Semisynchronous

Semisynchronous refers to a state or process that is partially synchronous. In computing and distributed systems, it typically describes a communication protocol or a transaction mechanism where the sender receives an acknowledgment that the message or operation has been processed at some level, but not necessarily fully completed or committed by all participants. This differs from fully synchronous operations where the sender must wait for complete confirmation of the operation's success.

In a semisynchronous model, the sender might receive confirmation that a message has been received by the destination server, or that it has been written to a log, but not necessarily that it has been applied to a database or made visible to all replicas. This offers a compromise between the performance of asynchronous communication, where no acknowledgment is required, and the strong consistency guarantees of synchronous communication.

Semisynchronous operations can improve performance by reducing the latency experienced by the sender compared to fully synchronous methods. However, they introduce a degree of uncertainty. If a failure occurs after the acknowledgment is sent but before the operation is fully completed and durable, the sender might believe the operation succeeded when it actually failed. This makes semisynchronous approaches suitable for scenarios where a trade-off between immediate consistency and performance is acceptable, such as in certain distributed database replication strategies or distributed caching systems. The specific guarantees of a semisynchronous protocol depend heavily on its implementation details.