Editing Consistency model (section)

==== Primary-based protocols ====

[[File:Primary-backup-protocol.gif|thumb|Primary backup protocol]]
[[File:Local-write-backup-protocol.gif|thumb|Primary-backup protocol (local-write)]]

Primary-based protocols<ref name="Replica"/> can be considered as a class of consistency protocols that are simpler to implement. For instance, sequential ordering is a popular consistency model when consistent ordering of operations is considered. The sequential ordering can be determined as primary-based protocol. In these protocols, there is an associated primary for each data item in a data store to coordinate write operations on that data item.

===== Remote-write protocols =====

In the simplest primary-based protocol that supports replication, also known as primary-backup protocol, write operations are forwarded to a single server and read operations can be performed locally.
: '''Example:''' Tanenbaum et al., 2007<ref name="Replica"/> gives an example of a primary-backup protocol. The diagram of primary-backup protocol shows an example of this protocol. When a client requests a write, the write request is forwarded to a primary server. The primary server sends request to backups to perform the update. The server then receives the update acknowledgement from all backups and sends the acknowledgement of completion of writes to the client. Any client can read the last available update locally. The trade-off of this protocol is that a client who sends the update request might have to wait so long to get the acknowledgement in order to continue. This problem can be solved by performing the updates locally, and then asking other backups to perform their updates. The non-blocking primary-backup protocol does not guarantee the consistency of update on all backup servers. However, it improves the performance. In the primary-backup protocol, all processes will see the same order of write operations since this protocol orders all incoming writes based on a globally unique time. Blocking protocols guarantee that processes view the result of the last write operation.

===== Local-write protocols =====

In primary-based local-write protocols,<ref name="Replica"/> primary copy moves between processes willing to perform an update. To update a data item, a process first moves it to its location. As a result, in this approach, successive write operations can be performed locally while each process can read their local copy of data items. After the primary finishes its update, the update is forwarded to other replicas and all perform the update locally. This non-blocking approach can lead to an improvement.
The diagram of the local-write protocol depicts the local-write approach in primary-based protocols. A process requests a write operation in a data item x. The current server is considered as the new primary for a data item x. The write operation is performed and when the request is finished, the primary sends an update request to other backup servers. Each backup sends an acknowledgment to the primary after finishing the update operation.