Setting Synchronization-Based Durability Policies

Whenever a node performs a write operation, the node must know whether it should wait for the data to be written to stable storage before successfully returning from the operation.

As a part of performing a write operation, the data modification is first made to an in-memory cache. It is then written to the filesystem's data buffers. And, finally, the contents of the data buffers are synchronized to stable storage (typically, a hard drive).

You can control how much of this process the master node will wait to complete before it returns from the write operation with a normal status. There are three different levels of synchronization durability that you can require:

Notice that in all cases, the data is eventually written to stable storage (assuming some failure does not occur to prevent it). The only question is, how much of this process will be completed before the write operation returns and your application can proceed to its next operation.