Snowflake

Warning: this is alpha quality software and subject to dramatic changes until its more stable.

Snowflake is a network service for generating unique ID numbers at high scale with some simple guarantees.

Motivation

As we at Twitter move away from Mysql towards Cassandra, we've needed a new way to generate id numbers. There is no sequential id generation facility in Cassandra, nor should there be.

Requirements

Performance

• minimum 10k ids per process
• response rate 2ms (plus network latency)

Uncoordinated

For high availability within and across data centers, machines generating ids should not have to coordinate with each other.

(Roughly) Time Ordered

We have a number of API resources that assume an ordering (they let you look things up "since this id").

However, as a result of a large number of asynchronous operations, we already don't guarantee in-order delivery.

We can guarantee, however, that the id numbers will be k-sorted (references: https://portal.acm.org/citation.cfm?id=70413.70419 and https://portal.acm.org/citation.cfm?id=110778.110783) within a reasonable bound (we're promising 1s, but shooting for 10's of ms).

Directly Sortable

The ids should be sortable without loading the full objects that the represent. This sorting should be the above ordering.

Compact

There are many otherwise reasonable solutions to this problem that require 128bit numbers. For various reasons, we need to keep our ids under 64bits.

Highly Available

The id generation scheme should be at least as available as our related services (like our storage services).

Solution

• Thrift Server written in Scala
• id is composed of:
  • time - 41 bits (millisecond precision w/ a custom epoch gives us 69 years)
  • configured machine id - 10 bits - gives us up to 1024 machines
  • sequence number - 12 bits - rolls over every 4096 per machine (with protection to avoid rollover in the same ms)