Github user pwendell commented on a diff in the pull request:
https://github.com/apache/spark/pull/42#discussion_r10238954
--- Diff: core/src/main/scala/org/apache/spark/ui/UISparkListener.scala ---
@@ -0,0 +1,214 @@
+/*
+ * 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 org.apache.spark.ui
+
+import scala.collection.mutable
+import scala.collection.mutable.ArrayBuffer
+
+import org.json4s.jackson.JsonMethods._
+
+import org.apache.spark.scheduler._
+import org.apache.spark.storage._
+import org.apache.spark.util.FileLogger
+import org.apache.spark.util.JsonProtocol
+import org.apache.spark.SparkContext
+
+private[ui] trait UISparkListener extends SparkListener
+
+/**
+ * A SparkListener that serves as an entry point for all events posted to
the UI.
+ *
+ * GatewayUISparkListener achieves two functions:
+ *
+ * (1) If the UI is live, GatewayUISparkListener posts each event to all
attached listeners
+ * then logs it as JSON. This centralizes event logging and avoids
having all attached
+ * listeners log the events on their own.
+ *
+ * (2) If the UI is rendered from disk, GatewayUISparkListener replays
each event deserialized
+ * from the event logs to all attached listeners.
+ */
+private[ui] class GatewayUISparkListener(parent: SparkUI, sc:
SparkContext) extends SparkListener {
--- End diff --
This class doesn't seem particularly specific to the UI. It might be a good
idea to expose this to the rest of Spark as a general utility that could be
used potentially by other higher level applications that hook into the Listener
interface (e.g. not just Spark's own UI). Like basically build a listener class
that can either get data from a live context or read it from a file, if the
file is present. Then you could have a class that extends that and adds the
more UI specific stuff, like dispatching to all these internal listeners.
The doc here also implies that the GatewayUISparkListener is responsible
for replaying events, but as far as I can tell that happens in another location.
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