"Due to our choice of using passive replication, mutations are executed on a primary node before being committed into the trans- action log. If a commit fails, for example due to network isolation, the change must not be acknowledged and must not become visible. Other database engines use isolation mechanisms like Multi-Version Concurrency Control (MVCC) to achieve this, but Redis data struc- tures do not support this functionality, and it cannot be readily decoupled from the database engine itself. Instead, MemoryDB adds a layer of client blocking. After a client sends a mutation, the reply from the mutation operation is stored in a tracker until the transaction log acknowledges persistence and only then sent to the client. Meanwhile, the Redis workloop can process other operations. Non-mutating operations can be executed immediately but must consult the tracker to determine if their results must also be delayed until a particular log write completes. Hazards are detected at the key level. If the value or data-structure in a key has been modified by an operation which is not yet persisted, the responses to read operations on that key are delayed until all data in that response is persisted. Replica nodes do not require blocking as mutations are only visible once committed to three AZs"