Redis Bit Data & Operations

This page provides an Introduction to Redis Bit Data & Operations. These are my notes from the RU101 Redis University course.

Bitfield

The motivation behind this command is to store many small integers as a single large bitmap, which is highly memory-efficient and opens new use cases for Redis. Bit operations are performed on the string datatype because strings are binary-safe.

Histogram Counter Example

You can use bitfield to represent a histogram of values, we will have 24, integer counters for each hour of the day.

Create a histogram counter and set 0^th hour count to 0.

BITFIELD histogram:counter:machine1 SET u32 0 0

Now, Suppose we want to increment the 24^th hour counter by 20. This can be done in two ways: either by directly using the offset 736(736 = 23 * 32), or by using the pound symbol (#) to specify the relative offset position.

BITFIELD histogram:counter:machine1 INCRBY u32 768 20

BITFIELD histogram:counter:machine1 INCRBY u32 #23 20

Bitfield historgram counter will looks like below,

Player Path Example

You can use bitfield to track a player's path in a 2D matrix. A 1 bit means a tile has been visited, and the default 0 means it hasn't. To calculate the bit offset, use the formula Y * width + X, so coordinates (2, 2) correspond to an offset of 8.

Create a bitfield for player:1

BITFIELD player:1 SET u2 0 0
1) (integer) 0

Now, let's create a path for player:1 as show in the image below.

Here we can use BITFIELD SET command like we used for histogram example.

BITFIELD player:1 SET u1 0 1 SET u1 4 1 SET u1 7 1 SET u1 8 1
1) (integer) 0
2) (integer) 0
3) (integer) 0
4) (integer) 0

Or we can use SETBIT command.

SETBIT player:1 0 1
(integer) 0

SETBIT player:1 4 1
(integer) 0

SETBIT player:1 7 1
(integer) 0

SETBIT player:1 8 1
(integer) 0

Let's create a path for player:2 as show in the image below.

BITFIELD player:2 SET u1 0 1 SET u1 3 1 SET u1 6 1 SET u1 7 1 SET u1 8 1
1) (integer) 0
2) (integer) 0
3) (integer) 0
4) (integer) 0
5) (integer) 0

If we want to calculate path that is coverd by both player:1 and player:2 combined we can use AND operator.

BITOP AND player:and:12 player:1 player:2
(integer) 2

Now, if we want to count tiles which are visited by both player:1 and player:2 combined we can use BITCOUNT command.

BITCOUNT player:and:12
(integer) 3

Similarly, we can calculate path that is covered by either player:1 or player:2 we can use OR operator.

BITOP OR player:or:12 player:1 player:2
(integer) 2