Master Redis Bitmaps: SETBIT, GETBIT, BITCOUNT, BITOP Explained

This article shares Redis's advanced feature: bit operations (bitmap).

Operating System: macOS 64‑bit Redis version: 5.0.7 64‑bit Mode: standalone

Redis Bit Operations

Redis bit operations, also called bitmap, provide four commands: SETBIT, GETBIT, BITCOUNT, and BITOP.

SETBIT

Syntax: SETBIT key offset value (value must be 0 or 1). The command writes a binary value at the specified offset in the bitmap.

GETBIT

Syntax: GETBIT key offset. It retrieves the binary value stored at the given offset.

BITCOUNT

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

BITOP

Syntax: BITOP operation destkey key [key…]. It performs AND, OR, XOR on multiple bitmaps and stores the result in destkey.

Underlying Data Structure Analysis

Redis stores strings using SDS (Simple Dynamic String), a binary‑safe structure.

struct sdshdr {
  int len;   // used bytes (string length)
  int free;  // unused bytes
  char buff[]; // actual data
}

The bitmap is stored as a Redis string, which internally uses SDS.

Time Complexity

GETBIT and SETBIT: O(1)

BITCOUNT: O(n)

BITOP: O(n) or O(n²) depending on operation.

Explanation: SETBIT calculates the target byte as offset / 8 and the bit position as offset % 8, both constant‑time operations.

Storage Space Calculation

Each bit occupies 1/8 byte. For 1 billion bits, memory needed is:

1 000 000 000 ÷ 8 ÷ 1024 ÷ 1024 ≈ 119.21 MB

Thus 1 billion entries require roughly 119 MB, well within modern Redis clusters.

Be careful not to use excessively large offsets for small data sets, as this wastes memory.

Use Cases

User Sign‑In Tracking

Store each day as a key; user ID maps to an offset. Setting the bit to 1 records a sign‑in.

Active User Counting

Daily keys record whether a user was active (bit = 1). Same approach works for monthly active users.

Online Status Monitoring

Map user IDs to bits; 1 indicates online, 0 offline. Allows quick online/offline queries and total online count.

App Notification Red Dots

Use a bitmap to flag users with unread messages, enabling efficient global notification checks.