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What Is Snapshot Isolation?

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  • Written By: Jean Marie Asta
  • Edited By: Jessica Seminara
  • Last Modified Date: 26 August 2016
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    2003-2016
    Conjecture Corporation
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Snapshot isolation is a property in transaction processing and databases that guarantees all reads that are made in a transaction see the same database "snapshot." This means that the transaction will commit itself to successfully processing the request if its updates do not conflict with any other concurrent updates. Snapshot isolation is a control algorithm for multi-version concurrency that provides isolation levels that avoid the problems that commonly occur with concurrency. This isolation has been implemented by several different structured query language (SQL) servers, operating with minimal amounts of serialization anomalies. Snapshot isolations will not, however, guarantee complete serializability in a server.

Many major data management systems contain default isolation levels that are not serializable and often encounter anomalies of serialization without snapshot isolation. This can cause large systems to have numerous occurrences of isolation errors every day, which can lead to data being corrupted, especially that which is found in warehouse applications of data. The reason a system might operate in this state is because applications that run on lower isolation levels may improve efficiency if no serious errors occur. Including snapshot isolation in a system reduces these anomalies and — when taking operation time and the amount of errors that occur into account — increases efficiency even further.

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Some databases offer snapshot isolation as opposed to full serializability, but there are also anomalies that can occur within a database utilizing this type of isolation. These anomalies may lead to data consistency violations because transactions that maintain consistency interleave, or are arranged in alternating layers. The one way to resolve the problem and prevent anomalies such as these is to manipulate applications with an introduction of artificial locks and conflicting updates, which were followed by analysis of the conflicts between every pair of transactions. A different way to resolve these anomalies, however, is to modify the algorithms of a database system’s concurrency control to do automatic detections and preventions of anomalies of snapshot isolation at runtime. This can be done for important or arbitrary applications, but provides isolation that is serializable.

Newer SQL servers have been able to introduce new levels of isolation as well as new levels of snapshot isolation for the enhancement of concurrency in certain applications. Where early versions of the SQL server used locking as the basis for concurrency, snapshot isolation is dependent on enhancements of row versioning intended for the improvement of performance. It improves this performance when it encounters scenarios with read or write blocking by avoiding them.

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