mumrah commented on a change in pull request #9580:
URL: https://github.com/apache/kafka/pull/9580#discussion_r520851137



##########
File path: core/src/main/scala/kafka/server/ForwardingManager.scala
##########
@@ -0,0 +1,85 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kafka.server
+
+import kafka.metrics.KafkaMetricsGroup
+import kafka.network.RequestChannel
+import org.apache.kafka.clients.ClientResponse
+import org.apache.kafka.common.metrics.Metrics
+import org.apache.kafka.common.protocol.Errors
+import org.apache.kafka.common.requests.{AbstractRequest, AbstractResponse, 
EnvelopeRequest, EnvelopeResponse}
+import org.apache.kafka.common.utils.Time
+
+import scala.compat.java8.OptionConverters._
+
+class ForwardingManager(metadataCache: kafka.server.MetadataCache,
+                        time: Time,
+                        metrics: Metrics,
+                        config: KafkaConfig,
+                        threadNamePrefix: Option[String] = None) extends
+  BrokerToControllerChannelManagerImpl(metadataCache, time, metrics,
+    config, "forwardingChannel", threadNamePrefix) with KafkaMetricsGroup {
+
+  private val forwardingMetricName = "NumRequestsForwardingToControllerPerSec"
+
+  def forwardRequest(request: RequestChannel.Request,
+                     responseCallback: AbstractResponse => Unit): Unit = {
+    val principalSerde = request.context.principalSerde.asScala.getOrElse(

Review comment:
       nit: should we do this get-or-else-throw thing, or do a match/case?

##########
File path: core/src/main/scala/kafka/server/ForwardingManager.scala
##########
@@ -0,0 +1,85 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kafka.server
+
+import kafka.metrics.KafkaMetricsGroup
+import kafka.network.RequestChannel
+import org.apache.kafka.clients.ClientResponse
+import org.apache.kafka.common.metrics.Metrics
+import org.apache.kafka.common.protocol.Errors
+import org.apache.kafka.common.requests.{AbstractRequest, AbstractResponse, 
EnvelopeRequest, EnvelopeResponse}
+import org.apache.kafka.common.utils.Time
+
+import scala.compat.java8.OptionConverters._
+
+class ForwardingManager(metadataCache: kafka.server.MetadataCache,
+                        time: Time,
+                        metrics: Metrics,
+                        config: KafkaConfig,
+                        threadNamePrefix: Option[String] = None) extends
+  BrokerToControllerChannelManagerImpl(metadataCache, time, metrics,
+    config, "forwardingChannel", threadNamePrefix) with KafkaMetricsGroup {
+
+  private val forwardingMetricName = "NumRequestsForwardingToControllerPerSec"
+
+  def forwardRequest(request: RequestChannel.Request,
+                     responseCallback: AbstractResponse => Unit): Unit = {
+    val principalSerde = request.context.principalSerde.asScala.getOrElse(
+      throw new IllegalArgumentException(s"Cannot deserialize principal from 
request $request " +
+        "since there is no serde defined")
+    )
+    val serializedPrincipal = 
principalSerde.serialize(request.context.principal)
+    val forwardRequestBuffer = request.buffer.duplicate()
+    forwardRequestBuffer.flip()
+    val envelopeRequest = new EnvelopeRequest.Builder(
+      forwardRequestBuffer,
+      serializedPrincipal,
+      request.context.clientAddress.getAddress
+    )
+
+    def onClientResponse(clientResponse: ClientResponse): Unit = {
+      val envelopeResponse = 
clientResponse.responseBody.asInstanceOf[EnvelopeResponse]
+      val envelopeError = envelopeResponse.error()
+
+      val response = if (envelopeError != Errors.NONE) {
+        // An envelope error indicates broker misconfiguration (e.g. the 
principal serde
+        // might not be defined on the receiving broker). In this case, we do 
not return
+        // the error directly to the client since it would not be expected. 
Instead we
+        // return `UNKNOWN_SERVER_ERROR` so that the user knows that there is 
a problem
+        // on the broker.
+        debug(s"Forwarded request $request failed with an error in envelope 
response $envelopeError")
+        
request.body[AbstractRequest].getErrorResponse(Errors.UNKNOWN_SERVER_ERROR.exception())
+      } else {
+        AbstractResponse.deserializeBody(envelopeResponse.responseData, 
request.header)
+      }
+      responseCallback(response)
+    }
+
+    sendRequest(envelopeRequest, onClientResponse)
+  }
+
+  override def start(): Unit = {
+    super.start()
+    newGauge(forwardingMetricName, () => requestQueue.size())
+  }
+
+  override def shutdown(): Unit = {
+    super.shutdown()
+    removeMetric(forwardingMetricName)

Review comment:
       nit: I normally reverse the order of stuff in shutdown methods so when 
the super is called anything the child class has done is undone. Probably 
doesn't matter in this case

##########
File path: core/src/main/scala/kafka/server/ForwardingManager.scala
##########
@@ -0,0 +1,85 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kafka.server
+
+import kafka.metrics.KafkaMetricsGroup
+import kafka.network.RequestChannel
+import org.apache.kafka.clients.ClientResponse
+import org.apache.kafka.common.metrics.Metrics
+import org.apache.kafka.common.protocol.Errors
+import org.apache.kafka.common.requests.{AbstractRequest, AbstractResponse, 
EnvelopeRequest, EnvelopeResponse}
+import org.apache.kafka.common.utils.Time
+
+import scala.compat.java8.OptionConverters._
+
+class ForwardingManager(metadataCache: kafka.server.MetadataCache,
+                        time: Time,
+                        metrics: Metrics,
+                        config: KafkaConfig,
+                        threadNamePrefix: Option[String] = None) extends
+  BrokerToControllerChannelManagerImpl(metadataCache, time, metrics,

Review comment:
       Should we extend the BrokerToControllerChannel here? It looks like we 
only ever call `forwardRequest`, so maybe it's better for ForwardingManager 
_not_ to extend BrokerToControllerChannel and instead take it as a dependency. 
Otherwise we are exposing the `sendRequest` method which we don't really want 
anyone to use on the `forwardingManager` instance.

##########
File path: core/src/main/scala/kafka/server/ForwardingManager.scala
##########
@@ -0,0 +1,85 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kafka.server
+
+import kafka.metrics.KafkaMetricsGroup
+import kafka.network.RequestChannel
+import org.apache.kafka.clients.ClientResponse
+import org.apache.kafka.common.metrics.Metrics
+import org.apache.kafka.common.protocol.Errors
+import org.apache.kafka.common.requests.{AbstractRequest, AbstractResponse, 
EnvelopeRequest, EnvelopeResponse}
+import org.apache.kafka.common.utils.Time
+
+import scala.compat.java8.OptionConverters._
+
+class ForwardingManager(metadataCache: kafka.server.MetadataCache,
+                        time: Time,
+                        metrics: Metrics,
+                        config: KafkaConfig,
+                        threadNamePrefix: Option[String] = None) extends
+  BrokerToControllerChannelManagerImpl(metadataCache, time, metrics,
+    config, "forwardingChannel", threadNamePrefix) with KafkaMetricsGroup {
+
+  private val forwardingMetricName = "NumRequestsForwardingToControllerPerSec"

Review comment:
       Wdyt about also adding a "forwarded" boolean to the request log? (in 
`RequestChannel#updateRequestMetrics`)
   
   Would this be useful? cc @cmccabe 




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