Unlock Redis Power: Master Bitmaps, Commands, and Real-World Use Cases

Redis Bitmap Operations Overview

This article introduces Redis’s advanced bitmap feature, covering the four bitmap commands (SETBIT, GETBIT, BITCOUNT, BITOP), their syntax and usage examples, the underlying SDS data structure, time‑complexity analysis, memory‑size calculation, and typical application scenarios such as user sign‑in tracking, active‑user counting, online‑user statistics, and notification red‑dot indicators.

Environment

Operating System: macOS 64‑bit Redis version: 5.0.7 64‑bit Running mode: standalone

Bitmap Commands

SETBIT

Syntax: SETBIT key offset value – writes a binary value (0 or 1) at the specified offset (starting at 0). Non‑binary values cause the command to fail.

GETBIT

Syntax: GETBIT key offset – retrieves the bit value at the given offset.

BITCOUNT

Syntax: BITCOUNT key – returns the number of bits set to 1 in the bitmap.

BITOP

Syntax: BITOP operation destkey key [key...] – performs bitwise AND, OR, XOR, or NOT on one or more bitmaps and stores the result in destkey.

Underlying Data Structure (SDS)

Redis stores strings using Simple Dynamic String (SDS), a binary‑safe structure that provides O(1) length access, prevents buffer overflows, reduces reallocations, offers binary‑safe APIs, and is compatible with many C string functions.

Time Complexity

GETBIT – O(1)

SETBIT – O(1)

BITCOUNT – O(N)

BITOP – O(N) (or O(N²) for certain operations)

For SETBIT/GETBIT, the byte index is calculated as offset / 8 and the bit position as offset % 8, resulting in constant‑time operations.

Memory Calculation

Each bit occupies one position; 8 bits = 1 byte, 1 KB = 1024 bytes, etc. Example: 1 billion bits → 1 000 000 000 ÷ 8 ÷ 1024 ÷ 1024 ≈ 119.21 MB, which easily fits in modern Redis instances.

Typical Application Scenarios

User daily sign‑in tracking (key = date, offset = user ID)

Active‑user (DAU/MAU) counting and retention analysis

Online‑user presence and total online count

App notification red‑dot indicator for unread messages