BITFIELD key [GET type offset] [SET type offset value] [INCRBY type offset increment] [OVERFLOW WRAP|SAT|FAIL]
The command treats a Redis string as a array of bits, and is capable of addressing specific integer fields of varying bit widths and arbitrary non (necessary) aligned offset. In practical terms using this command you can set, for example, a signed 5 bits integer at bit offset 1234 to a specific value, retrieve a 31 bit unsigned integer from offset 4567. Similarly the command handles increments and decrements of the specified integers, providing guaranteed and well specified overflow and underflow behavior that the user can configure.
BITFIELD is able to operate with multiple bit fields in the same command call. It takes a list of operations to perform, and returns an array of replies, where each array matches the corresponding operation in the list of arguments.
For example the following command increments an 5 bit signed integer at bit offset 100, and gets the value of the 4 bit unsigned integer at bit offset 0:
> BITFIELD mykey INCRBY i5 100 1 GET u4 0 1) (integer) 1 2) (integer) 0
The following is the list of supported commands.
<offset>-- Returns the specified bit field.
<value>-- Set the specified bit field and returns its old value.
<increment>-- Increments or decrements (if a negative increment is given) the specified bit field and returns the new value.
There is another subcommand that only changes the behavior of successive INCRBY subcommand calls by setting the overflow behavior:
Where an integer type is expected, it can be composed by prefixing with
i for signed integers and
u for unsigned integers with the number of bits of our integer type. So for example
u8 is an unsigned integer of 8 bits and
i16 is a signed integer of 16 bits.
The supported types are up to 64 bits for signed integers, and up to 63 bits for unsigned integers. This limitation with unsigned integers is due to the fact that currently the Redis protocol is unable to return 64 bit unsigned integers as replies.
There are two ways in order to specify offsets in the bitfield command. If a number without any prefix is specified, it is used just as a zero based bit offset inside the string.
However if the offset is prefixed with a
# character, the specified offset is multiplied by the integer type width, so for example:
BITFIELD mystring SET i8 #0 100 SET i8 #1 200
Will set the first i8 integer at offset 0 and the second at offset 8. This way you don't have to do the math yourself inside your client if what you want is a plain array of integers of a given size.
OVERFLOW command the user is able to fine-tune the behavior of the increment or decrement overflow (or underflow) by specifying one of the following behaviors:
i8integer is set to the value 127, incrementing it by 1 will yield
i8integer starting from value 120 with an increment of 10, will result into the value 127, and further increments will always keep the value at 127. The same happens on underflows, but towards the value is blocked at the most negative value.
Note that each
OVERFLOW statement only affects the INCRBY commands that follow it in the list of subcommands, up to the next
By default, WRAP is used if not otherwise specified.
> BITFIELD mykey incrby u2 100 1 OVERFLOW SAT incrby u2 102 1 1) (integer) 1 2) (integer) 1 > BITFIELD mykey incrby u2 100 1 OVERFLOW SAT incrby u2 102 1 1) (integer) 2 2) (integer) 2 > BITFIELD mykey incrby u2 100 1 OVERFLOW SAT incrby u2 102 1 1) (integer) 3 2) (integer) 3 > BITFIELD mykey incrby u2 100 1 OVERFLOW SAT incrby u2 102 1 1) (integer) 0 2) (integer) 3
The command returns an array with each entry being the corresponding result of the sub command given at the same position.
OVERFLOW subcommands don't count as generating a reply.
The following is an example of
OVERFLOW FAIL returning NULL.
> BITFIELD mykey OVERFLOW FAIL incrby u2 102 1 1) (nil)
The motivation for this command is that the ability to store many small integers as a single large bitmap (or segmented over a few keys to avoid having huge keys) is extremely memory efficient, and opens new use cases for Redis to be applied, especially in the field of real time analytics. This use cases are supported by the ability to specify the overflow in a controlled way.
Fun fact: Reddit's 2017 April fools' project r/place was built using the Redis BITFIELD command in order to take an in-memory representation of the collaborative canvas.
Usually BITFIELD is a fast command, however note that addressing far bits of currently short strings will trigger an allocation that may be more costly than executing the command on bits already existing.
The representation used by BITFIELD considers the bitmap as having the bit number 0 to be the most significant bit of the first byte, and so forth, so for example setting a 5 bits unsigned integer to value 23 at offset 7 into a bitmap previously set to all zeroes, will produce the following representation:
+--------+--------+ |00000001|01110000| +--------+--------+
When offsets and integer sizes are aligned to bytes boundaries, this is the same as big endian, however when such alignment does not exist, its important to also understand how the bits inside a byte are ordered.
© 2009–2020 Salvatore Sanfilippo
Licensed under the Creative Commons Attribution-ShareAlike License 4.0.