Table of Contents

Setting Headers

Both Kafka and MQTT 5 support custom headers (called user properties in MQTT). With Silverback there are many ways to set headers.

Using WrapAndPublish / WrapAndPublishBatch

The WrapAndPublish, WrapAndPublishAsync, WrapAndPublishBatch and WrapAndPublishBatchAsync methods of the IPublisher can be used to wrap the messages in an envelope and add additional metadata, such as headers.

await _publisher.WrapAndPublishAsync(
    new MyMessage { ... },
    envelope => envelope
        .AddHeader("x-source", "my-app")
        .AddHeader("x-priority", envelope.Message.Priority));

Enrichers in the Endpoint Configuration

Using an enricher configured for the endpoint you can add static headers to all produced messages.

services.AddSilverback()
    .WithConnectionToMessageBroker(options => options.AddKafka())
    .AddKafkaClients(clients => clients
        .WithBootstrapServers("PLAINTEXT://localhost:9092")
        .AddProducer("producer1", producer => producer
            .Produce<MyMessage>("endpoint1", endpoint => endpoint
                .ProduceTo("my-topic")
                .AddHeader("header1", "value1")
                .AddHeader("header2", "value2"))));

Or you can add headers whose values are computed from the message (or the envelope).

services.AddSilverback()
    .WithConnectionToMessageBroker(options => options.AddKafka())
    .AddKafkaClients(clients => clients
        .WithBootstrapServers("PLAINTEXT://localhost:9092")
        .AddProducer("producer1", producer => producer
            .Produce<MyMessage>("endpoint1", endpoint => endpoint
                .ProduceTo("my-topic")
                .AddHeader("header1", message => message.Property1)
                .AddHeader("header2", envelope => envelope.Message.Property2))));

HeaderAttribute on the Message Model

Using the HeaderAttribute is straightforward: simply decorate the properties you want to publish as headers and specify a header name.

using Silverback.Messaging.Messages;

namespace Sample
{
    public class OrderCreatedEvent
    {
        public List<LineItems> Items { get; set; }

        [Header("x-order-type", PublishDefaultValue = true)]
        [JsonIgnore]
        public OrderType OrderType { get; set; }

        [Header("x-books-order")]
        public bool ContainsBooks => Items.Any(item => item.Type == "book");

        [Header("x-dvd-order")]
        public bool ContainsDvd => Items.Any(item => item.Type == "dvd");
    }
}
Note

The PublishDefaultValue boolean property defines whether the header has to be published even if the property is set to the default value for its data type. The default is false.

Note

The JsonIgnoreAttribute can be used to prevent the same properties to be serialized in the JSON body, when using the JsonMessageSerializer.

Important

Only the message type will be scanned, therefore the properties decorated with the HeaderAttribute must be in the root of the message object.

Additional Resources

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