Developing applications that run correctly under weak consistency is challenging because the execution of concurrent updates may lead the application to an incorrect state.
To address this challenge, we proposed explicit consistency (aka just-right consistency), a novel consistency model where the state of all replicas always enforces a set of invariants specified by the application.
Additionally, we explored two complementary approaches to enforce explicit consistency.
First, a conflict-avoidance approach where the execution of operations that may lead to invariant-violation is controlled by a reservation system. Each replica can obtain reservations that allow it to execute operations locally.
Second, an invariant-preservation approach where operations are modified to guarantee that the execution of concurrent operations never leads to an invariant-violation. When no concurrent operation is executed, the modified version of the operations have the exact same effect on the database. When concurrent updates occur, the modifications guarantee that invariants are maintained in the final database state.
To help programmers develop applications using these approaches, we have developed
We have applied these ideas to implement AQL, a SQL interface for a NoSQL database, AntidoteDB, that enforces some of the most common SQL constraints: primary key, check and foreign key constraints.
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