gaodayue commented on a change in pull request #6402: Thread-safe QueryMetrics
URL: https://github.com/apache/incubator-druid/pull/6402#discussion_r221675798
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File path:
processing/src/main/java/org/apache/druid/query/DefaultQueryMetrics.java
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@@ -24,50 +24,89 @@
import com.google.common.collect.ImmutableMap;
import org.apache.druid.collections.bitmap.BitmapFactory;
import org.apache.druid.java.util.common.StringUtils;
+import org.apache.druid.java.util.emitter.EmittingLogger;
import org.apache.druid.java.util.emitter.service.ServiceEmitter;
import org.apache.druid.java.util.emitter.service.ServiceMetricEvent;
import org.apache.druid.query.filter.Filter;
import org.joda.time.Interval;
+import javax.annotation.concurrent.GuardedBy;
+import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;
/**
- * DefaultQueryMetrics is unsafe for use from multiple threads. It fails with
RuntimeException on access not from the
- * thread where it was constructed. To "transfer" DefaultQueryMetrics from one
thread to another {@link #ownerThread}
- * field should be updated.
+ * A basic thread-safe implementation of the {@link QueryMetrics} interface.
+ *
+ * <p><b>Note for inheritence:</b> Subclass should override {@link
#makeCopy()} method to return
+ * an instance of that subclass. See {@link
org.apache.druid.query.groupby.DefaultGroupByQueryMetrics}
+ * for an example.
*/
public class DefaultQueryMetrics<QueryType extends Query<?>> implements
QueryMetrics<QueryType>
{
- protected final ObjectMapper jsonMapper;
- protected final ServiceMetricEvent.Builder builder = new
ServiceMetricEvent.Builder();
- protected final Map<String, Number> metrics = new HashMap<>();
+ private static final EmittingLogger log = new
EmittingLogger(DefaultQueryMetrics.class);
- /** Non final to give subclasses ability to reassign it. */
- protected Thread ownerThread = Thread.currentThread();
+ protected final ObjectMapper jsonMapper;
+ protected final Object lock = new Object();
+ @GuardedBy("lock") private final Map<String, String> singleValueDims = new
HashMap<>();
+ @GuardedBy("lock") private final Map<String, String[]> multiValueDims = new
HashMap<>();
+ @GuardedBy("lock") private final Map<String, Number> metrics = new
HashMap<>();
public DefaultQueryMetrics(ObjectMapper jsonMapper)
{
this.jsonMapper = jsonMapper;
}
- protected void checkModifiedFromOwnerThread()
+ // copy constructor, used by makeCopy()
+ public DefaultQueryMetrics(DefaultQueryMetrics that)
{
- if (Thread.currentThread() != ownerThread) {
- throw new IllegalStateException(
- "DefaultQueryMetrics must not be modified from multiple threads. If
it is needed to gather dimension or "
- + "metric information from multiple threads or from an async thread,
this information should explicitly be "
- + "passed between threads (e. g. using Futures), or this
DefaultQueryMetrics's ownerThread should be "
- + "reassigned explicitly");
- }
+ this.jsonMapper = that.jsonMapper;
+ this.singleValueDims.putAll(that.singleValueDims);
+ this.multiValueDims.putAll(that.multiValueDims);
+ this.metrics.putAll(that.metrics);
}
protected void setDimension(String dimension, String value)
{
- checkModifiedFromOwnerThread();
- builder.setDimension(dimension, value);
+ synchronized (lock) {
Review comment:
I used ConcurrentHashMap at first but switched to this coarse grained lock
because
1. Lock contention should be very low on QueryMetrics object. Normally it is
used by single thread. Concurrent runners would create separate QueryMetrics
for each child runner.
2. Imagine a thread adds a dimension and a metric, in order for the
subsequent emit() reflects the newly added dimension and metric, making each
Map concurrent is not enough. We need synchronization across these fields.
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