Redis Data Structures and Underlying Implementation

Tue, 25 Aug 2026 00:00:00 +0000

Redis exposes five common types to users, but underneath there are multiple encodings (encoding). Automatically switching encodings based on data size is the key to balancing memory and performance.

Core Underlying Structures

SDS (Simple Dynamic String): records length and pre-allocates space compared to C strings, avoiding buffer overflow and supporting binary safety.
ziplist / listpack: compact contiguous-memory lists that save space and suit small data volumes.
linkedlist: a doubly-linked list that replaces ziplist when there are many elements.
dict (dictionary): a hash table using chaining for collisions, with progressive rehash to avoid blocking.
skiplist (skip list): a multi-level ordered linked list, combined with dict to implement zset for efficient range queries.

Encodings and Conversions per Type

Type	Small-data encoding	Large-data encoding	Conversion threshold (illustrative)
string	int / embstr	raw	length > 44 bytes becomes raw
hash	ziplist/listpack	hashtable	element count or single value exceeds threshold
list	quicklist (ziplist segments)	quicklist	—
set	intset	hashtable	contains non-integers or too many elements
zset	ziplist	skiplist+dict	element count or value length exceeds threshold

Taking zset as an example, small data uses a compact ziplist (member and score adjacent); when the element count or a single member length exceeds the threshold it converts to skiplist + dict, ensuring ordered traversal and member lookup are O(1)/O(log n).

Internals on Zhuifeng Notes

Redis Data Structures and Underlying Implementation

Core Underlying Structures

Encodings and Conversions per Type