SYNCHRONIZATION

Synchronization (computer science)

In computer science, synchronization refers to one of two distinct, but related concepts: synchronization of processes, and synchronization of data. Process synchronization refers to the idea that multiple processes are to join up or handshake at a certain point, so as to reach an agreement or commit to a certain sequence of action. Data synchronization refers to the idea of keeping multiple copies of a dataset in coherence with one another, or to maintain data integrity. Process synchronization primitives are commonly used to implement data synchronization.

Process synchronization
Process synchronization refers to the coordination of simultaneous threads or processes to complete a task in order to get correct runtime order and avoid unexpected race conditions.
There are many types of synchronization:
barrier
lock/semaphore
thread join
mutex
non-blocking synchronization
synchronous communication operations (see: Comparison of synchronous and asynchronous signalling)
A synchronization point is the location, in a process or collection of threads or processes, where the synchronization occurs. The interaction of synchronization points is one of the objects of study in the theory of concurrent computing.

Data synchronization
A distinctly different (but related) concept is that of data synchronization. This refers to the need to keep multiple copies of a set of data coherent with one another.
Examples include:
File synchronization, such as syncing a hand-held MP3 player to a desktop computer.
Cluster file systems, which are file systems that maintain data or indexes in a coherent fashion across a whole computing cluster.
Cache coherency, maintaining multiple copies of data in sync across multiple caches.
RAID, where data is written in a redundant fashion across multiple disks, so that the loss of any one disk does not lead to a loss of data.
Database replication, where copies of data on a database are kept in sync, despite possible large geographical separation.
Journalling, a technique used by many modern file systems to make sure that file metadata are updated on a disk in a coherent, consistent manner.