[GitHub] [spark] WweiL commented on a diff in pull request #40937: [SPARK-42940][SS][CONNECT] Improve session management for streaming queries

["WweiL (via GitHub)" <gi...@apache.org>]

WweiL commented on code in PR #40937:
URL: https://github.com/apache/spark/pull/40937#discussion_r1178400574


##########
connector/connect/server/src/main/scala/org/apache/spark/sql/connect/service/SparkConnectStreamingQueryCache.scala:
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@@ -0,0 +1,203 @@
+/*
+ * 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.sql.connect.service
+
+import java.util.concurrent.Executors
+import java.util.concurrent.ScheduledExecutorService
+import java.util.concurrent.TimeUnit
+import javax.annotation.concurrent.GuardedBy
+
+import scala.collection.mutable
+import scala.concurrent.duration.Duration
+import scala.concurrent.duration.DurationInt
+import scala.util.control.NonFatal
+
+import org.apache.spark.internal.Logging
+import org.apache.spark.sql.SparkSession
+import org.apache.spark.sql.streaming.StreamingQuery
+import org.apache.spark.util.Clock
+import org.apache.spark.util.SystemClock
+
+/**
+ * Caches Spark-Connect streaming query references and the sessions. When a query is stopped (i.e.
+ * no longer active), it is cached for 1 hour so that it is accessible from the client side. It
+ * runs a background thread to run a periodic task that does the following:
+ *   - Check the status of the queries, and drops those that expired (1 hour after being stopped).
+ *   - Keep the associated session active by invoking supplied function `sessionKeepAliveFn`.
+ *
+ * This class helps with supporting following semantics for streaming query sessions:
+ *   - Keep the session and session mapping at connect server alive as long as a streaming query
+ *     is active. Even if the client side has disconnected.
+ *     - This matches how streaming queries behave in Spark. The queries continue to run if
+ *       notebook or job session is lost.
+ *   - Once a query is stopped, the reference and mappings are maintained for 1 hour and will be
+ *     accessible from the client. This allows time for client to fetch status.
+ *     - During this time if the query is restarted (i.e. has a new run id), the reference to
+ *       previous run is dropped. As a result logical query has only the most recent query
+ *       reference cached. This policy can be revisited to cache multiple runs for a query.
+ *   - Note that these semantics are evolving and might change before being finalized in Connect.
+ *   - Future improvements:
+ *     - Provide an API for a users to access session even after they lose session id.
+ *       - Once a user is properly authenticated, the API could return list of sessions that are
+ *         still alive in the connect server for that user.
+ */
+private[connect] class SparkConnectStreamingQueryCache(
+    val sessionKeepAliveFn: (String, String) => Unit, // (userId, sessionId) => Unit.
+    val clock: Clock = new SystemClock(),
+    private val stoppedQueryCachePeriod: Duration = 1.hour, // Configurable for testing.
+    private val sessionPollingPeriod: Duration = 1.minute // Configurable for testing.
+) extends Logging {
+
+  import SparkConnectStreamingQueryCache._
+
+  def registerNewStreamingQuery(sessionHolder: SessionHolder, query: StreamingQuery): Unit = {
+    queryCacheLock.synchronized {
+      val value = QueryCacheValue(
+        userId = sessionHolder.userId,
+        sessionId = sessionHolder.sessionId,
+        session = sessionHolder.session,
+        query = query,
+        expiresAtMs = None)
+
+      queryCache.put(QueryCacheKey(query.id.toString), value) match {
+        case Some(existing) => // Query is being replaced. Can happen when a query is restarted.
+          log.info(
+            s"Replacing existing query in the cache. Query Id: ${query.id}." +
+              s"Existing value $existing, new value $value.")
+        case None =>
+          log.info(s"Adding new query to the cache. Query Id ${query.id}, value $value.")
+      }
+
+      schedulePeriodicChecks() // Starts the scheduler thread if it hasn't started.

Review Comment:
   Oh I thought doing a lock() inside a lock() would block...? That's not the case in scala...?



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