Misplaced Pages

#82917

29-397: In a database management system, a transaction is a single unit of logic or work, sometimes made up of multiple operations. Any logical calculation done in a consistent mode in a database is known as a transaction. One example is a transfer from one bank account to another: the complete transaction requires subtracting the amount to be transferred from one account and adding that same amount to

58-517: 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

87-463: A layered system architecture (e.g., with one operation at the database-engine level, one operation at the operating-system level). Another type of transaction is the compensating transaction . Transactions are available in most SQL database implementations, though with varying levels of robustness. For example, MySQL began supporting transactions from early version 3.23, but the InnoDB storage engine

116-407: A transaction ID or XID. There are multiple varying ways for transactions to be implemented other than the simple way documented above. Nested transactions , for example, are transactions which contain statements within them that start new transactions (i.e. sub-transactions). Multi-level transactions are a variant of nested transactions where the sub-transactions take place at different levels of

145-438: A transaction might consist of one or more data-manipulation statements and queries, each reading and/or writing information in the database. Users of database systems consider consistency and integrity of data as highly important. A simple transaction is usually issued to the database system in a language like SQL wrapped in a transaction, using a pattern similar to the following: A transaction commit operation persists all

174-405: Is serializability , which guarantees that the effect of concurrent transactions is equivalent to their serial (i.e. sequential) execution. Most modern relational database management systems support transactions. NoSQL databases prioritize scalability along with supporting transactions in order to guarantee data consistency in the event of concurrent updates and accesses. In a database system,

203-478: Is crucial, as it offers a virtually infinite pool of storage resources, accommodating a range of cloud-based data store classes with varying availability, scalability, and ACID properties. This integration is essential for achieving higher availability, lower response time, and cost efficiency in data-intensive applications deployed across cloud-based data stores. The Namesys Reiser4 filesystem for Linux supports transactions, and as of Microsoft Windows Vista ,

232-437: Is now extremely rare. Compensating transaction The execution of a business process consists of one or more transactions . Each transaction may consist of several individual operations yet, as a whole, it moves the system between consistent states. There are two groups of systems where compensating transaction may be applied: 1. In the context of a database this is often easily achieved using transactions and

261-454: Is one of the ACID ( Atomicity, Consistency , Isolation , Durability ) transaction properties. An atomic transaction is an indivisible and irreducible series of database operations such that either all occur, or none occur. A guarantee of atomicity prevents partial database updates from occurring, because they can cause greater problems than rejecting the whole series outright. As

290-531: Is only a workaround which has to be implemented manually and cannot guarantee that the system always ends in a consistent state. The system designer may need to consider what happens if the compensating transaction also fails. Compensating transactions are also used in case where a transaction is long lived (commonly called Saga Transactions ), for instance in a business process requiring user input. In such cases, data will be committed to permanent storage, but may subsequently need to be rolled back, perhaps due to

319-418: The commit / rollback mechanism. Compensating transaction logic could be implemented as additional on top of database supporting commit/rollback. In that case, we can decrease business transaction granularity. 2. For systems without a commit/rollback mechanism available, one can undo a failed transaction with a compensating transaction , which will bring the system back to its initial state. Typically, this

SECTION 10

#1732787641083

348-464: The ACID properties over multiple nodes, and might include systems such as databases, storage managers, file systems, messaging systems, and other data managers. In a distributed transaction there is typically an entity coordinating all the process to ensure that all parts of the transaction are applied to all relevant systems. Moreover, the integration of Storage as a Service (StaaS) within these environments

377-574: The Microsoft NTFS filesystem supports distributed transactions across networks. There is occurring research into more data coherent filesystems, such as the Warp Transactional Filesystem (WTF). Atomicity (database systems) In database systems , atomicity ( / ˌ æ t ə ˈ m ɪ s ə t i / ; from Ancient Greek : ἄτομος , romanized :  átomos , lit.   'undividable')

406-417: The acronym ACID . Databases and other data stores which treat the integrity of data as paramount often include the ability to handle transactions to maintain the integrity of data. A single transaction consists of one or more independent units of work, each reading and/or writing information to a database or other data store. When this happens it is often important to ensure that all such processing leaves

435-508: The database is always in a consistent state, even in the event of concurrent updates and failures. All the write operations within a transaction have an all-or-nothing effect, that is, either the transaction succeeds and all writes take effect, or otherwise, the database is brought to a state that does not include any of the writes of the transaction. Transactions also ensure that the effect of concurrent transactions satisfies certain guarantees, known as isolation level . The highest isolation level

464-493: The database or data store in a consistent state. Examples from double-entry accounting systems often illustrate the concept of transactions. In double-entry accounting every debit requires the recording of an associated credit. If one writes a check for $ 100 to buy groceries, a transactional double-entry accounting system must record the following two entries to cover the single transaction: A transactional system would make both entries pass or both entries would fail. By treating

493-507: 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 in database systems, where it instead refers to the concept that the values for fields may not consist of multiple smaller values to be decomposed, such as a string into which multiple names, numbers, dates, or other types may be packed. Atomicity does not behave completely orthogonally with regard to

522-549: The enclosing transaction may cause an isolation or consistency failure. 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 ). Several filesystems have developed methods for avoiding the need to keep multiple copies of data, using journaling (see journaling file system ). Databases usually implement this using some form of logging/journaling to track changes. The system synchronizes

551-610: The file-system level, POSIX -compliant systems provide system calls such as open(2) and flock(2) that allow applications to atomically open or lock a file. At the process level, POSIX Threads provide adequate synchronization primitives. The hardware level requires atomic operations such as Test-and-set , Fetch-and-add , Compare-and-swap , or Load-Link/Store-Conditional , together with memory barriers . Portable operating systems cannot simply block interrupts to implement synchronization, since hardware that lacks concurrent execution such as hyper-threading or multi-processing

580-403: The logs (often the metadata ) 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

609-416: The lowest level ( READ UNCOMMITTED ), which may occasionally be used to ensure high concurrency, such changes will be immediately visible. Relational databases are traditionally composed of tables with fixed-size fields and records. Object databases comprise variable-sized blobs , possibly serializable or incorporating a mime-type . The fundamental similarities between Relational and Object databases are

SECTION 20

#1732787641083

638-403: The other ACID 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 ; consistency 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

667-411: The other. A database transaction, by definition, must be atomic (it must either be complete in its entirety or have no effect whatsoever), consistent (it must conform to existing constraints in the database), isolated (it must not affect other transactions) and durable (it must get written to persistent storage). Database practitioners often refer to these properties of database transactions using

696-421: The recording of multiple entries as an atomic transactional unit of work the system maintains the integrity of the data recorded. In other words, nobody ends up with a situation in which a debit is recorded but no associated credit is recorded, or vice versa. A transactional database is a DBMS that provides the ACID properties for a bracketed set of database operations (begin-commit). Transactions ensure that

725-441: The results of data manipulations within the scope of the transaction to the database. A transaction rollback operation does not persist the partial results of data manipulations within the scope of the transaction to the database. In no case can a partial transaction be committed to the database since that would leave the database in an inconsistent state. Internally, multi-user databases store and process transactions, often by using

754-480: The start and the commit or rollback . After starting a transaction, database records or objects are locked, either read-only or read-write. Reads and writes can then occur. Once the transaction is fully defined, changes are committed or rolled back atomically , such that at the end of the transaction there is no inconsistency . Database systems implement distributed transactions as transactions accessing data over multiple nodes. A distributed transaction enforces

783-463: The user opting to cancel the operation. Unlike conventional rollbacks, specific business logic will typically be required to roll back a long lived transaction and restore the system to its original state. This type of transaction differs from distributed transactions (often implemented using the two-phase-commit protocol ), because although both types of transactions can result in multiple data stores being updated, compensating transactions allows for

812-405: Was disabled with the start of a transaction, autocommit will also be re-enabled with the end of the transaction. One can set the isolation level for individual transactional operations as well as globally. At the highest level ( READ COMMITTED ), the result of any operation performed after a transaction has started will remain invisible to other database users until the transaction has ended. At

841-506: Was not default before version 5.5. The earlier available storage engine, MyISAM does not support transactions. A transaction is typically started using the command BEGIN (although the SQL standard specifies START TRANSACTION ). When the system processes a COMMIT statement, the transaction ends with successful completion. A ROLLBACK statement can also end the transaction, undoing any work performed since BEGIN . If autocommit

#82917