In
database system
In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases spa ...
s, atomicity (; from grc, ἄτομος, átomos, undividable) is one of the
ACID
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
(''Atomicity,
Consistency
In classical deductive logic, a consistent theory is one that does not lead to a logical contradiction. The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent ...
,
Isolation,
Durability
Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime. There are several measures of durability in us ...
'')
transaction properties. An atomic transaction is an ''indivisible'' and ''
irreducible
In philosophy, systems theory, science, and art, emergence occurs when an entity is observed to have properties its parts do not have on their own, properties or behaviors that emerge only when the parts interact in a wider whole.
Emergence ...
'' series of database operations such that either ''all'' occurs, or ''nothing'' occurs. A guarantee of atomicity prevents updates to the database occurring only partially, which can cause greater problems than rejecting the whole series outright. As a consequence, the transaction cannot be observed to be in progress by another database client. At one moment in time, it has not yet happened, and at the next it has already occurred in whole (or nothing happened if the transaction was cancelled in progress).
An example of an atomic transaction is a monetary transfer from bank account A to account B. It consists of two operations, withdrawing the money from account A and saving it to account B. Performing these operations in an atomic transaction ensures that the database remains in a
consistent state, that is, money is neither lost nor created if either of those two operations fails.
The same term is also used in the definition of
First normal form
First normal form (1NF) is a property of a relation in a relational database. A relation is in first normal form if and only if no attribute domain has relations as elements. Or more informally, that no table column can have tables as values (or ...
in database systems, where it instead refers to the concept that the values for fields may not consist of multiple smaller value to be decomposed, such as a string into which multiple names, numbers, dates, or other types may be packed.
Orthogonality
Atomicity does not behave completely
orthogonally with regard to the other
ACID
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
properties of transactions. For example,
isolation relies on atomicity to roll back the enclosing transaction in the event of an isolation violation such as a
deadlock
In concurrent computing, deadlock is any situation in which no member of some group of entities can proceed because each waits for another member, including itself, to take action, such as sending a message or, more commonly, releasing a lo ...
;
consistency
In classical deductive logic, a consistent theory is one that does not lead to a logical contradiction. The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent ...
also relies on atomicity to roll back the enclosing transaction in the event of a consistency violation by an illegal transaction.
As a result of this, a failure to detect a violation and roll back the enclosing transaction may cause an isolation or consistency failure.
Implementation
Typically, systems implement Atomicity by providing some mechanism to indicate which transactions have started and which finished; or by keeping a copy of the data before any changes occurred (
read-copy-update In computer science, read-copy-update (RCU) is a synchronization mechanism that avoids the use of lock primitives while multiple threads concurrently read and update elements that are linked through pointers and that belong to shared data structur ...
). Several filesystems have developed methods for avoiding the need to keep multiple copies of data, using journaling (see
journaling file system
A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the goal of such changes in a data structure known as a "journal", which is usually a circular log. In the even ...
). Databases usually implement this using some form of logging/journaling to track changes. The system synchronizes the logs (often the
metadata
Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including:
* Descriptive metadata – the descriptive ...
) as necessary after changes have successfully taken place. Afterwards, crash recovery ignores incomplete entries. Although implementations vary depending on factors such as concurrency issues, the principle of atomicity – i.e. complete success or complete failure – remain.
Ultimately, any application-level implementation relies on
operating-system functionality. At the file-system level,
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming interf ...
-compliant systems provide
system call
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
s such as
open(2)
and
flock(2)
that allow applications to atomically open or lock a file. At the process level,
POSIX Threads
POSIX Threads, commonly known as pthreads, is an execution model that exists independently from a language, as well as a parallel execution model. It allows a program to control multiple different flows of work that overlap in time. Each flow of ...
provide adequate synchronization primitives.
The hardware level requires
atomic operations
In concurrent programming, an operation (or set of operations) is linearizable if it consists of an ordered list of invocation and response events (event), that may be extended by adding response events such that:
# The extended list can be re-e ...
such as
Test-and-set In computer science, the test-and-set instruction is an instruction used to write (set) 1 to a memory location and return its old value as a single atomic (i.e., non-interruptible) operation. The caller can then "test" the result to see if the stat ...
,
Fetch-and-add In computer science, the fetch-and-add CPU instruction (FAA) atomically increments the contents of a memory location by a specified value.
That is, fetch-and-add performs the operation
:increment the value at address by , where is a memory loca ...
,
Compare-and-swap
In computer science, compare-and-swap (CAS) is an atomic instruction used in multithreading to achieve synchronization. It compares the contents of a memory location with a given value and, only if they are the same, modifies the contents of tha ...
, or
Load-Link/Store-Conditional
In computer science, load-linked/store-conditional (LL/SC), sometimes known as load-reserved/store-conditional (LR/SC), are a pair of instructions used in multithreading to achieve synchronization. Load-link returns the current value of a memory ...
, together with
memory barrier
In computing, a memory barrier, also known as a membar, memory fence or fence instruction, is a type of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued b ...
s. Portable operating systems cannot simply block interrupts to implement synchronization, since hardware that lacks concurrent execution such as
hyper-threading
Hyper-threading (officially called Hyper-Threading Technology or HT Technology and abbreviated as HTT or HT) is Intel's proprietary simultaneous multithreading (SMT) implementation used to improve parallelization of computations (doing multip ...
or
multi-processing
Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There are ...
is now extremely rare.
See also
*
Atomic operation
In concurrent programming, an operation (or set of operations) is linearizable if it consists of an ordered list of invocation and response events (event), that may be extended by adding response events such that:
# The extended list can be re-e ...
*
Transaction processing
Transaction processing is information processing in computer science that is divided into individual, indivisible operations called ''transactions''. Each transaction must succeed or fail as a complete unit; it can never be only partially comple ...
*
Long-running transaction
Long-running transactions (also known as the saga interaction pattern) are computer database transactions that avoid lock (computer science), locks on non-local resources, use compensation to handle failures, potentially aggregate smaller ACID tran ...
*
Read-copy-update In computer science, read-copy-update (RCU) is a synchronization mechanism that avoids the use of lock primitives while multiple threads concurrently read and update elements that are linked through pointers and that belong to shared data structur ...
References
{{DEFAULTSORT:Atomicity (Database Systems)
Data management
Transaction processing