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Date: Tue, 26 Apr 2016 17:29:13 +0000 (UTC)
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Subject: [jira] [Commented] (ACCUMULO-4229) BatchWriter Locator cache
 out-of-sync when shared with tserver
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    [ https://issues.apache.org/jira/browse/ACCUMULO-4229?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15258508#comment-15258508 ] 

ASF GitHub Bot commented on ACCUMULO-4229:
------------------------------------------

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

    https://github.com/apache/accumulo/pull/96#discussion_r61128227
  
    --- 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 --
    
    Aargh, headache.  I'm not completely an expert on the Java memory model.  And even if I were, trying to reason about partial memory barriers makes my head hurt.  
    
    Every code path I can see should be threadsafe, given that `TabletLocator.isValid` is also volatile, and `TabletLocator.getLocator(...)` is synchronized.  But given all that, taking out the lock on `this` here is effectively a no-op.  It's difficult to guess what the consequences would be for an arbitrary `GetLocatorFunction`


> BatchWriter Locator cache out-of-sync when shared with tserver
> --------------------------------------------------------------
>
>                 Key: ACCUMULO-4229
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-4229
>             Project: Accumulo
>          Issue Type: Bug
>          Components: client
>    Affects Versions: 1.6.5, 1.7.1
>            Reporter: Dylan Hutchison
>            Assignee: Dylan Hutchison
>
> BatchWriters that run a long time have write rates that sometimes mysteriously decrease after the table it is writing to goes through a major compaction or a split.  The decrease can be as bad as reducing throughput to 0.
> This was first first mentioned in this [email thread|https://mail-archives.apache.org/mod_mbox/accumulo-user/201406.mbox/%3CCAMz+DuvmmHegOn9EJeHR9H_rRpP50L2QZ53BbdruVO0pirArQw@mail.gmail.com%3E] for major compactions.  
> I discovered this in this [email thread|https://mail-archives.apache.org/mod_mbox/accumulo-dev/201604.mbox/%3CCAPx%3DJkaY7fVh-U0O%2Bysx2d98LOGMcA4oEQOYgoPxR-0em4hdvg%40mail.gmail.com%3E] for splits.  See the thread for some log messages.
> I turned on TRACE logs and I think I pinned it down: the TabletLocator cached by a BatchWriter gets out of sync with the static cache of TabletLocators.
> # The TabletServerBatchWriter caches a TabletLocator from the static collection of TabletLocators when it starts writing.  Suppose it is writing to tablet T1.
> # The TabletServerBatchWriter uses its locally cached TabletLocator inside its `binMutations` method for its entire lifespan; this cache is never refreshed or updated to sync up with the static collection of TabletLocators.
> # Every hour, the static collection of TabletLocators clears itself.  The next call to get a TabletLocator from the static collection allocates a new TabletLocator.  Unfortunately, the TabletServerBatchWriter does not reflect this change and continues to use the old, locally cached TabletLocator.
> # Tablet T1 splits into T2 and T3, which closes T1.  As such, it no longer exists and the tablet server that receives the entries meant to go to T1 all fail to write because T1 is closed.
> # The TabletServerBatchWriter receives the response from the tablet server that all entries failed to write.  It invalidates the cache of the *new* TabletLocator obtained from the static collection of TabletLocators.  The old TabletLocator that is cached locally does not get invalidated.
> # The TabletServerBatchWriter re-queues the failed entries and tries to write them to the same closed tablet T1, because it is still looking up tablets using the old TabletLocator.
> This behavior subsumes the circumstances William wrote about in the thread he mentioned.  The problem would occur as a result of either splits or major compactions.  It would only stop the BatchWriter if its entire memory filled up with writes to the same tablet that was closed as a result of a majc or split; otherwise it would just slow down the BatchWriter by failing to write some number of entries with every RPC.
> There are a few solutions we can think of.  
> # Not have the MutationWriter inside the TabletServerBatchWriter locally cache TabletLocators.  I suspect this was done for performance reasons, so it's probably not a good solution. 
> # Have all the MutationWriters clear their cache at the same time the static TabletLocator cache clears.  We could store a reference to the Map that each MutationWriter has inside a static synchronized WeakHashMap.  The only time the weak map needs to be accessed is:
> ## When a MutationWriter is constructed (from constructing a TabletServerBatchWriter), add its new local TabletLocator cache to the weak map.
> ## When the static TabletLocator cache is cleared, also clear every map in the weak map.
> # Another solution is to make the invalidate calls on the local TabletLocator cache rather than the global static one.  If we go this route we should double check the idea to make sure it does not impact the correctness of any other pieces of code that use the cache. 
> # Perhaps the simplest solution is to put an extra Boolean variable inside the Locators indicating whether they are valid.  When they are cleared, their Boolean variables set to false.  Before a client uses a locator from cache, it checks its Boolean indicator.
> The TimeoutTabletLocator does not help when no timeout is set on the BatchWriter (the default behavior).


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