...
Database Cluster
While the row raw "truth" data of a table is stored in S3, and tracked in a single MySQL database, a cluster of MySQL databases is used to support table queries. A secondary worker (see Worker Cluster above) listens for table change events and attempts to build an index for a table in one of the database instances in the query cluster. This worker will update a table's index while holding an exclusive lock on the table. This workers primary job is to determine the current version of each row and ensure it is reflected in the index. In the future we plan to "rebalanced" the query database cluster by redistributing table incidences based on the load of the of each database machine. Currently every table is indexed in the cluster but we plan to only index "active" tables in the future.
...