MetadataClient is a thin wrapper for @apidoc[org.apache.pekko.kafka.KafkaConsumerActor$] hiding the ask calls and mapping to the correct response types.
To access the Kafka consumer metadata you need to create the @apidoc[org.apache.pekko.kafka.KafkaConsumerActor$] as described in the @refConsumer documentation pass it to MetadataClient's factory method create.
Another approach to create metadata client is passing the ConsumerSettings and ActorSystem objects to the factory method. Then the metadata client manages the internal actor and stops it when the close method is called.
The metadata the MetadataClient provides is documented in the @javadocKafka Consumer API API.
The supported metadata are
| Metadata | Response type |
|---|---|
| Topics list | @scala[Future[Map[String, List[PartitionInfo]]]]@java[CompletionStage[java.util.Map[java.lang.String, java.util.List[PartitionInfo]]]] |
| Partitions | @scala[Future[List[PartitionInfo]]]@java[CompletionStage[java.util.List[PartitionInfo]]] |
| Beginning offsets | @scala[Future[Map[TopicPartition, Long]]]@java[CompletionStage[java.util.Map[TopicPartition, java.lang.Long]]] |
| End offsets | @scala[Future[Map[TopicPartition, Long]]]@java[CompletionStage[java.util.Map[TopicPartition, java.lang.Long]]] |
| Committed offsets | @scala[Future[Map[TopicPartition, OffsetAndMetadata]]]@java[CompletionStage[java.util.Map[TopicPartition, OffsetAndMetadata]]] |
@@@ warning
Processing of these requests blocks the actor loop. The @apidoc[org.apache.pekko.kafka.KafkaConsumerActor$] is configured to run on its own dispatcher, so just as the other remote calls to Kafka, the blocking happens within a designated thread pool.
However, calling these during consuming might affect performance and even cause timeouts in extreme cases.
Please consider to use a dedicated @apidoc[org.apache.pekko.kafka.KafkaConsumerActor$] to create metadata client requests against.
@@@
Example:
Scala : @@ snip snip { #metadataClient }
Java : @@ snip snip { #metadataClient }
To access the Kafka consumer metadata you need to create the @apidoc[org.apache.pekko.kafka.KafkaConsumerActor$] as described in the @refConsumer documentation and send messages from @apidoc[Metadata$] to it.
The supported metadata are
| Request | Reply |
|---|---|
| ListTopics | Topics |
| GetPartitionsFor | PartitionsFor |
| GetBeginningOffsets | BeginningOffsets |
| GetEndOffsets | EndOffsets |
| GetOffsetsForTimes | OffsetsForTimes |
| GetCommittedOffsets | CommittedOffsets |
These requests are blocking within the Kafka client library up to a timeout configured by metadata-request-timeout or ConsumerSettings.withMetadataRequestTimeout respectively.
@@@ warning
Accessing the Kafka consumer metadata using the KafkaConsumerActor is not a recommended approach. It is reasonable only when you need to perform a request GetOffsetsForTimes which is not supported by the MetadataClient yet.
@@@
Example:
Scala : @@ snip snip { #metadata }
Java : @@ snip snip { #metadata }