splett2 commented on a change in pull request #9386:
URL: https://github.com/apache/kafka/pull/9386#discussion_r521856417



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File path: core/src/main/scala/kafka/network/SocketServer.scala
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@@ -1324,7 +1401,59 @@ class ConnectionQuotas(config: KafkaConfig, time: Time, 
metrics: Metrics) extend
   private[network] def updateBrokerMaxConnectionRate(maxConnectionRate: Int): 
Unit = {
     // if there is a connection waiting on the rate throttle delay, we will 
let it wait the original delay even if
     // the rate limit increases, because it is just one connection per 
listener and the code is simpler that way
-    updateConnectionRateQuota(maxConnectionRate)
+    updateConnectionRateQuota(maxConnectionRate, BrokerQuotaEntity)
+  }
+
+  /**
+   * Update the connection rate quota for a given IP and updates quota configs 
for updated IPs.
+   * If an IP is given, metric config will be updated only for the given IP, 
otherwise
+   * all metric configs will be checked and updated if required.
+   *
+   * @param ip ip to update or default if None
+   * @param maxConnectionRate new connection rate, or resets entity to default 
if None
+   */
+  def updateIpConnectionRateQuota(ip: Option[String], maxConnectionRate: 
Option[Int]): Unit = {
+    def isIpConnectionRateMetric(metricName: MetricName) = {
+      metricName.name == ConnectionRateMetricName &&
+      metricName.group == MetricsGroup &&
+      metricName.tags.containsKey(IpMetricTag)
+    }
+
+    def shouldUpdateQuota(metric: KafkaMetric, quotaLimit: Int) = {
+      quotaLimit != metric.config.quota.bound
+    }
+    counts.synchronized {

Review comment:
       I'm thinking on this a bit more, and I think that only locking on 
updates to `defaultConnectionRatePerIp` should be sufficient for correctness.
   
   ZK dynamic config changes are processed within one thread, so we will only 
have one thread executing in `updateIpConnectionRateQuota`. If we want to be 
really careful about this, we can have `IpConfigHandler` synchronize on 
`ConnectionQuotas`.
   
   The case we want to avoid by synchronizing on `counts` is that another 
thread `defaultConnectionRatePerIp` as connection rate limit `A` while calling 
`inc()`, then we update connection rate and quota metrics to `B`, then the 
other thread creates a sensor/metric with quota limit `A` => inconsistency.
   
   If we synchronize on `counts` for only updates to 
`connectionRateForIp/defaultConnectionRate`, we know that any thread that has 
read a connection rate quota as `A` will finish creating quota metrics with 
limit `A` before we acquire the `counts` lock.
   
   After acquiring the `counts` lock and updating 
`connectionRateForIp/defaultConnectionRate` to `B`, any subsequent threads 
calling `inc()` will read the quota as `B` and create a metric as `B`. We will 
then be able to update any quota metrics from `A` to `B`, knowing that all new 
ones will be created as `B`.
   
   The only issue remaining is that we can get concurrent reads of 
`connectionRatePerIp`, but we can just replace `mutable.Map` with 
`ConcurrentHashMap`.
   
   Let me know if I'm missing something here with respect to thread safety.




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