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## Setting Acknowledgment-Based Durability Policies

Whenever a master node performs a write operation (create, update or delete), it must send that operation to its various replica nodes. The replica nodes then apply the write operation(s) to their local databases so that the replicas are consistent relative to the master node.

Upon successfully applying write operations to their local databases, replicas in primary zones send an acknowledgment message back to the master node. This message simply says that the write operation was received and successfully applied to the replica's local database. Replicas in secondary zones do not send these acknowledgement messages.

Note:

The exception to this are replicas in secondary zones, which will never acknowledge write operations.

An acknowledgment-based durability policy describes whether the master node will wait for these acknowledgments before considering the write operation to have completed successfully. You can require the master node to not wait for acknowledgments, or to wait for acknowledgments from a simple majority of replica nodes in primary zones, or to wait for acknowledgments from all replica nodes in primary zones.

The more acknowledgments the master requires, the slower its write performance will be. Waiting for acknowledgments means waiting for a write message to travel from the master to the replicas, then for the write operation to be performed at the replica (this may mean disk I/O), then for an acknowledgment message to travel from the replica back to the master. From a computer application's point of view, this can all take a long time.

When setting an acknowledgment-based durability policy, you can require acknowledgment from:

- All replicas. That is, all of the replica nodes in the shard that reside in a primary zone. Remember that your store has more than one shard, so the master node is not waiting for acknowledgments from every machine in the store.

- No replicas. In this case, the master returns with normal status from the write operation as soon as it has met its synchronization-based durability policy. These are described in the next section.

- A simple majority of replicas in primary zones. That is, if the shard has 5 replica nodes residing in primary zones, then the master will wait for acknowledgments from 3 nodes.