Transaction isolation levels supported in JDBC

Transaction isolation level is a value that determines the level at which inconsistent data is allowed in a transaction, that is, the degree of isolation of one transaction from another. A higher level of isolation improves the accuracy of the data, but it can reduce the number of concurrent transactions. On the other hand, a lower level of isolation allows more concurrent transactions, but reduces the accuracy of the data.

During the use of transactions, to ensure data integrity, the DBMS uses locks to block other access to the data involved in the transaction. Such locks are necessary to prevent:

  • "dirty" read - reading data, added or modified by a transaction, which later will not be confirmed (rolled back);
  • non-repeatable read - when re-reading within one transaction, previously read data is changed;
  • phantom reads - a situation when, when reading repeatedly within the same transaction, the same sample gives different sets of rows.

Transaction isolation levels are defined as constants of the java.sql.Connection interface:

  • TRANSACTION_NONE - the driver does not support transactions;
  • TRANSACTION_READ_UNCOMMITTED - allows transactions to see unsaved data changes: allows dirty, unchecked and phantom reads;
  • TRANSACTION_READ_COMMITTED - any change made in a transaction is not visible outside of it until it is saved: prevents dirty reads, but allows unchecked and phantom reads;
  • TRANSACTION_REPEATABLE_READ - prohibits dirty and unchecked, phantom reads are allowed;
  • TRANSACTION_SERIALIZABLE - Dirty, unchecked, and phantom reads are prohibited.

The database server may not support all isolation levels. The java.sql.DatabaseMetaData interface provides information about the transaction isolation levels supported by a given DBMS.

The transaction isolation level used by the DBMS can be set using the setTransactionIsolation() method of the java.sql.Connection object. The getTransactionIsolation() method will help you get information about the applied isolation level.

Read also:


Comments

Post a Comment

Popular posts from this blog

Methods for reading XML in Java

Image
DOM (Document Object Model) - object - reads XML, recreating it in memory in the form of an object structure, while the XML document is represented as a set of tags - nodes. Each node can have an unlimited number of child nodes. Each child can also contain several levels of descendants or none at all. Thus, the result is a kind of tree. Pluses: Easy to program. If there are many objects in XML with cross-references to each other, it is enough to go through the document twice: first time create objects without references and fill in the name-object dictionary, the second time - restore links. If an error occurs in XML, a half-created XML structure remains in memory, which will be automatically destroyed. Suitable for both reading and writing. Minuses: Low speed. Wastes a lot of memory. SAX (Simple API for XML) - event-driven - reads an XML document, responding to emerging events (opening or closing tag, string, attribute) by calling the event handlers provided by the appl...
Read more

XML, well-formed XML and valid XML

Image
XML, the eXtensible Markup Language, is a language with a simple formal syntax, well suited for creating and manipulating documents with programs, and at the same time easy to read and create documents by humans. XML is extensible, it does not fix the markup used in documents, and the developer is free to create markup in accordance with the needs of a particular area, being limited only by the syntax rules of the language. DTD A DTD, Document Type Definition, is a predefined set of rules that defines the relationships between elements and attributes. For example, the DTD for HTML says that the DIV tag must be inside the BODY tag and can appear multiple times, TITLE - in HEAD and only once, and SCRIPT - both there and there as many times as you want. A DTD is usually described directly in the document as a formulation line starting with <!DOCTYPE ... > or in a separate file. Difference between well-formed XML and valid XML Depending on the level of compliance, the document...
Read more

ArrayList and LinkedList in Java, memory usage and speed

Image
How much extra memory is needed when calling ArrayList.add()? If there is enough space in the array to accommodate a new element, then no additional memory is required. Otherwise, a new array is created, 1.5 times larger than the existing one (this is true for JDK above 1.7, in earlier versions the size of the increase is different). How much extra memory is allocated when you call LinkedList.add()? One new instance of the nested Node class is created. How much memory to store one byte primitive in LinkedList? Each LinkedList element stores a link to the previous element, the next element, and a data link. private static class Node<E> { E item; Node<E> next; Node<E> prev; //... } For 32-bit systems, each link is 32 bits (4 bytes). The object (header) of the nested Node class itself takes 8 bytes. 4 + 4 + 4 + 8 = 20 bytes, and since the size of each object in Java is a multiple of 8, so we get 24 bytes. A byte primitive occupies 1 by...
Read more