public boolean start()  {
        checkpointsPerConsumerGroup = readCheckpoints();
        isInitialized = true;
        if (log.isTraceEnabled()) {
            log.trace("CheckpointStore started, load success={}, map={}", loadSuccess, checkpointsPerConsumerGroup);
        } else {
            log.debug("CheckpointStore started, load success={}, map.size={}", loadSuccess, checkpointsPerConsumerGroup.size());
        }
        return loadSuccess;
    }
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