Most of us who deal with traditional databases take auto-increments for granted. While auto-increments are simple on consistent clusters, it can become a challenge in a cluster of independent nodes which donâ€™t use the same source for the unique-ids. Even bigger challenge is to do it in such a way so that they are roughly in sequence.
While this may be an old problem, I realized the importance of such a sequence only after using Cassandra in my own environment. Twitter, which has been using Cassandra in many interesting ways has proposed a solution for it which they are releasing as open source today.
Here are some interesting sections from their post announcing â€œSnowflakeâ€.
We currently use MySQL to store most of our online data. In the beginning, the data was in one small database instance which in turn became one large database instance and eventually many large database clusters. For various reasons, the details of which merit a whole blog post, weâ€™re working to replace many of these systems with the Cassandra distributed database or horizontally sharded MySQL (using gizzard).
Unlike MySQL, Cassandra has no built-in way of generating unique ids â€“ nor should it, since at the scale where Cassandra becomes interesting, it would be difficult to provide a one-size-fits-all solution for ids. Same goes for sharded MySQL.
Our requirements for this system were pretty simple, yet demanding:
We needed something that could generate tens of thousands of ids per second in a highly available manner. This naturally led us to choose an uncoordinated approach.
These ids need to be roughly sortable, meaning that if tweets A and B are posted around the same time, they should have ids in close proximity to one another since this is how we and most Twitter clients sort tweets.
Additionally, these numbers have to fit into 64 bits. Weâ€™ve been through the painful process of growing the number of bits used to store tweet ids before. Itâ€™s unsurprisingly hard to do when you have over 100,000 different codebases involved.
To generate the roughly-sorted 64 bit ids in an uncoordinated manner, we settled on a composition of: timestamp, worker number and sequence number.
Sequence numbers are per-thread and worker numbers are chosen at startup via zookeeper (though thatâ€™s overridable via a config file).
We encourage you to peruse and play with the code: youâ€™ll find it on github. Please remember, however, that it is currently alpha-quality software that we arenâ€™t yet running in production and is very likely to change.