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Subject [GitHub] jolynch commented on a change in pull request #283: CASSANDRA-14459: DynamicEndpointSnitch should never prefer latent replicas
Date Fri, 25 Jan 2019 03:05:21 GMT
jolynch commented on a change in pull request #283: CASSANDRA-14459: DynamicEndpointSnitch
should never prefer latent replicas
URL: https://github.com/apache/cassandra/pull/283#discussion_r250853939
 
 

 ##########
 File path: src/java/org/apache/cassandra/locator/DynamicEndpointSnitch.java
 ##########
 @@ -21,133 +21,505 @@
 import java.net.InetAddress;
 import java.net.UnknownHostException;
 import java.util.*;
+import java.util.concurrent.CancellationException;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.stream.Collectors;
 
-import com.codahale.metrics.ExponentiallyDecayingReservoir;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.MoreExecutors;
+import com.google.common.util.concurrent.RateLimiter;
+import com.google.common.util.concurrent.SettableFuture;
 
-import com.codahale.metrics.Snapshot;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import org.apache.cassandra.concurrent.DebuggableScheduledThreadPoolExecutor;
 import org.apache.cassandra.concurrent.ScheduledExecutors;
 import org.apache.cassandra.config.DatabaseDescriptor;
+import org.apache.cassandra.exceptions.RequestFailureReason;
 import org.apache.cassandra.gms.ApplicationState;
 import org.apache.cassandra.gms.EndpointState;
 import org.apache.cassandra.gms.Gossiper;
 import org.apache.cassandra.gms.VersionedValue;
+import org.apache.cassandra.locator.dynamicsnitch.DynamicEndpointSnitchHistogram;
+import org.apache.cassandra.net.IAsyncCallbackWithFailure;
+import org.apache.cassandra.net.LatencyMeasurementType;
+import org.apache.cassandra.net.MessageIn;
+import org.apache.cassandra.net.MessageOut;
 import org.apache.cassandra.net.MessagingService;
+import org.apache.cassandra.net.PingMessage;
 import org.apache.cassandra.service.StorageService;
 import org.apache.cassandra.utils.FBUtilities;
 import org.apache.cassandra.utils.MBeanWrapper;
 
+import static org.apache.cassandra.net.MessagingService.Verb.PING;
+import static org.apache.cassandra.net.async.OutboundConnectionIdentifier.ConnectionType.LARGE_MESSAGE;
+import static org.apache.cassandra.net.async.OutboundConnectionIdentifier.ConnectionType.SMALL_MESSAGE;
+
+
 /**
  * A dynamic snitch that sorts endpoints by latency with an adapted phi failure detector
+ * Note that the subclasses (e.g. {@link DynamicEndpointSnitchHistogram}) are responsible
for actually measuring
+ * latency and providing an ISnitchMeasurement implementation back to this class.
  */
-public class DynamicEndpointSnitch extends AbstractEndpointSnitch implements ILatencySubscriber,
DynamicEndpointSnitchMBean
+public abstract class DynamicEndpointSnitch extends AbstractEndpointSnitch implements ILatencySubscriber,
DynamicEndpointSnitchMBean
 {
-    private static final boolean USE_SEVERITY = !Boolean.getBoolean("cassandra.ignore_dynamic_snitch_severity");
+    private static final Logger logger = LoggerFactory.getLogger(DynamicEndpointSnitch.class);
+
+    // Latency measurement and ranking. The samples contain latency measurements and the
scores are used for ranking
+    protected boolean registered = false;
+    protected static final boolean USE_SEVERITY = !Boolean.getBoolean("cassandra.ignore_dynamic_snitch_severity");
+    protected volatile Map<InetAddressAndPort, Double> scores = new HashMap<>();
 
-    private static final double ALPHA = 0.75; // set to 0.75 to make EDS more biased to towards
the newer values
-    private static final int WINDOW_SIZE = 100;
+    protected final Map<InetAddressAndPort, AnnotatedMeasurement> samples = new ConcurrentHashMap<>();
 
-    private volatile int dynamicUpdateInterval = DatabaseDescriptor.getDynamicUpdateInterval();
-    private volatile int dynamicResetInterval = DatabaseDescriptor.getDynamicResetInterval();
-    private volatile double dynamicBadnessThreshold = DatabaseDescriptor.getDynamicBadnessThreshold();
+    // Latency probe functionality for actively probing endpoints that we haven't measured
recently but are ranking
+    public static final long MAX_PROBE_INTERVAL_MS = Long.getLong("cassandra.dynamic_snitch_max_probe_interval_ms",
60 * 10 * 1000L);
+    public static final long MIN_PROBE_INTERVAL_MS = Long.getLong("cassandra.dynamic_snitch_min_probe_interval_ms",
60 * 1000L) ;
+    // The probe rate is set later when configuration is read in applyConfigChanges
+    protected static final RateLimiter probeRateLimiter = RateLimiter.create(1);
+    protected static final DebuggableScheduledThreadPoolExecutor latencyProbeExecutor = new
DebuggableScheduledThreadPoolExecutor("LatencyProbes");
+    private long lastUpdateSamplesNanos = System.nanoTime();
+
+    // User configuration of the snitch tunables
+    protected volatile int dynamicUpdateInterval = -1;
+    protected volatile int dynamicSampleUpdateInterval = -1;
+    protected volatile double dynamicBadnessThreshold = 0;
 
     // the score for a merged set of endpoints must be this much worse than the score for
separate endpoints to
     // warrant not merging two ranges into a single range
     private static final double RANGE_MERGING_PREFERENCE = 1.5;
 
     private String mbeanName;
-    private boolean registered = false;
-
-    private volatile HashMap<InetAddressAndPort, Double> scores = new HashMap<>();
-    private final ConcurrentHashMap<InetAddressAndPort, ExponentiallyDecayingReservoir>
samples = new ConcurrentHashMap<>();
+    private boolean mbeanRegistered = false;
 
     public final IEndpointSnitch subsnitch;
 
-    private volatile ScheduledFuture<?> updateSchedular;
-    private volatile ScheduledFuture<?> resetSchedular;
-
-    private final Runnable update;
-    private final Runnable reset;
+    private volatile ScheduledFuture<?> updateScoresScheduler = null;
+    private volatile ScheduledFuture<?> updateSamplesScheduler = null;
 
     public DynamicEndpointSnitch(IEndpointSnitch snitch)
     {
         this(snitch, null);
     }
 
-    public DynamicEndpointSnitch(IEndpointSnitch snitch, String instance)
+    protected DynamicEndpointSnitch(IEndpointSnitch snitch, String instance)
     {
         mbeanName = "org.apache.cassandra.db:type=DynamicEndpointSnitch";
         if (instance != null)
             mbeanName += ",instance=" + instance;
         subsnitch = snitch;
-        update = new Runnable()
+
+        if (DatabaseDescriptor.isDaemonInitialized())
         {
-            public void run()
+            open();
+        }
+    }
+
+    /**
+     * Update configuration of the background tasks and restart the various scheduler tasks
+     * if the configured rates for these tasks have changed.
+     */
+    public void applyConfigChanges(int newDynamicUpdateInternal, int newDynamicSampleUpdateInterval,
double newDynamicBadnessThreshold)
+    {
+        if (DatabaseDescriptor.isDaemonInitialized())
+        {
+            if (dynamicUpdateInterval != newDynamicUpdateInternal || updateScoresScheduler
== null)
             {
-                updateScores();
+                cancelAndWait(updateScoresScheduler, Math.max(1, dynamicUpdateInterval),
TimeUnit.MILLISECONDS);
+                updateScoresScheduler = ScheduledExecutors.scheduledTasks.scheduleWithFixedDelay(this::updateScores,
newDynamicUpdateInternal, newDynamicUpdateInternal, TimeUnit.MILLISECONDS);
             }
-        };
-        reset = new Runnable()
+
+            if (dynamicSampleUpdateInterval != newDynamicSampleUpdateInterval || updateSamplesScheduler
== null)
+            {
+                cancelAndWait(updateSamplesScheduler, Math.max(1, dynamicSampleUpdateInterval),
TimeUnit.MILLISECONDS);
+                if (newDynamicSampleUpdateInterval > 0)
+                    updateSamplesScheduler = ScheduledExecutors.scheduledTasks.scheduleWithFixedDelay(this::updateSamples,
newDynamicSampleUpdateInterval, newDynamicSampleUpdateInterval, TimeUnit.MILLISECONDS);
+            }
+        }
+
+        dynamicUpdateInterval = newDynamicUpdateInternal;
+        dynamicSampleUpdateInterval = newDynamicSampleUpdateInterval;
+        dynamicBadnessThreshold = newDynamicBadnessThreshold;
+
+        if (dynamicSampleUpdateInterval > 0)
+            probeRateLimiter.setRate(dynamicSampleUpdateInterval);
+    }
+
+    public synchronized void open()
+    {
+        applyConfigChanges(DatabaseDescriptor.getDynamicUpdateInterval(),
+                           DatabaseDescriptor.getDynamicSampleUpdateInterval(),
+                           DatabaseDescriptor.getDynamicBadnessThreshold());
+
+        MBeanWrapper.instance.registerMBean(this, mbeanName);
+        mbeanRegistered = true;
+    }
+
+    public synchronized void close()
+    {
+        cancelAndWait(updateScoresScheduler, Math.max(1, dynamicUpdateInterval), TimeUnit.MILLISECONDS);
+        cancelAndWait(updateSamplesScheduler, Math.max(1, dynamicSampleUpdateInterval), TimeUnit.MILLISECONDS);
+        updateScoresScheduler = null;
+        updateSamplesScheduler = null;
+
+        for (AnnotatedMeasurement measurement : samples.values())
         {
-            public void run()
+            cancelAndWait(measurement.probeFuture, PING.getTimeout(), TimeUnit.MILLISECONDS);
+
+            measurement.millisSinceLastMeasure.set(0);
+            measurement.millisSinceLastRequest.set(MAX_PROBE_INTERVAL_MS);
+            measurement.nextProbeDelayMillis = 0;
+        }
+
+        if (mbeanRegistered)
+            MBeanWrapper.instance.unregisterMBean(mbeanName);
+
+        mbeanRegistered = false;
+    }
+
+    private static void cancelAndWait(ScheduledFuture future, long timeout, TimeUnit unit)
+    {
+        if (future != null)
+        {
+            future.cancel(false);
+            try
             {
-                // we do this so that a host considered bad has a chance to recover, otherwise
would we never try
-                // to read from it, which would cause its score to never change
-                reset();
+                future.get(timeout, unit);
             }
-        };
+            catch (CancellationException | InterruptedException | ExecutionException | TimeoutException
ignored)
+            {
+                // Exception is expected to happen eventually due to the cancel -> get
+            }
+        }
+    }
 
-        if (DatabaseDescriptor.isDaemonInitialized())
+    /**
+     * Allows subclasses to inject new ways of measuring latency back to this abstract base
class.
+     */
+    protected interface ISnitchMeasurement
+    {
+        void sample(long value);
+        double measure();
+        Iterable<Double> measurements();
+    }
+
+    /**
+     * Adds some boookeeping that the DES uses over top of the various metrics techniques
used by the
+     * implementations. This is used to allow CASSANDRA-14459 latency probes as well as further
safe
+     * experimentation on new latency measurement techniques in CASSANDRA-14817
+     *
+     * {@link AnnotatedMeasurement#millisSinceLastRequest} is set to zero through
+     * {@link DynamicEndpointSnitch#sortedByProximity(InetAddressAndPort, ReplicaCollection)}.
It defaults to
+     * the maximum interval so that we only start probing once it has been ranked at least
once
+     * {@link AnnotatedMeasurement#millisSinceLastMeasure} is set to zero from
+     * {@link DynamicEndpointSnitch#receiveTiming(InetAddressAndPort, long, LatencyMeasurementType)}.
+     *
+     * {@link AnnotatedMeasurement#millisSinceLastMeasure and {@link AnnotatedMeasurement#nextProbeDelayMillis
}
+     * are incremented via {@link DynamicEndpointSnitch#updateSamples()}
+     */
+    protected static class AnnotatedMeasurement
+    {
+        // Used to optimally target latency probes only on nodes that are both requested
for ranking
+        // and are not being measured. For example with token aware clients a large portion
of the cluster will never
+        // be ranked at all and therefore we won't probe them.
+        public AtomicLong millisSinceLastRequest = new AtomicLong(MAX_PROBE_INTERVAL_MS);
+        public AtomicLong millisSinceLastMeasure = new AtomicLong(0);
+        public volatile long nextProbeDelayMillis = 0;
+        public volatile ScheduledFuture<?> probeFuture = null;
+
+        // The underlying measurement technique. E.g. a median filter (histogram) or an EMA
filter, or ...
+        public final ISnitchMeasurement measurement;
+        public volatile double cachedMeasurement;
 
 Review comment:
   I believe the only potential concurrent access is from the `toString` method if the logger
is set to Trace ... doesn't seem likely.
   
   Removing the volatile.

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