leventov commented on a change in pull request #6402: Thread-safe QueryMetrics
URL: https://github.com/apache/incubator-druid/pull/6402#discussion_r229344228
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File path: processing/src/main/java/org/apache/druid/query/QueryMetrics.java
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@@ -92,8 +94,24 @@
* 100% guarantee of compatibility, because methods could not only be added to
QueryMetrics, existing methods could also
* be changed or removed.
*
- * <p>QueryMetrics is designed for use from a single thread, implementations
shouldn't care about thread-safety.
+ * Thread safety consideration
+ * ---------------------------
+ * <p><b>All implementations of QueryMetrics must be Thread-safe.</b> This is
to facilitate some use cases (like JDBC)
+ * that needs to use QueryMetrics from multiple threads.
*
+ * <p>However, allowing multiple threads to use QueryMetrics can lead to bugs
that could be difficult to notice. e.g.,
+ * Segment processing threads could erroneously share one QueryMetrics,
overriding metrics of each other.
+ *
+ * <p>In order to catch such bugs ASAP, <b>all implementations should add the
following checks.</b> (see
+ * {@link DefaultQueryMetrics} for an example)
+ *
+ * 1. For all methods that add dimension: if the value of one dimension is
updated to another value, throws
+ * IllegalStateException.
+ * 2. For all methods that add metrics: if one metric is regitered more than
once, throws IllegalStateException
Review comment:
"registered"
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