public Map<String, Map<TopicPartition, OffsetAndMetadata>> computeConvertedUpstreamOffset() {
        Map<String, Map<TopicPartition, OffsetAndMetadata>> result = new HashMap<>();

        for (Map.Entry<String, Map<TopicPartition, Checkpoint>> entry : checkpointsPerConsumerGroup.entrySet()) {
            String consumerId = entry.getKey();
            Map<TopicPartition, OffsetAndMetadata> convertedUpstreamOffset = new HashMap<>();
            for (Checkpoint checkpoint : entry.getValue().values()) {
                convertedUpstreamOffset.put(checkpoint.topicPartition(), checkpoint.offsetAndMetadata());
            }
            result.put(consumerId, convertedUpstreamOffset);
        }
        return result;
    }
Reads once the Kafka log for checkpoints and populates a map of
checkpoints per consumer group.

The Kafka log is closed after the initial load and only the in memory map is
used after start.
connect/mirror/src/main/java/org/apache/kafka/connect/mirror/CheckpointStore.java