Module crc 

CRC16 for slot placement and CRC32C for integrity.

Two different polynomials for two different jobs. CRC16 is Redis’s XMODEM variant and it exists here because slot = crc16(key) & 0x3FFF is how a key reaches a shard (04 section 1). Getting it wrong does not corrupt anything, it just makes us incompatible with every Redis cluster client, so the hash tag rules are implemented here too.

CRC32C is Castagnoli, the same polynomial SSE4.2 and the ARM CRC extension implement in hardware, and it is what guards pages and superblocks (07).

Constants

SLOT_COUNT

The number of slots, which is Redis’s 16384 and is not configurable.

Functions

crc16

Redis’s CRC16, the XMODEM variant.

crc32c

CRC32C over data, continuing from crc. Start with 0.

hash_tag

The part of a key that decides its slot.

slot_of

The cluster slot a key belongs to, hash tags included.