FrankChen021 commented on code in PR #20255:
URL: https://github.com/apache/druid/pull/20255#discussion_r3940642339
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processing/src/main/java/org/apache/druid/query/cache/CacheKeyBuilder.java:
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@@ -323,7 +327,7 @@ public byte[] build()
buffer.put(id);
for (Item item : items) {
- buffer.put(item.typeKey).put(item.item);
+ buffer.put(item.typeKey).putInt(item.item.length).put(item.item);
Review Comment:
[P1] Version the new cache-key format
This changes the serialized bytes for every CacheKeyBuilder key, but the
output still uses only the caller-supplied id and does not distinguish the new
format from the old one. During a rolling upgrade, shared caches such as
Memcached can retain old entries; the two formats are not disjoint, so a new
key can be an old-format key for a different public builder sequence (for
example, old appendByte(0).appendByte(0).appendByteArray(new byte[]{v})
produces the same bytes as new appendByte(v) for the same id). That can turn a
miss into a false hit and return another query's result. Add a format/version
discriminator or explicitly isolate/invalidate old entries, and add a
mixed-version collision regression test.
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