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Several previous works have proven that there is no way of guaranteeing a snapshot isolation level in symmetrical replicated database systems without blocking transactions when they are started. As a result of this, the generalized snapshot isolation (GSI) level was defined, relaxing a bit the freshness of the snapshot being taken when a transaction is initiated in its local replica. This enhances performance, since transactions do not need to get blocked, but in some cases will increase the abortion rate. This paper proposes a flexible protocol that is able to bound the degree of snapshot outdateness from a relaxed GSI to the strict one-copy equivalent SI. Additionally, it proposes an optimistic solution where transactions do not block, and only need to be re-initiated when their optimistic start fails. Such re-initialization is made very soon and only rolls back the first transaction accesses, without waiting for the transaction completion. Finally, if 1CSI is not enough, this protocol is also able to manage transactions with serializable isolation, if such a level is requested. |
