A bitmap (also known as a bit array, or bit vector) is an array of bits. A Redis bitmap is not a new data type; its actual underlying data type is string. Since a string is inherently a binary blob, it can be viewed as a bitmap. A bitmap saves enormous memory space for storing boolean information under certain circumstances.
In this recipe, we will use a bitmap to store a flag, whether or not a user has ever used a feature in Relp. Suppose that in Relp every user has a unique and incremental id, which can be denoted by the bitmap offset. The flag, which is a boolean attribute, can be denoted by the bit value in the bitmap.
