public Map<TopicPartition, Checkpoint> get(String group) {
        Map<TopicPartition, Checkpoint> result = checkpointsPerConsumerGroup.get(group);
        return result == null ? null : Collections.unmodifiableMap(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