A druuid comprises:
A 41-bit timestamp (which has millisecond precision for over 69 years after a defined epoch); and
23 random bits.
For example, a druuid generated at midnight on February 4, 2012, may look something like 11142943683383068069. In binary:
This ID can be displayed compactly in base 36: 2cnpvvfkm56ed.
64-bit IDs can be stored in BIGINT database columns, which are generally more efficient to index (and index uniquely) than VARCHAR.
The timestamp component allows for efficient date-based queries and easy cursor-based pagination.
23 bits of randomness contains much less entropy than traditional, 128-bit UUIDs, so precautions must be taken to avoid collisions between druuids generated in the same millisecond (e.g., a database constraint). The probability, within a millisecond, can be calculated with the Birthday problem (where n is the number of IDs generated per millisecond and 23 represents the number of random bits):
IDs generated in different milliseconds cannot collide, but at a rate of 10 IDs per millisecond (10,000 IDs per second), the probability a collision will occur within any given millisecond approaches 0.000596%, which is about once every few minutes if that rate is constant.
$ gem install druuid
require 'druuid' # Druuid.epoch = Time.utc 1970 # change the default (Unix) epoch uuid = Druuid.gen # => 11142943683383068069 time = Druuid.time uuid # => 2012-02-04 00:00:00 -0800 Druuid.min_for_time time # => 11142943683379200000
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