MongoSource from Kafka connect create weird _data key - mongodb

Question

I'm using KafkaConnect - MongoSource with the following configuration:
curl -X PUT http://localhost:8083/connectors/mongo-source2/config -H "Content-Type: application/json" -d '{
"name":"mongo-source2",
"tasks.max":1,
"connector.class":"com.mongodb.kafka.connect.MongoSourceConnector",
"key.converter":"org.apache.kafka.connect.storage.StringConverter",
"value.converter":"org.apache.kafka.connect.storage.StringConverter",
"connection.uri":"mongodb://xxx:xxx#localhost:27017/mydb",
"database":"mydb",
"collection":"claimmappingrules.66667777-8888-9999-0000-666677770000",
"pipeline":"[{\"$addFields\": {\"something\":\"xxxx\"} }]",
"transforms":"dropTopicPrefix",
"transforms.dropTopicPrefix.type":"org.apache.kafka.connect.transforms.RegexRouter",
"transforms.dropTopicPrefix.regex":".*",
"transforms.dropTopicPrefix.replacement":"my-topic"
}'
For some reason, when I consume messages, I'm getting a weird key:
"_id": {
"_data": "825DFD2A53000000012B022C0100296E5A1004060C0FB7484A4990A7363EF5F662CF8D465A5F6964005A1003F9974744D06AFB498EF8D78370B0CD440004"
}
I have no Idea where did it come from, My mongo document's _id is UUID, When I'm consuming messages, I was expected to see the documentKey field at my consumer key.
Here is a message example of what the connector published into kafka:
{
"_id": {
"_data": "825DFD2A53000000012B022C0100296E5A1004060C0FB7484A4990A7363EF5F662CF8D465A5F6964005A1003F9974744D06AFB498EF8D78370B0CD440004"
},
"operationType": "replace",
"clusterTime": {
"$timestamp": {
"t": 1576872531,
"i": 1
}
},
"fullDocument": {
"_id": {
"$binary": "+ZdHRNBq+0mO+NeDcLDNRA==",
"$type": "03"
},
...
},
"ns": {
"db": "security",
"coll": "users"
},
"documentKey": {
"_id": {
"$binary": "+ZdHRNBq+0mO+NeDcLDNRA==",
"$type": "03"
}
}
}

Answer 1

The documentation related to schema for Kafka connect configuration is really limited out there. I know it is too late to reply but lately I have also been facing the same issue and found out the solution by trial and error.
I added these two configuration to my mongodb-kafka-connect configuration -
"output.format.key": "schema",
"output.schema.key": "{\"name\":\"sampleId\",\"type\":\"record\",\"namespace\":\"com.mongoexchange.avro\",\"fields\":[{\"name\":\"documentKey._id\",\"type\":\"string\"}]}",
But still even after this I don't know whether resume_token of change stream as a key for kafka partition allotment had any significance in term of performance or even for the case where resume_token gets expired due long time of inactivity.
P.S. - The final version of my kafka connect configuration for mongodb as a source is this -
{
"tasks.max": 1,
"connector.class": "com.mongodb.kafka.connect.MongoSourceConnector",
"key.converter": "org.apache.kafka.connect.storage.StringConverter",
"value.converter": "org.apache.kafka.connect.storage.StringConverter",
"connection.uri": "mongodb://example-mongodb-0:27017,example-mongodb-1:27017,example-mongodb-2:27017/?replicaSet=replicaSet",
"database": "exampleDB",
"collection": "exampleCollection",
"output.format.key": "schema",
"output.schema.key": "{\"name\":\"ClassroomId\",\"type\":\"record\",\"namespace\":\"com.mongoexchange.avro\",\"fields\":[{\"name\":\"documentKey._id\",\"type\":\"string\"}]}",
"change.stream.full.document": "updateLookup",
"copy.existing": "true",
"topic.prefix": "mongodb"
}