chia7712 commented on code in PR #15836:
URL: https://github.com/apache/kafka/pull/15836#discussion_r1590078150


##########
core/src/main/scala/kafka/server/FetchSession.scala:
##########
@@ -690,6 +702,10 @@ class FetchSessionCache(private val maxEntries: Int,
     * 2. B is considered "stale" because it has been inactive for a long time, 
or
     * 3. A contains more partitions than B, and B is not recently created.
     *
+    * Prior to KAFKA-9401, the session cache was not sharded and we looked at 
all

Review Comment:
   This docs is great. Could you please update this also?
   
   
https://github.com/apache/kafka/blob/9b8aac22ec7ce927a2ceb2bfe7afd57419ee946c/core/src/main/scala/kafka/server/KafkaConfig.scala#L182



##########
core/src/main/scala/kafka/server/FetchSession.scala:
##########
@@ -787,9 +803,37 @@ class FetchSessionCache(private val maxEntries: Int,
     }
   }
 }
+object FetchSessionCache {
+  private[server] val metricsGroup = new 
KafkaMetricsGroup(classOf[FetchSessionCache])
+  private val counter = new AtomicLong(0)
+}
+
+class FetchSessionCache(private val cacheShards: Seq[FetchSessionCacheShard]) {
+  // Set up metrics.
+  
FetchSessionCache.metricsGroup.newGauge(FetchSession.NUM_INCREMENTAL_FETCH_SESSIONS,
 () => cacheShards.map(_.size).sum)
+  
FetchSessionCache.metricsGroup.newGauge(FetchSession.NUM_INCREMENTAL_FETCH_PARTITIONS_CACHED,
 () => cacheShards.map(_.totalPartitions).sum)
+
+  def getCacheShard(sessionId: Int): FetchSessionCacheShard = {
+    val shard = sessionId / cacheShards.head.sessionIdRange

Review Comment:
   It assumes the `cacheShards` is sorted by the `shardNum`, right? If so, 
could you please add comments for it?



##########
core/src/main/scala/kafka/server/FetchSession.scala:
##########
@@ -787,9 +803,37 @@ class FetchSessionCache(private val maxEntries: Int,
     }
   }
 }
+object FetchSessionCache {
+  private[server] val metricsGroup = new 
KafkaMetricsGroup(classOf[FetchSessionCache])
+  private val counter = new AtomicLong(0)
+}
+
+class FetchSessionCache(private val cacheShards: Seq[FetchSessionCacheShard]) {
+  // Set up metrics.
+  
FetchSessionCache.metricsGroup.newGauge(FetchSession.NUM_INCREMENTAL_FETCH_SESSIONS,
 () => cacheShards.map(_.size).sum)
+  
FetchSessionCache.metricsGroup.newGauge(FetchSession.NUM_INCREMENTAL_FETCH_PARTITIONS_CACHED,
 () => cacheShards.map(_.totalPartitions).sum)
+
+  def getCacheShard(sessionId: Int): FetchSessionCacheShard = {
+    val shard = sessionId / cacheShards.head.sessionIdRange
+    cacheShards(shard)
+  }
+
+  // Returns the shard in round-robin
+  def getNextCacheShard: FetchSessionCacheShard = {
+    val shardNum = (FetchSessionCache.counter.getAndIncrement() % size).toInt

Review Comment:
   As `int` is enough to this case, maybe we can use `AtomicInteger`?



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