Database Buckets and Data Model of Spanner

Buckets and placement

Spanner provides an additional layer of abstraction over the bag of key-value mappings in the form of a directory or bucket—a group of adjacent keys that all begin with the same prefix. Applications that support buckets manage the data locality by carefully selecting keys.

The basic organizational structure for data is a bucket. All the bucket's data share the same replication settings. Consider the illustration below. The data is transferred between Paxos groups bucket by bucket. To minimize the load on a Paxos group, we can relocate the frequently accessed buckets into the same Paxos group or place a bucket geographically closer to its accessors. Changing a bucket's location doesn't have to interrupt service for the client. Normally, copying 50 MB of data to a new bucket would take a few seconds.