junrao commented on code in PR #15621:
URL: https://github.com/apache/kafka/pull/15621#discussion_r1548427028
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clients/src/main/java/org/apache/kafka/common/record/MemoryRecordsBuilder.java:
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@@ -240,25 +240,54 @@ public MemoryRecords build() {
return builtRecords;
}
+
/**
- * Get the max timestamp and its offset. The details of the offset
returned are a bit subtle.
- * Note: The semantic for the offset of max timestamp is the first offset
with the max timestamp if there are multi-records having same timestamp.
- *
- * If the log append time is used, the offset will be the first offset of
the record.
- *
- * If create time is used, the offset will always be the offset of the
record with the max timestamp.
- *
- * If it's NO_TIMESTAMP (i.e. MAGIC_VALUE_V0), we'll return offset -1
since no timestamp info in records.
- *
- * @return The max timestamp and its offset
+ * There are three cases of finding max timestamp to return:
+ * 1) version 0: The max timestamp is NO_TIMESTAMP (-1)
+ * 2) LogAppendTime: All records have same timestamp, and so the max
timestamp is equal to logAppendTime
+ * 3) CreateTime: The max timestamp of record
+ * <p>
+ * Let's talk about OffsetOfMaxTimestamp. There are some paths that we
don't try to find the OffsetOfMaxTimestamp
+ * to avoid expensive records iteration. Those paths include follower
append and index recovery. In order to
+ * avoid inconsistent time index, we let all paths find
shallowOffsetOfMaxTimestamp instead of OffsetOfMaxTimestamp.
+ * <p>
+ * Let's define the shallowOffsetOfMaxTimestamp: It is last offset of the
batch having max timestamp. If there are
+ * many batches having same max timestamp, we pick up the earliest batch.
+ * <p>
+ * There are five cases of finding shallowOffsetOfMaxTimestamp to return:
+ * 1) version 0: It is always the -1
+ * 2) LogAppendTime with single batch: It is the offset of last record
+ * 3) LogAppendTime with many single-record batches: Those single-record
batches have same max timestamp, so the
+ * base offset is equal
with the last offset of earliest batch
Review Comment:
so the base offset is equal with the last offset of earliest batch => so we
return the base offset, which is equal to the last offset of earliest batch
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