a second constructor that handles this case. This saves a bit of 
code.  Will push shortly. 


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