OffsetSync syncFor(TopicPartition topicPartition, int syncIdx) {
        OffsetSync[] syncs = offsetSyncs.get(topicPartition);
        if (syncs == null)
            throw new IllegalArgumentException("No syncs present for " + topicPartition);
        if (syncIdx >= syncs.length)
            throw new IllegalArgumentException(
                    "Requested sync " + (syncIdx + 1) + " for " + topicPartition
                            + " but there are only " + syncs.length + " syncs available for that topic partition"
            );
        return syncs[syncIdx];
    }
Start the OffsetSyncStore, blocking until all previous Offset Syncs have been read from backing storage.
connect/mirror/src/main/java/org/apache/kafka/connect/mirror/OffsetSyncStore.java