Exception throw during Kafka message key deserialization causes fatal consumer error

[danmalcolm]

If an error occurs during deserialization of the Kafka message key, the consumer stops and the following error message is logged:

"Fatal error occurred processing the consumed message. The consumer will be stopped."

When using Schema Registry serializers, such as Avro, there are a couple of potential key deserialization problems, which we'd prefer not to cause a consumer to stop:

• Malformed message key content - Avro deserializer expects key bytes to contain magic byte and schema id
• Schema Registry outage - Deserializer makes a REST API call to SR before caching the schema for the topic key

This is specific to message key deserialization. Currently, keys are deserialized during an earlier consumer step in Silverback.Messaging.Broker.KafkaConsumer, where there is no opportunity to handle the error. Errors encountered during message value deserialization are thrown within the consumer behaviour pipeline and can be handled by the error policy (move bad message to dlq topic, retry if there is a transient Schema Registry connectivity error). Ideally key deserialization errors could be handled in the same way.

I can help with a PR, but will wait to hear if maintainers have any advice on the best approach.

One suggestion:

• Extend RawInboundEnvelope with optional RawKey property
• Extend KafkaConsumer to populate the RawKey property (via Consumer)
• Introduce a new IMessageKeySerializer interface type that extends IMessageSerializer with key serde methods. IKafkaMessageKeySerializer inherits from this. This is just to avoid having Kafka-specific stuff in behaviors, which work with messaging abstractions.
• Extend IConsumerEndpoint with a MessageKey property (MessageKeySettings with HasKey and KeyHeader properties)
• Add a new KeyDeserializerConsumerBehavior to default pipeline. If endpoint's MessageKeySettings indicate that key is applicable, raw key is available and endpoint's serializer implements IMessageKeySerializer, deserialize the key and add the headers
• Configuration to enable new key deserialization behaviour to preserve backwards compatibility

[BEagle1984]

Thank you for bringing this up, your analysis is spot on.

Handling key deserialization in a way that aligns with how value deserialization is already managed (in the pipeline) makes a lot of sense. Using a behavior is probably the right approach here and in hindsight, it’s likely how it should’ve been implemented from the start.

I’d like to take a bit of time to explore how this could be integrated in a clean and flexible way, staying consistent with Silverback’s abstractions.

I’ll follow up soon, once I’ve had a chance to think it through further.

[BEagle1984]

Hi @danmalcolm, thanks again for the detailed analysis, it's much appreciated.

As I started digging into this, I realized that in Silverback v5 we actually removed support for message key serialization altogether. We currently always treat the key as a UTF-8 string, which implicitly avoids issues like malformed Avro headers or schema registry calls failing, since there's no complex deserialization step involved.

That got me wondering: could you share a bit more about your use case? Are you actually relying on structured message keys serialized/deserialized via the schema registry (e.g., Avro), or are you just using primitive types (like strings)? I ask because if it's mostly primitives, the current design may already suffice, but if there’s a need for structured keys, we probably need to rethink this a bit and potentially make it more flexible (supporting both structured and primitive keys and allowing schema-registry-based deserialization when needed).

Thank you in advance for any insights you can share, they’ll help me shape the best path forward.

[danmalcolm]

If you serialize a key with one of the Confluent Kafka Schema Registry serdes, then it will manage a subject in Schema Registry for the topic key and include a magic byte and id of the schema in the serialized data. During serialization, you specify the message component (key or value) on the SerializationContext that is passed to the serializer and this is used to create the name of the subject ("mytopicname-key") that is stored in Schema Registry.

Our solution uses strings as keys throughout, but we opted to use Schema Registry serializer. Our take at the time was that, if you're using Schema Registry serialization to protect the integrity and backwards compatibility of messages in a topic, then it makes sense to gatekeep both keys and values. We were also influenced by the implementation of AvroMessageSerializer<TMessage> in Silverback (v4), which uses Schema Registry Avro serialization for keys.

It would be a reasonable choice to use basic UTF-8 string serialization for keys throughout a solution of course.

For a framework like Silverback, it would be better to offer the same flexibility that we have for value serialization.

[danmalcolm]

Hi. Just wondering if you had any thoughts about implementation? I'd be happy to submit a PR for a change (to v4) as a starting point.

[BEagle1984]

I'm working on it for v5 as it requires some refactoring to do it properly. To have an elegant way to move the key (de)serialization down the behaviors pipeline. I promise that I'll post a draft PR in the upcoming weeks to give an idea.