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From a..@apache.org
Subject [2/3] hadoop git commit: HADOOP-13263. Reload cached groups in background after expiry. (Contributed bt Stephen O'Donnell)
Date Mon, 27 Jun 2016 16:54:03 GMT
HADOOP-13263. Reload cached groups in background after expiry. (Contributed bt Stephen O'Donnell)


Project: http://git-wip-us.apache.org/repos/asf/hadoop/repo
Commit: http://git-wip-us.apache.org/repos/asf/hadoop/commit/f73e7487
Tree: http://git-wip-us.apache.org/repos/asf/hadoop/tree/f73e7487
Diff: http://git-wip-us.apache.org/repos/asf/hadoop/diff/f73e7487

Branch: refs/heads/branch-2
Commit: f73e748754f66d47a938cac453b5d69dc70dbdbc
Parents: 20ee619
Author: Arpit Agarwal <arp@apache.org>
Authored: Mon Jun 27 09:49:04 2016 -0700
Committer: Arpit Agarwal <arp@apache.org>
Committed: Mon Jun 27 09:49:04 2016 -0700

----------------------------------------------------------------------
 .../fs/CommonConfigurationKeysPublic.java       |  13 +
 .../java/org/apache/hadoop/security/Groups.java | 119 +++++++-
 .../src/main/resources/core-default.xml         |  22 ++
 .../src/site/markdown/GroupsMapping.md          |   4 +
 .../hadoop/security/TestGroupsCaching.java      | 269 ++++++++++++++++++-
 5 files changed, 423 insertions(+), 4 deletions(-)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/hadoop/blob/f73e7487/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/CommonConfigurationKeysPublic.java
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/CommonConfigurationKeysPublic.java
b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/CommonConfigurationKeysPublic.java
index e16c0ba..ceb8086 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/CommonConfigurationKeysPublic.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/CommonConfigurationKeysPublic.java
@@ -301,6 +301,19 @@ public class CommonConfigurationKeysPublic {
   public static final long HADOOP_SECURITY_GROUPS_CACHE_WARN_AFTER_MS_DEFAULT =
     5000;
   /** See <a href="{@docRoot}/../core-default.html">core-default.xml</a> */
+  public static final String HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD =
+      "hadoop.security.groups.cache.background.reload";
+  /** See <a href="{@docRoot}/../core-default.html">core-default.xml</a>. */
+  public static final boolean
+      HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD_DEFAULT = false;
+  /** See <a href="{@docRoot}/../core-default.html">core-default.xml</a>. */
+  public static final String
+      HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD_THREADS =
+          "hadoop.security.groups.cache.background.reload.threads";
+  /** See <a href="{@docRoot}/../core-default.html">core-default.xml</a>. */
+  public static final int
+      HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD_THREADS_DEFAULT = 3;
+  /** See <a href="{@docRoot}/../core-default.html">core-default.xml</a>.*/
   public static final String  HADOOP_SECURITY_AUTHENTICATION =
     "hadoop.security.authentication";
   /** See <a href="{@docRoot}/../core-default.html">core-default.xml</a> */

http://git-wip-us.apache.org/repos/asf/hadoop/blob/f73e7487/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Groups.java
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Groups.java
b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Groups.java
index 11d8f2a..5414ba5 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Groups.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Groups.java
@@ -24,8 +24,13 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLong;
 
 import org.apache.htrace.core.TraceScope;
 import org.apache.htrace.core.Tracer;
@@ -35,6 +40,11 @@ import com.google.common.cache.CacheBuilder;
 import com.google.common.cache.Cache;
 import com.google.common.cache.CacheLoader;
 import com.google.common.cache.LoadingCache;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.common.util.concurrent.ListeningExecutorService;
+import com.google.common.util.concurrent.MoreExecutors;
+import com.google.common.util.concurrent.ThreadFactoryBuilder;
+
 import org.apache.hadoop.HadoopIllegalArgumentException;
 import org.apache.hadoop.classification.InterfaceAudience;
 import org.apache.hadoop.classification.InterfaceAudience.Private;
@@ -71,6 +81,17 @@ public class Groups {
   private final long warningDeltaMs;
   private final Timer timer;
   private Set<String> negativeCache;
+  private final boolean reloadGroupsInBackground;
+  private final int reloadGroupsThreadCount;
+
+  private final AtomicLong backgroundRefreshSuccess =
+      new AtomicLong(0);
+  private final AtomicLong backgroundRefreshException =
+      new AtomicLong(0);
+  private final AtomicLong backgroundRefreshQueued =
+      new AtomicLong(0);
+  private final AtomicLong backgroundRefreshRunning =
+      new AtomicLong(0);
 
   public Groups(Configuration conf) {
     this(conf, new Timer());
@@ -93,6 +114,18 @@ public class Groups {
     warningDeltaMs =
       conf.getLong(CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_WARN_AFTER_MS,
         CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_WARN_AFTER_MS_DEFAULT);
+    reloadGroupsInBackground =
+      conf.getBoolean(
+          CommonConfigurationKeys.
+              HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD,
+          CommonConfigurationKeys.
+              HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD_DEFAULT);
+    reloadGroupsThreadCount  =
+      conf.getInt(
+          CommonConfigurationKeys.
+              HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD_THREADS,
+          CommonConfigurationKeys.
+              HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD_THREADS_DEFAULT);
     parseStaticMapping(conf);
 
     this.timer = timer;
@@ -187,6 +220,22 @@ public class Groups {
     }
   }
 
+  public long getBackgroundRefreshSuccess() {
+    return backgroundRefreshSuccess.get();
+  }
+
+  public long getBackgroundRefreshException() {
+    return backgroundRefreshException.get();
+  }
+
+  public long getBackgroundRefreshQueued() {
+    return backgroundRefreshQueued.get();
+  }
+
+  public long getBackgroundRefreshRunning() {
+    return backgroundRefreshRunning.get();
+  }
+
   /**
    * Convert millisecond times from hadoop's timer to guava's nanosecond ticker.
    */
@@ -208,11 +257,41 @@ public class Groups {
    * Deals with loading data into the cache.
    */
   private class GroupCacheLoader extends CacheLoader<String, List<String>> {
+
+    private ListeningExecutorService executorService;
+
+    GroupCacheLoader() {
+      if (reloadGroupsInBackground) {
+        ThreadFactory threadFactory = new ThreadFactoryBuilder()
+            .setNameFormat("Group-Cache-Reload")
+            .setDaemon(true)
+            .build();
+        // With coreThreadCount == maxThreadCount we effectively
+        // create a fixed size thread pool. As allowCoreThreadTimeOut
+        // has been set, all threads will die after 60 seconds of non use
+        ThreadPoolExecutor parentExecutor =  new ThreadPoolExecutor(
+            reloadGroupsThreadCount,
+            reloadGroupsThreadCount,
+            60,
+            TimeUnit.SECONDS,
+            new LinkedBlockingQueue<Runnable>(),
+            threadFactory);
+        parentExecutor.allowCoreThreadTimeOut(true);
+        executorService = MoreExecutors.listeningDecorator(parentExecutor);
+      }
+    }
+
     /**
      * This method will block if a cache entry doesn't exist, and
      * any subsequent requests for the same user will wait on this
      * request to return. If a user already exists in the cache,
-     * this will be run in the background.
+     * and when the key expires, the first call to reload the key
+     * will block, but subsequent requests will return the old
+     * value until the blocking thread returns.
+     * If reloadGroupsInBackground is true, then the thread that
+     * needs to refresh an expired key will not block either. Instead
+     * it will return the old cache value and schedule a background
+     * refresh
      * @param user key of cache
      * @return List of groups belonging to user
      * @throws IOException to prevent caching negative entries
@@ -247,6 +326,44 @@ public class Groups {
     }
 
     /**
+     * Override the reload method to provide an asynchronous implementation. If
+     * reloadGroupsInBackground is false, then this method defers to the super
+     * implementation, otherwise is arranges for the cache to be updated later
+     */
+    @Override
+    public ListenableFuture<List<String>> reload(final String key,
+                                                 List<String> oldValue)
+        throws Exception {
+      if (!reloadGroupsInBackground) {
+        return super.reload(key, oldValue);
+      }
+
+      backgroundRefreshQueued.incrementAndGet();
+      ListenableFuture<List<String>> listenableFuture =
+          executorService.submit(new Callable<List<String>>() {
+            @Override
+            public List<String> call() throws Exception {
+              boolean success = false;
+              try {
+                backgroundRefreshQueued.decrementAndGet();
+                backgroundRefreshRunning.incrementAndGet();
+                List<String> results = load(key);
+                success = true;
+                return results;
+              } finally {
+                backgroundRefreshRunning.decrementAndGet();
+                if (success) {
+                  backgroundRefreshSuccess.incrementAndGet();
+                } else {
+                  backgroundRefreshException.incrementAndGet();
+                }
+              }
+            }
+          });
+      return listenableFuture;
+    }
+
+    /**
      * Queries impl for groups belonging to the user. This could involve I/O and take awhile.
      */
     private List<String> fetchGroupList(String user) throws IOException {

http://git-wip-us.apache.org/repos/asf/hadoop/blob/f73e7487/hadoop-common-project/hadoop-common/src/main/resources/core-default.xml
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/main/resources/core-default.xml b/hadoop-common-project/hadoop-common/src/main/resources/core-default.xml
index ead7b50..980889d 100644
--- a/hadoop-common-project/hadoop-common/src/main/resources/core-default.xml
+++ b/hadoop-common-project/hadoop-common/src/main/resources/core-default.xml
@@ -175,6 +175,28 @@
 </property>
 
 <property>
+  <name>hadoop.security.groups.cache.background.reload</name>
+  <value>false</value>
+  <description>
+    Whether to reload expired user->group mappings using a background thread
+    pool. If set to true, a pool of
+    hadoop.security.groups.cache.background.reload.threads is created to
+    update the cache in the background.
+  </description>
+</property>
+
+<property>
+  <name>hadoop.security.groups.cache.background.reload.threads</name>
+  <value>3</value>
+  <description>
+    Only relevant if hadoop.security.groups.cache.background.reload is true.
+    Controls the number of concurrent background user->group cache entry
+    refreshes. Pending refresh requests beyond this value are queued and
+    processed when a thread is free.
+  </description>
+</property>
+
+<property>
   <name>hadoop.security.group.mapping.ldap.connection.timeout.ms</name>
   <value>60000</value>
   <description>

http://git-wip-us.apache.org/repos/asf/hadoop/blob/f73e7487/hadoop-common-project/hadoop-common/src/site/markdown/GroupsMapping.md
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/site/markdown/GroupsMapping.md b/hadoop-common-project/hadoop-common/src/site/markdown/GroupsMapping.md
index b0508f8..f2e04b9 100644
--- a/hadoop-common-project/hadoop-common/src/site/markdown/GroupsMapping.md
+++ b/hadoop-common-project/hadoop-common/src/site/markdown/GroupsMapping.md
@@ -63,6 +63,10 @@ Caching/Negative caching
 --------
 Since the group mapping resolution relies on external mechanisms, the NameNode performance
may be impacted. To reduce the impact due to repeated lookups, Hadoop caches the groups returned
by the service provider. The cache invalidate is configurable via `hadoop.security.groups.cache.secs`,
and the default is 300 seconds.
 
+With the default caching implementation, after `hadoop.security.groups.cache.secs` when the
cache entry expires, the next thread to request group membership will query the group mapping
service provider to lookup the current groups for the user. While this lookup is running,
the thread that initiated it will block, while any other threads requesting groups for the
same user will retrieve the previously cached values. If the refresh fails, the thread performing
the refresh will throw an exception and the process will repeat for the next thread that requests
a lookup for that value. If the lookup repeatedly fails, and the cache is not updated, after
`hadoop.security.groups.cache.secs * 10` seconds the cached entry will be evicted and all
threads will block until a successful reload is performed.
+
+To avoid any threads blocking when the cached entry expires, set `hadoop.security.groups.cache.background.reload`
to true. This enables a small thread pool of `hadoop.security.groups.cache.background.reload.threads`
threads having 3 threads by default. With this setting, when the cache is queried for an expired
entry, the expired result is returned immediately and a task is queued to refresh the cache
in the background. If the background refresh fails a new refresh operation will be queued
by the next request to the cache, until `hadoop.security.groups.cache.secs * 10` when the
cached entry will be evicted and all threads will block for that user until a successful reload
occurs.
+
 To avoid spamming NameNode with unknown users, Hadoop employs negative caching so that if
the result of the lookup is empty, return an empty group directly instead of performing more
group mapping queries,
 The cache invalidation is configurable via `hadoop.security.groups.negative-cache.secs`.
The default is 30 seconds, so if group mapping service providers returns no group for a user,
no lookup will be performed for the same user within 30 seconds.
 

http://git-wip-us.apache.org/repos/asf/hadoop/blob/f73e7487/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestGroupsCaching.java
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestGroupsCaching.java
b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestGroupsCaching.java
index 3686694..5a5596e 100644
--- a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestGroupsCaching.java
+++ b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestGroupsCaching.java
@@ -50,8 +50,8 @@ public class TestGroupsCaching {
   private Configuration conf;
 
   @Before
-  public void setup() {
-    FakeGroupMapping.resetRequestCount();
+  public void setup() throws IOException {
+    FakeGroupMapping.clearAll();
     ExceptionalGroupMapping.resetRequestCount();
 
     conf = new Configuration();
@@ -66,13 +66,18 @@ public class TestGroupsCaching {
     private static Set<String> blackList = new HashSet<String>();
     private static int requestCount = 0;
     private static long getGroupsDelayMs = 0;
+    private static boolean throwException;
 
     @Override
     public List<String> getGroups(String user) throws IOException {
       LOG.info("Getting groups for " + user);
+      delayIfNecessary();
+
       requestCount++;
 
-      delayIfNecessary();
+      if (throwException) {
+        throw new IOException("For test");
+      }
 
       if (blackList.contains(user)) {
         return new LinkedList<String>();
@@ -102,6 +107,15 @@ public class TestGroupsCaching {
       blackList.clear();
     }
 
+    public static void clearAll() throws IOException {
+      LOG.info("Resetting FakeGroupMapping");
+      blackList.clear();
+      allGroups.clear();
+      requestCount = 0;
+      getGroupsDelayMs = 0;
+      throwException = false;
+    }
+
     @Override
     public void cacheGroupsAdd(List<String> groups) throws IOException {
       LOG.info("Adding " + groups + " to groups.");
@@ -124,6 +138,10 @@ public class TestGroupsCaching {
     public static void setGetGroupsDelayMs(long delayMs) {
       getGroupsDelayMs = delayMs;
     }
+
+    public static void setThrowException(boolean throwIfTrue) {
+      throwException = throwIfTrue;
+    }
   }
 
   public static class ExceptionalGroupMapping extends ShellBasedUnixGroupsMapping {
@@ -404,6 +422,251 @@ public class TestGroupsCaching {
   }
 
   @Test
+  public void testThreadNotBlockedWhenExpiredEntryExistsWithBackgroundRefresh()
+      throws Exception {
+    conf.setLong(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_SECS, 1);
+    conf.setBoolean(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD,
+        true);
+    FakeTimer timer = new FakeTimer();
+    final Groups groups = new Groups(conf, timer);
+    groups.cacheGroupsAdd(Arrays.asList(myGroups));
+    groups.refresh();
+    FakeGroupMapping.clearBlackList();
+
+    // We make an initial request to populate the cache
+    groups.getGroups("me");
+    // Further lookups will have a delay
+    FakeGroupMapping.setGetGroupsDelayMs(100);
+    // add another groups
+    groups.cacheGroupsAdd(Arrays.asList("grp3"));
+    int startingRequestCount = FakeGroupMapping.getRequestCount();
+
+    // Then expire that entry
+    timer.advance(4 * 1000);
+
+    // Now get the cache entry - it should return immediately
+    // with the old value and the cache will not have completed
+    // a request to getGroups yet.
+    assertEquals(groups.getGroups("me").size(), 2);
+    assertEquals(startingRequestCount, FakeGroupMapping.getRequestCount());
+
+    // Now sleep for over the delay time and the request count should
+    // have completed
+    Thread.sleep(110);
+    assertEquals(startingRequestCount + 1, FakeGroupMapping.getRequestCount());
+    // Another call to get groups should give 3 groups instead of 2
+    assertEquals(groups.getGroups("me").size(), 3);
+  }
+
+  @Test
+  public void testThreadBlockedWhenExpiredEntryExistsWithoutBackgroundRefresh()
+      throws Exception {
+    conf.setLong(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_SECS, 1);
+    conf.setBoolean(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD,
+        false);
+    FakeTimer timer = new FakeTimer();
+    final Groups groups = new Groups(conf, timer);
+    groups.cacheGroupsAdd(Arrays.asList(myGroups));
+    groups.refresh();
+    FakeGroupMapping.clearBlackList();
+
+    // We make an initial request to populate the cache
+    groups.getGroups("me");
+    // Further lookups will have a delay
+    FakeGroupMapping.setGetGroupsDelayMs(100);
+    // add another group
+    groups.cacheGroupsAdd(Arrays.asList("grp3"));
+    int startingRequestCount = FakeGroupMapping.getRequestCount();
+
+    // Then expire that entry
+    timer.advance(4 * 1000);
+
+    // Now get the cache entry - it should block and return the new
+    // 3 group value
+    assertEquals(groups.getGroups("me").size(), 3);
+    assertEquals(startingRequestCount + 1, FakeGroupMapping.getRequestCount());
+  }
+
+  @Test
+  public void testExceptionOnBackgroundRefreshHandled() throws Exception {
+    conf.setLong(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_SECS, 1);
+    conf.setBoolean(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD,
+        true);
+    FakeTimer timer = new FakeTimer();
+    final Groups groups = new Groups(conf, timer);
+    groups.cacheGroupsAdd(Arrays.asList(myGroups));
+    groups.refresh();
+    FakeGroupMapping.clearBlackList();
+
+    // We make an initial request to populate the cache
+    groups.getGroups("me");
+
+    // add another group
+    groups.cacheGroupsAdd(Arrays.asList("grp3"));
+    int startingRequestCount = FakeGroupMapping.getRequestCount();
+    // Arrange for an exception to occur only on the
+    // second call
+    FakeGroupMapping.setThrowException(true);
+
+    // Then expire that entry
+    timer.advance(4 * 1000);
+
+    // Now get the cache entry - it should return immediately
+    // with the old value and the cache will not have completed
+    // a request to getGroups yet.
+    assertEquals(groups.getGroups("me").size(), 2);
+    assertEquals(startingRequestCount, FakeGroupMapping.getRequestCount());
+
+    // Now sleep for a short time and re-check the request count. It should have
+    // increased, but the exception means the cache will not have updated
+    Thread.sleep(50);
+    FakeGroupMapping.setThrowException(false);
+    assertEquals(startingRequestCount + 1, FakeGroupMapping.getRequestCount());
+    assertEquals(groups.getGroups("me").size(), 2);
+
+    // Now sleep another short time - the 3rd call to getGroups above
+    // will have kicked off another refresh that updates the cache
+    Thread.sleep(50);
+    assertEquals(startingRequestCount + 2, FakeGroupMapping.getRequestCount());
+    assertEquals(groups.getGroups("me").size(), 3);
+  }
+
+
+  @Test
+  public void testEntriesExpireIfBackgroundRefreshFails() throws Exception {
+    conf.setLong(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_SECS, 1);
+    conf.setBoolean(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD,
+        true);
+    FakeTimer timer = new FakeTimer();
+    final Groups groups = new Groups(conf, timer);
+    groups.cacheGroupsAdd(Arrays.asList(myGroups));
+    groups.refresh();
+    FakeGroupMapping.clearBlackList();
+
+    // We make an initial request to populate the cache
+    groups.getGroups("me");
+
+    // Now make all calls to the FakeGroupMapper throw exceptions
+    FakeGroupMapping.setThrowException(true);
+
+    // The cache entry expires for refresh after 1 second
+    // It expires for eviction after 1 * 10 seconds after it was last written
+    // So if we call getGroups repeatedly over 9 seconds, 9 refreshes should
+    // be triggered which will fail to update the key, but the keys old value
+    // will be retrievable until it is evicted after about 10 seconds.
+    for(int i=0; i<9; i++) {
+      assertEquals(groups.getGroups("me").size(), 2);
+      timer.advance(1 * 1000);
+    }
+    // Wait until the 11th second. The call to getGroups should throw
+    // an exception as the key will have been evicted and FakeGroupMapping
+    // will throw IO Exception when it is asked for new groups. In this case
+    // load must be called synchronously as there is no key present
+    timer.advance(2 * 1000);
+    try {
+      groups.getGroups("me");
+      fail("Should have thrown an exception here");
+    } catch (Exception e) {
+      // pass
+    }
+
+    // Finally check groups are retrieve again after FakeGroupMapping
+    // stops throw exceptions
+    FakeGroupMapping.setThrowException(false);
+    assertEquals(groups.getGroups("me").size(), 2);
+  }
+
+  @Test
+  public void testBackgroundRefreshCounters()
+      throws IOException, InterruptedException {
+    conf.setLong(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_SECS, 1);
+    conf.setBoolean(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD,
+        true);
+    conf.setInt(
+        CommonConfigurationKeys.
+            HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD_THREADS,
+        2);
+    FakeTimer timer = new FakeTimer();
+    final Groups groups = new Groups(conf, timer);
+    groups.cacheGroupsAdd(Arrays.asList(myGroups));
+    groups.refresh();
+    FakeGroupMapping.clearBlackList();
+
+    // populate the cache
+    String[] grps = {"one", "two", "three", "four", "five"};
+    for (String g: grps) {
+      groups.getGroups(g);
+    }
+
+    // expire the cache
+    timer.advance(2*1000);
+    FakeGroupMapping.setGetGroupsDelayMs(40);
+
+    // Request all groups again, as there are 2 threads to process them
+    // 3 should get queued and 2 should be running
+    for (String g: grps) {
+      groups.getGroups(g);
+    }
+    Thread.sleep(20);
+    assertEquals(groups.getBackgroundRefreshQueued(), 3);
+    assertEquals(groups.getBackgroundRefreshRunning(), 2);
+
+    // After 120ms all should have completed running
+    Thread.sleep(120);
+    assertEquals(groups.getBackgroundRefreshQueued(), 0);
+    assertEquals(groups.getBackgroundRefreshRunning(), 0);
+    assertEquals(groups.getBackgroundRefreshSuccess(), 5);
+
+    // Now run again, this time throwing exceptions but no delay
+    timer.advance(2*1000);
+    FakeGroupMapping.setGetGroupsDelayMs(0);
+    FakeGroupMapping.setThrowException(true);
+    for (String g: grps) {
+      groups.getGroups(g);
+    }
+    Thread.sleep(20);
+    assertEquals(groups.getBackgroundRefreshQueued(), 0);
+    assertEquals(groups.getBackgroundRefreshRunning(), 0);
+    assertEquals(groups.getBackgroundRefreshSuccess(), 5);
+    assertEquals(groups.getBackgroundRefreshException(), 5);
+  }
+
+  @Test
+  public void testExceptionCallingLoadWithoutBackgroundRefreshReturnsOldValue()
+      throws Exception {
+    conf.setLong(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_SECS, 1);
+    conf.setBoolean(
+        CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_BACKGROUND_RELOAD,
+        false);
+    FakeTimer timer = new FakeTimer();
+    final Groups groups = new Groups(conf, timer);
+    groups.cacheGroupsAdd(Arrays.asList(myGroups));
+    groups.refresh();
+    FakeGroupMapping.clearBlackList();
+
+    // First populate the cash
+    assertEquals(groups.getGroups("me").size(), 2);
+
+    // Advance the timer so a refresh is required
+    timer.advance(2 * 1000);
+
+    // This call should throw an exception
+    FakeGroupMapping.setThrowException(true);
+    assertEquals(groups.getGroups("me").size(), 2);
+  }
+
+  @Test
   public void testCacheEntriesExpire() throws Exception {
     conf.setLong(
       CommonConfigurationKeys.HADOOP_SECURITY_GROUPS_CACHE_SECS, 1);


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