Скачать или смотреть Understanding Why Redis Uses Skiplist to Implement Zset

Explore the benefits and reasons behind Redis's choice to use `skiplist` over `ziplist` for implementing `zset`, especially regarding performance and memory usage.
---
This video is based on the question https://stackoverflow.com/q/66914001/ asked by the user 'Dennis' ( https://stackoverflow.com/u/11160245/ ) and on the answer https://stackoverflow.com/a/66916617/ provided by the user 'Gawain' ( https://stackoverflow.com/u/9012649/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: why Redis use skiplist to implement zset

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Why Redis Uses Skiplist to Implement Zset

Redis is renowned for its fast and efficient data handling capabilities, backing its functionality with robust data structures. One of the pivotal structures employed by Redis is the zset (sorted set), which elegantly combines elements of both sets and sorted data. But why does Redis opt to use a skiplist to implement zset instead of its alternative, the ziplist? In this guide, we will break down the reasons behind this choice, focusing on two specific scenarios where the use of skiplist becomes essential for optimal performance.

The Foundation: Understanding Zset in Redis

Before diving into the core of the discussion, let's clarify what a Redis zset is. A zset is a collection of unique elements mapped to a score, which determines their order within the set. This structure allows for efficient range queries, ranked retrievals, and score manipulations.

Data Structures Behind Zset

Skiplist: An advanced data structure that maintains multiple pointers to allow faster access to elements. The average time complexity for performing search operations in a skiplist is O(log N).

Ziplist: A more memory-efficient structure that arranges elements linearly in memory. However, its search and update time complexities can climb to O(n), making it slower for larger datasets.

The Scenarios for Using Skiplist

Redis smartly decides which data structure to use based on the characteristics of the data being stored in zset. There are two key scenarios where Redis leverages the benefits of skiplist:

1. Large Number of Members in Zset

When a zset contains many members, maintaining efficient access and retrieval becomes crucial. In this context:

Performance: The time complexity becomes a significant factor. With a skiplist, operations like searching and updating can be performed in O(log N) time, which enhances performance, especially when dealing with larger datasets.

Concurrent Operations: The efficiency of concurrent read and write operations is vital. The skiplist's logarithmic performance can avoid bottlenecks in systems where many users access data simultaneously.

2. Long Strings as Members

In scenarios where the members of the zset are long strings, the following benefits of using a skiplist come into play:

Memory Usage vs. Speed Trade-off: While a ziplist can save memory due to its linear structure with no additional pointers, the risk is high when handling long strings because the search operation's O(n) complexity leads to performance degradation. The skiplist, despite having a higher memory overhead, offers significantly better performance for lookups and updates.

Operational Efficiency: Longer strings require more processing time when accessed linearly, and utilizing a skiplist mitigates this issue by providing quicker access paths.

Why Not Use One Data Structure for All Cases?

The choice for Redis to implement both skiplist and ziplist arises from the need to balance memory efficiency with performance. Here’s why a one-size-fits-all solution is not optimal:

Efficiency for Small Sizes: For small zset with a few members, the simplicity and lower memory usage of a ziplist far outweigh the benefits of using a skiplist. For example, if a zset has only ten members, the difference between O(n) and O(log N) is negligible.

Scalability: As the number of elements grows, the advantages of using a skiplist become apparent. Redis aims to provide the best performance regardless of the situation, making it adaptable to the data's nature.

Conclusion

In summary, Redis employs skiplist for implementing zset in scenarios where performance is critical due to a large number of members or long strings. While ziplist serves well in memory-