jackjlli commented on a change in pull request #4338: Within a partition, only
allow querying the first CONSUMING segment for the real-time table routing
URL: https://github.com/apache/incubator-pinot/pull/4338#discussion_r295126848
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File path:
pinot-broker/src/main/java/org/apache/pinot/broker/routing/builder/LowLevelRoutingTableBuilderUtil.java
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@@ -32,59 +32,49 @@
public class LowLevelRoutingTableBuilderUtil {
/**
- * Compute the map of allowed 'consuming' segments for each partition.
+ * Compute the map of allowed CONSUMING segments for each partition for
routing purpose.
+ * <p>Within a partition, we only allow querying the first CONSUMING segment
(segment with instances in CONSUMING
+ * state) for the following reasons:
+ * <ul>
+ * <li>
+ * If within a partition, there are multiple CONSUMING segments
(typically caused by the delay of CONSUMING to
+ * ONLINE state transition), we can only query the first CONSUMING
segment because it might contain records that
+ * overlapped with the records in the next segment (over-consumed).
+ * </li>
+ * <li>
+ * If the instance states for a segment is partial ONLINE and partial
CONSUMING (some instances finished the
+ * CONSUMING to ONLINE state transition, others didn't), we count the
segment as CONSUMING segment (most likely
+ * the first CONSUMING segment because it is already committed and is
performing the CONSUMING to ONLINE state
+ * transition). If we don't count the segment as CONSUMING segment, then
this segment is not allowed to be in the
+ * CONSUMING state for routing purpose, and we will route all queries to
the ONLINE instances which can
+ * potentially overwhelm instance.
+ * </li>
+ * <li>
+ * It is possible that the latest CONSUMING segment is not allowed for
routing purpose and we won't query it, but
+ * it should only last for a short period of time. Once the older
CONSUMING segment becomes ONLINE (all instances
+ * finished the CONSUMING to ONLINE state transition), the latest
CONSUMING segment will become the first
+ * CONSUMING segment and will be allowed for routing purpose.
+ * </li>
+ * </ul>
*
* @param externalView helix external view
Review comment:
external view of realtime table.
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