NIFI-7889 - ConsumeMQTT - use offer instead of add

This closes #4578.

Signed-off-by: Peter Turcsanyi <turcsanyi@apache.org>

nifi-nar-bundles/nifi-mqtt-bundle/nifi-mqtt-processors/src/main/java/org/apache/nifi/processors/mqtt/ConsumeMQTT.java

@@ -19,6 +19,8 @@ package org.apache.nifi.processors.mqtt;
 
 import org.apache.commons.lang3.StringUtils;
 import org.apache.nifi.annotation.behavior.InputRequirement;
+import org.apache.nifi.annotation.behavior.SystemResource;
+import org.apache.nifi.annotation.behavior.SystemResourceConsideration;
 import org.apache.nifi.annotation.behavior.TriggerSerially;
 import org.apache.nifi.annotation.behavior.WritesAttribute;
 import org.apache.nifi.annotation.behavior.WritesAttributes;
@@ -58,6 +60,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import static org.apache.nifi.processors.mqtt.ConsumeMQTT.BROKER_ATTRIBUTE_KEY;
@@ -82,6 +85,9 @@ import static org.apache.nifi.processors.mqtt.common.MqttConstants.ALLOWABLE_VAL
     @WritesAttribute(attribute=IS_DUPLICATE_ATTRIBUTE_KEY, description="Whether or not this message might be a duplicate of one which has already been received."),
     @WritesAttribute(attribute=IS_RETAINED_ATTRIBUTE_KEY, description="Whether or not this message was from a current publisher, or was \"retained\" by the server as the last message published " +
             "on the topic.")})
+@SystemResourceConsideration(resource = SystemResource.MEMORY, description = "The 'Max Queue Size' specifies the maximum number of messages that can be hold in memory by NiFi by a single "
+        + "instance of this processor. A high value for this property could represent a lot of data being stored in memory.")
+
 public class ConsumeMQTT extends AbstractMQTTProcessor  implements MqttCallback {
 
     public final static String BROKER_ATTRIBUTE_KEY =  "mqtt.broker";
@@ -118,15 +124,13 @@ public class ConsumeMQTT extends AbstractMQTTProcessor  implements MqttCallback
 
     public static final PropertyDescriptor PROP_MAX_QUEUE_SIZE = new PropertyDescriptor.Builder()
             .name("Max Queue Size")
-            .description("The MQTT messages are always being sent to subscribers on a topic. If the 'Run Schedule' is significantly behind the rate at which the messages are arriving to this " +
+            .description("The MQTT messages are always being sent to subscribers on a topic regardless of how frequently the processor is scheduled to run. If the 'Run Schedule' is "
-                    "processor then a back up can occur. This property specifies the maximum number of messages this processor will hold in memory at one time.")
+                    + "significantly behind the rate at which the messages are arriving to this processor, then a back up can occur in the internal queue of this processor. This property "
+                    + "specifies the maximum number of messages this processor will hold in memory at one time in the internal queue. This data would be lost in case of a NiFi restart.")
             .required(true)
             .addValidator(StandardValidators.POSITIVE_INTEGER_VALIDATOR)
             .build();
 
-
-    private volatile long maxQueueSize;
-
     private volatile int qos;
     private volatile String topicPrefix = "";
     private volatile String topicFilter;
@@ -217,11 +221,11 @@ public class ConsumeMQTT extends AbstractMQTTProcessor  implements MqttCallback
         return descriptors;
     }
 
+    @Override
     @OnScheduled
     public void onScheduled(final ProcessContext context) {
         super.onScheduled(context);
         qos = context.getProperty(PROP_QOS).asInteger();
-        maxQueueSize = context.getProperty(PROP_MAX_QUEUE_SIZE).asLong();
         topicFilter = context.getProperty(PROP_TOPIC_FILTER).evaluateAttributeExpressions().getValue();
 
         if (context.getProperty(PROP_GROUPID).isSet()) {
@@ -347,10 +351,8 @@ public class ConsumeMQTT extends AbstractMQTTProcessor  implements MqttCallback
             }
         }
 
-        if (mqttQueue.size() >= maxQueueSize){
+        if(!mqttQueue.offer(new MQTTQueueMessage(topic, message), 1, TimeUnit.SECONDS)) {
             throw new IllegalStateException("The subscriber queue is full, cannot receive another message until the processor is scheduled to run.");
-        } else {
-            mqttQueue.add(new MQTTQueueMessage(topic, message));
         }
     }
 

nifi-nar-bundles/nifi-mqtt-bundle/nifi-mqtt-processors/src/main/resources/docs/org.apache.nifi.processors.mqtt.ConsumeMQTT/additionalDetails.html

@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html lang="en">
+<!--
+  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.
+-->
+<head>
+    <meta charset="utf-8" />
+    <title>ConsumeMQTT</title>
+
+    <link rel="stylesheet" href="../../../../../css/component-usage.css" type="text/css" />
+</head>
+<body>
+    <p>The MQTT messages are always being sent to subscribers on a topic regardless of how frequently the processor is scheduled to run. 
+    If the 'Run Schedule' is significantly behind the rate at which the messages are arriving to this processor, then a back up can occur 
+    in the internal queue of this processor. Each time the processor is scheduled, the messages in the internal queue will be written to 
+    FlowFiles. In case the internal queue is full, the MQTT client will try for up to 1 second to add the message into the internal queue. 
+    If the internal queue is still full after this time, an exception saying that 'The subscriber queue is full' would be thrown, the 
+    message would be dropped and the client would be disconnected. In case the QoS property is set to 0, the message would be lost. In 
+    case the QoS property is set to 1 or 2, the message will be received after the client reconnects.</p>
+</body>
+</html>