Question

In a Rails app I need a source of unique, sequential (no gaps) integers to use as serial numbers. It must be persistent and allow concurrent access.

Database auto-increment isn’t adequate because most don’t guarentee the “no gaps” property.

In straight SQL I would just create a one-line table and say (in PostgreSQL) something like:

update sequence set value = value + 1 returning value

This is apparently standard practice in the SQL world. References exist.

In ActiveRecord I easily created a model the model and found .increment! and .increment_counter in the documentation. But I can’t figure out how to atomically retrieve the incremented value. Locks and transactions don’t seem to help.

Answer 1

If you want to use redis (and maybe you already are because of Resque or Sidekiq), you can do an INCR on a key, this is atomic and returns the new value.