Abstract: Clock times may be stored at nodes of a database system that are used to provide access to database data using Multi-Version Concurrency Control (MVCC) and implement isolation for distributed transactions. When an access request is received at a processing node of a database, the snapshot time of the access request is determined. The snapshot may be a range of time, with an earliest and latest timestamp. If the snapshot time has a latest time stamp greater than a current timestamp in the clock time, then the clock time may be updated. The clock time and snapshot time may be used to implement time-based MVCC to select a version of a tuple stored in the database to use for performing the access request.

Abstract: Distributed execution of transactions may be performed across shards of a scalable database table. Instructions to perform as part of a transaction with respect to one or more database tables may be received. The instructions may be evaluated with respect to metadata obtained for the database tables to determine an assignment distribution of computing resources of the database system to data that can satisfy the access request. The commit protocol for the transaction may be determined according to the assignment distribution of the computing resources. After a request to commit the transaction is received, the transaction may be committed according to the determined commit protocol.

Abstract: Lease-based consistency may be implemented for databases to handle failovers. A database node may obtain a consistency lease that describes a point in time determined from a time-to-live amount added to a consistent point in time for database data. While the consistency lease is valid, Multi-version Concurrency Control (MVCC) snapshots assigned by the database node can be used to handle requests to access the database data. Once expired, the database node may have to renew the consistency lease in order to continue to handle write and read requests.