WSO2 Kafka Inbound Endpoint Retry on Error - apache-kafka

Question

We try to use Kafka with WSO2 ESB server.
We've implemented and API which puts incoming messages to Kafka.
Then we've implemented an inbound endpoint that retrieves messages from Kafka and transfer these messages to other external sytems.
Everything works very well in happy path but when we test the "external systems down" scenario failed messages are not delivered when external systems up again.
How can we send failed messages to external systems?
API config:
<?xml version="1.0" encoding="UTF-8"?>
<api context="/api/event" name="EventAPI" xmlns="http://ws.apache.org/ns/synapse">
<resource methods="POST">
<inSequence>
<log category="DEBUG" description="" level="full"/>
<kafkaTransport.init>
<bootstrapServers>localhost:9092</bootstrapServers>
<keySerializerClass>org.apache.kafka.common.serialization.StringSerializer</keySerializerClass>
<valueSerializerClass>org.apache.kafka.common.serialization.StringSerializer</valueSerializerClass>
<maxPoolSize>20</maxPoolSize>
</kafkaTransport.init>
<kafkaTransport.publishMessages>
<topic>event_topic</topic>
</kafkaTransport.publishMessages>
<loopback/>
</inSequence>
<outSequence>
<payloadFactory media-type="json">
<format>{"result" : "OK"}</format>
</payloadFactory>
<property name="messageType" scope="axis2" type="STRING" value="application/json"/>
<send/>
</outSequence>
</resource>
</api>
Inbound config:
<?xml version="1.0" encoding="UTF-8"?>
<inboundEndpoint name="EventTransmitter" protocol="kafka"
sequence="transmit_sequence" suspend="false" onError="fault"
xmlns="http://ws.apache.org/ns/synapse">
<parameters>
<parameter name="interval">10</parameter>
<parameter name="coordination">true</parameter>
<parameter name="sequential">true</parameter>
<parameter name="zookeeper.connect">localhost:2181</parameter>
<parameter name="consumer.type">highlevel</parameter>
<parameter name="content.type">application/json</parameter>
<parameter name="topics">event_topic</parameter>
<parameter name="group.id">myconsumer</parameter>
<parameter name="consumer.id">myconsumer</parameter>
<parameter name="dual.commit.enabled">true</parameter>
<parameter name="auto.offset.reset">largest</parameter>
</parameters>
</inboundEndpoint>
Sequence:
<?xml version="1.0" encoding="UTF-8"?>
<sequence name="transmit_sequence" onError="fault" trace="disable" xmlns="http://ws.apache.org/ns/synapse">
<send receive="event_transmit_out_sequence">
<endpoint key="gov:endpoints/HandlerEndpoint.xml"/>
</send>
</sequence>