redis源码阅读之hash模型

redis当中哈希对象的底层存储使用的是ziplist和hashtable，其中个人觉得比较有意思的就是类型转换这一块了
首先，在redis当中（我读的是4.0.11）没有实现由hashtable转为ziplist的可能，具体见代码：

void hashTypeConvert(robj *o, int enc) {
    if (o->encoding == OBJ_ENCODING_ZIPLIST) {
        hashTypeConvertZiplist(o, enc);
    } else if (o->encoding == OBJ_ENCODING_HT) {
        serverPanic("Not implemented");
    } else {
        serverPanic("Unknown hash encoding");
    }
}

其次，发生转换的条件是：1）当前编码为ziplist；2）当前ziplist当中的entry个数大于512，或者ziplist当中最长的key/value占用字节大于64字节时。具体代码如下：

void hashTypeTryConversion(robj *o, robj **argv, int start, int end) {
    int i;

    if (o->encoding != OBJ_ENCODING_ZIPLIST) return;

    for (i = start; i <= end; i++) {
        if (sdsEncodedObject(argv[i]) &&
            sdslen(argv[i]->ptr) > server.hash_max_ziplist_value)
        {
            hashTypeConvert(o, OBJ_ENCODING_HT);
            break;
        }
    }
}

其中的512和64是可以通过redis.conf中的两个配置项更改的，具体如下：

# Hashes are encoded using a memory efficient data structure when they have a
# small number of entries, and the biggest entry does not exceed a given
# threshold. These thresholds can be configured using the following directives.
hash-max-ziplist-entries 512
hash-max-ziplist-value 64

转化的流程就是遍历ziplist当中的key/value对，逐个插入到新建的hashtable当中，最后释放原有数据，将object当中的ptr指向新的hashtable，具体代码如下：

void hashTypeConvertZiplist(robj *o, int enc) {
    serverAssert(o->encoding == OBJ_ENCODING_ZIPLIST);

    if (enc == OBJ_ENCODING_ZIPLIST) {
        /* Nothing to do... */

    } else if (enc == OBJ_ENCODING_HT) {
        hashTypeIterator *hi;
        dict *dict;
        int ret;

        hi = hashTypeInitIterator(o);
        dict = dictCreate(&hashDictType, NULL);

        while (hashTypeNext(hi) != C_ERR) {
            sds key, value;

            key = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_KEY);
            value = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_VALUE);
            ret = dictAdd(dict, key, value);
            if (ret != DICT_OK) {
                serverLogHexDump(LL_WARNING,"ziplist with dup elements dump",
                    o->ptr,ziplistBlobLen(o->ptr));
                serverPanic("Ziplist corruption detected");
            }
        }
        hashTypeReleaseIterator(hi);
        zfree(o->ptr);
        o->encoding = OBJ_ENCODING_HT;
        o->ptr = dict;
    } else {
        serverPanic("Unknown hash encoding");
    }
}

其余的命令实现，底层都是基于ziplist和hashtable的操作，参数解析什么的也都同字符串模型和list模型，这里就不再赘述了。

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