Redis is an open source, memory-based data structure storage system that can be used as a database, cache, and message broker. It supports many types of data structures such as strings, hash tables, lists, sets, sorted sets, etc. Today we will focus on an important data structure of Redis: Hash, also called hash table.
1. What is Redis Hash
Redis Hash is a collection of key-value pairs, which is a mapping table of string type field and value. Each of its hashes can store 43 billion key-value pairs (more than 4 billion).
2. The internal implementation principle of Redis Hash
In order to provide efficient data storage and query capabilities, Redis uses a special data structure for data storage. Specifically, Redis's Hash implementation uses two strategies: ziplist (compressed list) and hashtable (hash table). Which strategy to use is determined by the hash_max_ziplist_entries configuration parameter and the hash_max_ziplist_value configuration parameter.
1. Ziplist
When the number of elements of the Hash type and the size of a single element are small, Redis will choose ziplist as the storage structure. Ziplist is a special linear table that can achieve a good balance between space efficiency and query efficiency.
2. Hashtable
When the number of elements of the Hash type is large, or the size of a single element is large, Redis will use hashtable as the storage structure. The query efficiency of Hashtable is very high, but the space efficiency is low.
3. Operation performance of Redis Hash
- HSET : Set the hash field to the specified value, if the key does not exist, create a new one. The complexity is O(1).
- **HGET:** Get the value stored in the hash field. The complexity is O(1).
- HMSET : Set multiple hash fields at the same time. The complexity is O(N), where N is the number of fields.
- **HDEL:** Delete one or more hash fields. The complexity is O(N), where N is the number of fields.
4. Application scenarios of Redis Hash
Because Redis Hash provides efficient data storage and query capabilities, it is very useful in many scenarios, such as:
- **Store objects:** We can store each field of an object in Hash, and then query or modify the object through a key. This usage is very similar to traditional relational databases.
- **Cache:** Since Redis provides efficient query capabilities, we can store hot data in Redis and use it as a cache.
In general, the Hash structure of Redis is a flexible and powerful data structure, which plays an important role in many data structures of Redis. I hope this article can help you understand it better.