Table of Contents

KNet: library usage

To use KNet classes the developer can write code in .NET using the same classes available in the official Apache Kafka package. If classes or methods are not available yet it is possible to use the approach synthetized in What to do if an API was not yet implemented

Environment setup

KNet accepts many command-line switches to customize its behavior. The full list is available at Command line switch page.

JVM identification

One of the most important command-line switch is JVMPath and it is available in JCOBridge switches: it can be used to set-up the location of the JVM library if JCOBridge is not able to identify a suitable JRE/JDK installation. If a developer is using KNet within its own product it is possible to override the JVMPath property with a snippet like the following one:

    class MyKNetCore : KNetCore
    {
        public override string JVMPath
        {
            get
            {
                string pathToJVM = "Set here the path to JVM library or use your own search method";
                return pathToJVM;
            }
        }
    }

IMPORTANT NOTE: pathToJVM shall be escaped

  1. string pathToJVM = "C:\\Program Files\\Eclipse Adoptium\\jdk-11.0.18.10-hotspot\\bin\\server\\jvm.dll";
  2. string pathToJVM = @"C:\Program Files\Eclipse Adoptium\jdk-11.0.18.10-hotspot\bin\server\jvm.dll";

Special initialization conditions

JCOBridge try to identify a suitable JRE/JDK installation within the system using some standard mechanism of JRE/JDK: JAVA_HOME environment variable or Windows registry if available. However it is possible, on Windows operating systems, that the library raises an InvalidOperationException: Missing Java Key in registry: Couldn't find Java installed on the machine. This means that neither JAVA_HOME nor Windows registry contains information about a default installed JRE/JDK: some vendors may not setup them. If the developer/user encounter this condition can do the following steps:

  1. On a command prompt execute set | findstr JAVA_HOME and verify the result;
  2. If something was reported maybe the JAVA_HOME environment variable is not set at system level, but at a different level like user level which is not visible from the KNet process that raised the exception;
  3. Try to set JAVA_HOME at system level e.g. JAVA_HOME=C:\Program Files\Eclipse Adoptium\jdk-11.0.18.10-hotspot\;
  4. Try to set JCOBRIDGE_JVMPath at system level e.g. JCOBRIDGE_JVMPath=C:\Program Files\Eclipse Adoptium\jdk-11.0.18.10-hotspot\.

IMPORTANT NOTES:

  • One of JCOBRIDGE_JVMPath or JAVA_HOME environment variables or Windows registry (on Windows OSes) shall be available
  • JCOBRIDGE_JVMPath environment variable takes precedence over JAVA_HOME and Windows registry: you can set JCOBRIDGE_JVMPath to C:\Program Files\Eclipse Adoptium\jdk-11.0.18.10-hotspot\bin\server\jvm.dll and avoid to override JVMPath in your code
  • After first initialization steps, JVMPath takes precedence over JCOBRIDGE_JVMPath/JAVA_HOME environment variables or Windows registry

Producer example

Below the reader can found two different version of producer examples.

Simple producer

A basic producer can be like the following one:

using MASES.KNet;
using Org.Apache.Kafka.Clients.Producer;
using Java.Util;
using System;
using System.Threading;

namespace MASES.KNetTemplate.KNetProducer
{
    class Program
    {
        const string theServer = "localhost:9092";
        const string theTopic = "myTopic";

        static string serverToUse = theServer;
        static string topicToUse = theTopic;

        static readonly ManualResetEvent resetEvent = new ManualResetEvent(false);

        static void Main(string[] args)
        {
            KNetCore.CreateGlobalInstance();
            var appArgs = KNetCore.FilteredArgs;

            if (appArgs.Length != 0)
            {
                serverToUse = args[0];
            }

            /**** Direct mode ******
            Properties props = new Properties();
            props.Put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, serverToUse);
            props.Put(ProducerConfig.ACKS_CONFIG, "all");
            props.Put(ProducerConfig.RETRIES_CONFIG, 0);
            props.Put(ProducerConfig.LINGER_MS_CONFIG, 1);
            props.Put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
            props.Put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
            ******/

            Properties props = ProducerConfigBuilder.Create()
                                                    .WithBootstrapServers(serverToUse)
                                                    .WithAcks(ProducerConfig.Acks.All)
                                                    .WithRetries(0)
                                                    .WithLingerMs(1)
                                                    .WithKeySerializerClass("org.apache.kafka.common.serialization.StringSerializer")
                                                    .WithValueSerializerClass("org.apache.kafka.common.serialization.StringSerializer")
                                                    .ToProperties();

            Console.CancelKeyPress += Console_CancelKeyPress;
            Console.WriteLine("Press Ctrl-C to exit");

			using (KafkaProducer producer = new KafkaProducer(props))
			{
				int i = 0;
				while (!resetEvent.WaitOne(0))
				{
					var record = new ProducerRecord<string, string>(topicToUse, i.ToString(), i.ToString());
					var result = producer.Send(record);
					Console.WriteLine($"Producing: {record} with result: {result.Get()}");
					producer.Flush();
					i++;
				}
			}
        }

        private static void Console_CancelKeyPress(object sender, ConsoleCancelEventArgs e)
        {
            if (e.Cancel) resetEvent.Set();
        }
    }
}

The example above can be found in the templates package. Its behavior is:

  • during initialization prepares the properties,
  • create a producer using the properties
  • create ProducerRecord and send it
  • print out the produced data and the resulting RecordMetadata

Producer with Callback

A producer with Callback can be like the following one. In this example the reader can highlight a slightly difference from the corresponding Java code. Surf JVM callbacks to go into detail in the callback management from JVM.

using MASES.KNet;
using Org.Apache.Kafka.Clients.Producer;
using Java.Util;
using System;
using System.Threading;

namespace MASES.KNetTemplate.KNetProducer
{
    class Program
    {
        const string theServer = "localhost:9092";
        const string theTopic = "myTopic";

        static string serverToUse = theServer;
        static string topicToUse = theTopic;

        static readonly ManualResetEvent resetEvent = new ManualResetEvent(false);

        static void Main(string[] args)
        {
            KNetCore.CreateGlobalInstance();
            var appArgs = KNetCore.FilteredArgs;

            if (appArgs.Length != 0)
            {
                serverToUse = args[0];
            }

            /**** Direct mode ******
            Properties props = new Properties();
            props.Put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, serverToUse);
            props.Put(ProducerConfig.ACKS_CONFIG, "all");
            props.Put(ProducerConfig.RETRIES_CONFIG, 0);
            props.Put(ProducerConfig.LINGER_MS_CONFIG, 1);
            props.Put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
            props.Put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
            ******/

            Properties props = ProducerConfigBuilder.Create()
                                                    .WithBootstrapServers(serverToUse)
                                                    .WithAcks(ProducerConfig.Acks.All)
                                                    .WithRetries(0)
                                                    .WithLingerMs(1)
                                                    .WithKeySerializerClass("org.apache.kafka.common.serialization.StringSerializer")
                                                    .WithValueSerializerClass("org.apache.kafka.common.serialization.StringSerializer")
                                                    .ToProperties();

            Console.CancelKeyPress += Console_CancelKeyPress;
            Console.WriteLine("Press Ctrl-C to exit");

			using (KafkaProducer producer = new KafkaProducer(props))
			{
				int i = 0;
				using (var callback = new Callback((o1, o2) =>
				{
					if (o2 != null) Console.WriteLine(o2.ToString());
					else Console.WriteLine($"Produced on topic {o1.Topic} at offset {o1.Offset}");
				}))
				{
					while (!resetEvent.WaitOne(0))
					{
						var record = new ProducerRecord<string, string>(topicToUse, i.ToString(), i.ToString());
						var result = producer.Send(record, callback);
						Console.WriteLine($"Producing: {record} with result: {result.Get()}");
						producer.Flush();
						i++;
					}
				}
			}
        }

        private static void Console_CancelKeyPress(object sender, ConsoleCancelEventArgs e)
        {
            if (e.Cancel) resetEvent.Set();
        }
    }
}

The example above can be found in the templates package. Its behavior is:

  • during initialization prepares the properties
  • create a producer using the properties
  • create ProducerRecord and send it using the API Send with the attached Callback
  • when the operation completed the Callback is called:
    • if an Exception was raised it will be printed out
    • otherwise the RecordMetadata is printed out
  • print out the produced data and the resulting RecordMetadata

Consumer example

A basic consumer can be like the following one:

using MASES.KNet;
using Org.Apache.Kafka.Clients.Consumer;
using Java.Util;
using System;

namespace MASES.KNetTemplate.KNetConsumer
{
    class Program
    {
        const string theServer = "localhost:9092";
        const string theTopic = "myTopic";

        static string serverToUse = theServer;
        static string topicToUse = theTopic;

        static readonly ManualResetEvent resetEvent = new ManualResetEvent(false);

        static void Main(string[] args)
        {
            KNetCore.CreateGlobalInstance();
            var appArgs = KNetCore.FilteredArgs;

            if (appArgs.Length != 0)
            {
                serverToUse = args[0];
            }

            /**** Direct mode ******
            Properties props = new Properties();
            props.Put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, serverToUse);
            props.Put(ConsumerConfig.GROUP_ID_CONFIG, "test");
            props.Put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "true");
            props.Put(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG, "1000");
            props.Put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringDeserializer");
            props.Put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringDeserializer");
            *******/

            Properties props = ConsumerConfigBuilder.Create()
                                                    .WithBootstrapServers(serverToUse)
                                                    .WithGroupId("test")
                                                    .WithEnableAutoCommit(true)
                                                    .WithAutoCommitIntervalMs(1000)
                                                    .WithKeyDeserializerClass("org.apache.kafka.common.serialization.StringDeserializer")
                                                    .WithValueDeserializerClass("org.apache.kafka.common.serialization.StringDeserializer")
                                                    .ToProperties();

            Console.CancelKeyPress += Console_CancelKeyPress;
            Console.WriteLine("Press Ctrl-C to exit");

            using (var consumer = new KafkaConsumer<string, string>(props))
            {
                var topics = Collections.Singleton(topicToUse);
                consumer.Subscribe(topics);
                while (!resetEvent.WaitOne(0))
                {
                    var records = consumer.Poll((long)TimeSpan.FromMilliseconds(200).TotalMilliseconds);
                    foreach (var item in records)
                    {
                        Console.WriteLine($"Offset = {item.Offset}, Key = {item.Key}, Value = {item.Value}");
                    }
                }
                topics?.Dispose(); // needed to avoid Java.Lang.NullPointerException in some conditions where .NET GC retires topics too early
            }
        }

        private static void Console_CancelKeyPress(object sender, ConsoleCancelEventArgs e)
        {
            if (e.Cancel) resetEvent.Set();
        }
    }
}

The example above can be found in the templates package. Its behavior is:

  • during initialization prepares the properties,
  • create a consumer using the properties
  • subscribe and starts consume
  • when data are received it logs to the console the information.
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