riak_id is a very simple application that serves both to
duplicate functionality from Twitter’s “Snowflake” and to teach,
via a very simple example, how to build a riak_core application.
The core functionality of riak_id is to provide k-sorted unique
identifiers. The structure of the identifiers is very similar to
Snowflake. They are 64-bit integers with the following layout
(from highest to lowest):
| Bits | Description |
|---|---|
| 1 | Signedness flag, always 0 |
| 41 | Unix timestamp, down to the millisecond |
| 10 | Top 10 bits of partition number (see Caveats) |
| 12 | Per-partition strictly increasing counter |
- Decent NTP synchronization between machines is needed for
riak_idto work as expected. - Request failures (e.g. from partition, timeout) should simply be retried. You will eventually get an ID.
- Because only 10 bits are reserved for the partition number, there is a practical ring-size limit of 1024. This should not be a problem because the work is mostly CPU-bound.
- So that we can give reasonable guarantees of uniqueness, some requests may have a minimum latency of 1ms (to allow the timestamp to increment).
- It will run out of ID space within the century. Snowflake solves this using the “Twepoch”. I didn’t bother with that, but it would be a simple feature to add.
- I wrote
riak_idto learn, I hope it will be useful to others.
riak_id requires Erlang/OTP, rebar and a sane GNU build system.
- Clone the repository, copy the
rebarscript into the repository. - Run
make relfrom the command line. rel/riak_id/bin/riak_id consoleriak_id:next_id().from the console to generate IDs.
Thanks to Jesse Newland, Andy Gross, Ryan Zezeski for their work on the rebar template for riak_core apps, without which I would be lost. Special thanks to Andy for pointing out some egregious errors in an early version of the vnode code.
Copyright 2011, Sean Cribbs. Licensed under Apache License 2.0.