Transactions in the client-server EOS object store

The paper describes the client-server software architecture of the EOS storage manager and the concurrency control and recovery mechanisms it employs. Unlike most client-server storage systems that use the standard two-phase locking protocol, EOS offers a semi-optimistic locking scheme based on a multigranularity two-version two-phase locking protocol. Under this scheme, many readers are allowed to access a data item while it is being updated by a single writer. For recovery, EOS maintains a write-ahead redo-only log because of the potential benefits it offers in a client-server environment. First, there are no undo records, as log records of aborted transactions are never inserted in the log; this minimizes the I/O and network transfer costs associated with logging during normal transaction execution. Secondly, it reduces the space required for the log. Thirdly, it facilitates fast recovery from system crashes because only one forward scan of the log is required for installing the updates performed by transactions that committed prior to the crash. Performance results of the EOS recovery subsystem are also presented