private boolean initMemoryDB(final StoreEngineOptions opts) {
        MemoryDBOptions memoryOpts = opts.getMemoryDBOptions();
        if (memoryOpts == null) {
            memoryOpts = new MemoryDBOptions();
            opts.setMemoryDBOptions(memoryOpts);
        }
        final MemoryRawKVStore memoryRawKVStore = new MemoryRawKVStore();
        if (!memoryRawKVStore.init(memoryOpts)) {
            LOG.error("Fail to init [MemoryRawKVStore].");
            return false;
        }
        this.rawKVStore = memoryRawKVStore;
        return true;
    }
Storage engine, there is only one instance in a node,
containing one or more {@link RegionEngine}.

@author jiachun.fjc
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/StoreEngine.java