Github user ScrapCodes commented on a diff in the pull request:
https://github.com/apache/bahir/pull/13#discussion_r73649853
--- Diff:
sql-streaming-mqtt/src/main/scala/org/apache/bahir/sql/streaming/mqtt/MQTTStreamSource.scala
---
@@ -0,0 +1,191 @@
+/*
+ * 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.bahir.sql.streaming.mqtt
+
+import java.sql.Timestamp
+import java.text.SimpleDateFormat
+import java.util.Calendar
+import java.util.concurrent.CountDownLatch
+
+import scala.collection.concurrent.TrieMap
+import scala.collection.mutable.ArrayBuffer
+import scala.util.{Failure, Success, Try}
+
+import org.apache.bahir.utils.Logging
+import org.eclipse.paho.client.mqttv3._
+import org.eclipse.paho.client.mqttv3.persist.{MemoryPersistence,
MqttDefaultFilePersistence}
+
+import org.apache.spark.sql.{DataFrame, SQLContext}
+import org.apache.spark.sql.execution.streaming.{LongOffset, Offset,
Source}
+import org.apache.spark.sql.sources.{DataSourceRegister,
StreamSourceProvider}
+import org.apache.spark.sql.types.{StringType, StructField, StructType,
TimestampType}
+
+
+object MQTTStreamConstants {
+
+ val DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
+
+ val SCHEMA_DEFAULT = StructType(StructField("value", StringType)
+ :: StructField("timestamp", TimestampType) :: Nil)
+}
+
+class MQTTTextStreamSource(brokerUrl: String, persistence:
MqttClientPersistence,
+ topic: String, clientId: String, messageParser: Array[Byte] =>
(String, Timestamp),
+ sqlContext: SQLContext) extends Source with Logging {
+
+ override def schema: StructType = MQTTStreamConstants.SCHEMA_DEFAULT
+
+ private val store = new LocalMessageStore(persistence,
sqlContext.sparkContext.getConf)
+
+ private val messages = new TrieMap[Int, (String, Timestamp)]
+
+ private val initLock = new CountDownLatch(1)
+
+ private var offset = 0
+
+ private var client: MqttClient = _
+
+ private def fetchLastProcessedOffset(): Int = {
+ Try(store.maxProcessedOffset) match {
+ case Success(x) =>
+ log.info(s"Recovering from last stored offset $x")
+ x
+ case Failure(e) => 0
+ }
+ }
+
+ initialize()
+ private def initialize(): Unit = {
+
+ client = new MqttClient(brokerUrl, clientId, persistence)
+ val mqttConnectOptions: MqttConnectOptions = new MqttConnectOptions()
+ mqttConnectOptions.setAutomaticReconnect(true)
+ // This is required to support recovery. TODO: configurable ?
+ mqttConnectOptions.setCleanSession(false)
+
+ val callback = new MqttCallbackExtended() {
+
+ override def messageArrived(topic_ : String, message: MqttMessage):
Unit = synchronized {
--- End diff --
No, definitely not for all QOS levels. In case of a failure to return from
this method, a message may be redelivered.
In case of a sudden crash, we may anyway lose messages. What do you think
should be our strategy here ?
About message ordering,
According to this
[post](http://stackoverflow.com/questions/30955110/is-message-order-preserved),
message ordering can be ensured at the server end. Should we implement it at
client end ? And do you think message id can be used to ensure ordering ?
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