Debezium MongoDB connector does not perform initial snapshot - apache-kafka-connect

Question

I am using MongoDB atlas with a sharded replica set cluster, with the Debezium MongoDB connector as described in the documentation.
This is how my current config looks like (running a standalone setup):
name=dev-mongodb
connector.class=io.debezium.connector.mongodb.MongoDbConnector
tasks.max=4
mongodb.hosts=<some-url>.mongodb.net:27017
mongodb.name=mongodb
mongodb.user=<admin_user>
mongodb.password=<admin_user_pw>
database.include.list=<list_of_databases>
database.history.kafka.bootstrap.servers=<list_of_aws_msk_brokers>
database.history.kafka.topic=mongodb.history
include.schema.changes=true
mongodb.ssl.enabled=true
I can receive CDC events in kafka topic but the initial snapshot that the documentation describes is never made. I have tried with a different mongodb.name resulting in entirely different set of topics being created and used, but the same outcome.
The MongoDB oplog has ~2M rows, kafka topics have hardly a few thousand messages in total.
On further digging up, it seems the connector records an offset for the last position of the oplog. Is it possible to reset this offset?

Answer 1

It sounds to me like you're using the same connector name in your multiple deployments, which means that despite changing the configuration and trying to reset the connector's state, it continues to find the prior offsets and restores the oplog position.
There are two alternatives:
Create a new connector with a completely different connector name.
Manually clear the offsets for the connector
A lot of users prefer the first option simply because it is the easiest. Kafka records a connector's offsets based on the connector's name and therefore by simply adjusting the name of a connector will tell Kafka that the connector is completely brand new and it won't find any persisted offsets to be restored.
The second option is a bit involved because you need to first locate the Kafka topic that stores the offsets, typically this is connect-offsets by default but can be overridden. Once you know the topic, you should shutdown all connectors that are using this topic. If you adjust this topic while a connector is using it, it can lead to unexpected behavior.
Using the kafkacat tool available from Kafka, you'll want to run the following, which assumes the default connect offsets topic name, so adjust that accordingly:
$ kafkacat -b localhost:9092 -t connect-offsets -C -f '\nKey (%K bytes): %k
Value (%S bytes): %s
Timestamp: %T
Partition: %p
Offset: %o\n'
This will generate some output and its important to take note of both the "Key" and "Partition". In order to reset the offsets, you're going to want to effectively write a NULL (or tombstone) into the topic using the correct "Key" and "Partition" values.
Assuming the above provided this output:
% Reached end of topic connect-offsets [0] at offset 0
% Reached end of topic connect-offsets [1] at offset 0
[…]
Key (52 bytes): ["source-file-01",{"filename":"/data/testdata.txt"}]
Value (15 bytes): {"position":87}
Timestamp: 1565859303551
Partition: 20
Offset: 0
[…]
You would want to execute the following command:
$ echo '["source-file-01",{"filename":"/data/testdata.txt"}]#' | \
kafkacat -b localhost:9092 -t connect-offsets -P -Z -K# -p 20
In the echo statement, we specify the key followed by the key separator # defined by the kafkacat argument -K# and the -Z option which is to send an empty value as NULL. The -p argument is where the partition is to be specified and its important that the key and partition be set correctly.
After this is done, you can safely restart the connectors that used that offset topic and you should see that the connector acts like its a brand new deployment.
Be mindful that if you are working with a connector that uses a database history topic such as MySQL, SQL Server, or Oracle, the database history topic will also need to be cleared as well.
As I said earlier however, its just simplier to redeploy the connector using a new name to avoid needing to do all the kafka topic magic to arrive at the same outcome.