In computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, 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.

Thread or process synchronization

Thread synchronization or serialization, strictly defined, is the application of particular mechanisms to ensure that two concurrently-executing threads

Thread (computer science)

In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...

 or processes do not execute specific portions of a program at the same time. If one thread has begun to execute a serialized portion of the program, any other thread trying to execute this portion must wait until the first thread finishes. Synchronization is used to control access to state both in small-scale multiprocessing systems -- in multithreaded environments and multiprocessor computers -- and in distributed computers consisting of thousands of units -- in banking and database systems, in web servers, and so on.

See

• Lock (computer science)
  Lock (computer science)
  In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...
  
   and mutex
• Monitor (synchronization)
  Monitor (synchronization)
  In concurrent programming, a monitor is an object or module intended to be used safely by more than one thread. The defining characteristic of a monitor is that its methods are executed with mutual exclusion. That is, at each point in time, at most one thread may be executing any of its methods...
  
  
• Semaphore (programming)
  Semaphore (programming)
  In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment....
  
  
• Test-and-set
  Test-and-set
  In computer science, the test-and-set instruction is an instruction used to write to a memory location and return its old value as a single atomic operation. If multiple processes may access the same memory, and if a process is currently performing a test-and-set, no other process may begin...
  
  

synchronization is a process to keep a copy of data.

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
  File synchronization
  File synchronization in computing is the process of ensuring that computer files in two or more locations are updated via certain rules....
  
  , such as syncing a hand-held MP3 player to a desktop computer.
• Cluster file systems, which are file system
  File system
  A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
  
  s that maintain data or indexes in a coherent fashion across a whole computing cluster.
• Cache coherency
  Cache coherency
  In computing, cache coherence refers to the consistency of data stored in local caches of a shared resource.When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data. This is particularly true of CPUs in a multiprocessing system...
  
  , maintaining multiple copies of data in sync across multiple cache
  Cache
  In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
  
  s.
• RAID
  RAID
  RAID is a storage technology that combines multiple disk drive components into a logical unit...
  
  , 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
  Database
  A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
  
   are kept in sync, despite possible large geographical separation.
• Journaling
  Journaling file system
  A journaling file system is a file system that keeps track of the changes that will be made in a journal before committing them to the main file system...
  
  , a technique used by many modern file systems to make sure that file metadata are updated on a disk in a coherent, consistent manner.

Mathematical foundations

An abstract mathematical foundation for synchronization primitives is given by the history monoid

History monoid

In mathematics and computer science, a history monoid is a way of representing the histories of concurrently running computer processes as a collection of strings, each string representing the individual history of a process...

. There are also many higher-level theoretical devices, such as process calculi and Petri net

Petri net

A Petri net is one of several mathematical modeling languages for the description of distributed systems. A Petri net is a directed bipartite graph, in which the nodes represent transitions and places...

s, which can be built on top of the history monoid.

External links

• Anatomy of Linux synchronization methods at IBM developerWorks
• The Little Book of Semaphores, by Allen B. Downey