Simple C# Kafka Producer with Schema Regitry Implementation throws Invalid Receive Size

I'm trying to learn Kafka using C# on my local machine. I had a producer/consumer/stream that work with strings and am now trying to allow for complex types using schema registry.

I'm attempting to register a class using the following console app:

    static async Task Main(string[] args)
    {
        var schemaRegistryConfig = new SchemaRegistryConfig
        {
            Url = "http://localhost:29092"
        };

        var schemaRegistry = new CachedSchemaRegistryClient(schemaRegistryConfig);

        var schema = @"{
            ""type"": ""TotallyCoolCustomClass"",
            ""properties"": {
                ""FavouriteQuote"": {""type"": ""string""},
                ""FavouriteNumber"": {""type"": ""integer""}
            }
        }";

        var subject = "SimpleTest";
        var schemaId = await schemaRegistry.RegisterSchemaAsync(subject, schema);
    }

Whenever I run my schema registry app, I get an Http exception:

System.Net.Http.HttpRequestException: '[http://localhost:29092/] HttpRequestException: An error occurred while sending the request.'

and docker reports this under my kafka instance:

    WARN [SocketServer listenerType=ZK_BROKER, nodeId=1] Unexpected error from /172.22.0.1 (channelId=172.22.0.3:29092-172.22.0.1:34802-28); closing connection (org.apache.kafka.common.network.Selector)
org.apache.kafka.common.network.InvalidReceiveException: Invalid receive (size = 1347375956 larger than 104857600)
        at org.apache.kafka.common.network.NetworkReceive.readFrom(NetworkReceive.java:94)

My message is obviously small, and not breaking the default 100Mb - so I'm not sure why it thinks it is. Can anyone tell me why I'm unable to register a schema?

I've set up Kafka, Zookeeper and a Schema Registry using this:

   version: '2'
services:
  zookeeper:
    image: confluentinc/cp-zookeeper:latest
    environment:
      ZOOKEEPER_CLIENT_PORT: 2181
      ZOOKEEPER_TICK_TIME: 2000
    ports:
      - 22181:2181
  
  kafka:
    image: confluentinc/cp-kafka:latest
    depends_on:
      - zookeeper
    ports:
      - 29092:29092
    environment:
      KAFKA_BROKER_ID: 1
      KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181
      KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:9092,PLAINTEXT_HOST://localhost:29092
      KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT
      KAFKA_INTER_BROKER_LISTENER_NAME: PLAINTEXT
      KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1

  schema-registry:
    image: confluentinc/cp-schema-registry:latest
    depends_on: 
      - kafka
    ports:
      - 28081:28081
    environment:
      SCHEMA_REGISTRY_KAFKASTORE_CONNECTION_URL: 'zookeeper:2181'
      SCHEMA_REGISTRY_HOST_NAME: schema-registry
      SCHEMA_REGISTRY_KAFKASTORE_BOOTSTRAP_SERVERS: 'PLAINTEXT://kafka:9092'
      # SCHEMA_REGISTRY_LISTENERS: http://0.0.0.0:28081

And produce to it with this (though I think this is largely redundant for the question, as the schema isn't registered so the publish will never work):

public class KafkaPProducerHostedService : IHostedService
        {
            private readonly ILogger<KafkaPProducerHostedService> _logger;
            private readonly IProducer<string, TotallyCoolCustomClass> _producer;
            public KafkaPProducerHostedService(ILogger<KafkaPProducerHostedService> logger)
            {
                _logger = logger;
                var config = new ProducerConfig();
                config.BootstrapServers = "localhost:29092";

                var schemaRegistryConfig = new SchemaRegistryConfig
                {
                    Url = "http://localhost:28081/"
                };

                //video on serializers
                _producer = new ProducerBuilder<string, TotallyCoolCustomClass>(config)
                    .SetValueSerializer(new JsonSerializer<TotallyCoolCustomClass>(new CachedSchemaRegistryClient(schemaRegistryConfig)))
                    .Build();
            }

            public async Task StartAsync(CancellationToken cancellationToken)
            {
                int i = 0;

                var rand = new Random();

                while (true) {
                    Thread.Sleep(2000);
                    var customClass = new TotallyCoolCustomClass(rand);
                    var key = $"UpdatedKey-{i}";
                    await _producer.ProduceAsync("SimpleTest", new Message<string, TotallyCoolCustomClass>()
                    {
                        Key = key,
                        Value = customClass
                    },                    
                    cancellationToken);

                    Console.WriteLine($"Published: Key: {key} with favourite quote {customClass.FavouriteQuote} and favourite number {customClass.FavouriteNumber}");
                    i++;
                }
            }

            public Task StopAsync(CancellationToken cancellationToken)
            {
                _producer?.Dispose();
                return Task.CompletedTask;
            }
        }

You've configured your schema registry client to send data at the Kafka broker, which is not an HTTP server

Use port 28081

However you've commented SCHEMA_REGISTRY_LISTENERS, therefore the port mapping is incorrect, and should be the default 8081

> schema isn't registered so the publish will never work

Producers automatically register schemas. There's no need to do that manually unless you really need the schema ID somewhere