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, data can be changed before the transaction ends, resulting in non-repeated READ or phantom data. This option is the default value of SQL Server.B. READ UNCOMMITTED: Execute dirty READ or 0-level isolation lock, which means no shared lock is issued or the exclusive lock is not accepted. When this option is set, uncommitted or dirty reads can be performed on the data. Before the transaction ends, the values in the data can be changed, and the rows can

for the key that previously appeared in the map, the Put method returns null, and the Remove method returns false for key set, value collection, and entry Set checks, the number of elements generated is decreasing.3. Each key object in the Weakhashmap is stored indirectly as an indication object of a weak reference. Therefore, the key is removed automatically, either within the map or outside the map, only after the garbage collector clears the weak reference for a key.(ii) Four categories of r

sub-things, and thing B also has a number of sub-things, and things A and B in high concurrency, there will be a variety of problems. "A variety of problems," summed up the main is five: the first category of missing updates, the second category of missing updates, dirty read, non-repeatable read, Phantom read. Of the five, the first category of missing updates, the second category of missing updates is not important, do not talk about dirty reading,

Mysql lock mechanism usage summary, mysql Lock Mechanism 1. isolation level (1) Read uncommitted (RU) At this isolation level, transactions are completely isolated and dirty reads are generated. uncommitted records can be read, which is not used in actual cases. (2) Read committed (RC) Only committed records can be read. At this isolation level, phantom read occurs. Phantom read refers to executing the same

( //try last COMMIT Transaction} catch () {conn.rollback ();/ROLLBACK TRANSACTION} Transaction ISOLATION Level 1, transaction concurrent read problem dirty read: Read to another transaction uncommitted data (cannot be allowed to appear) Things) cannot be read repeatedly: Two read inconsistent Phantom read (virtual Read): Read to another transaction committed data 2, concurrent transaction problem because concurrent transactions cause problems with ro

the same transaction may have new commits during the instance processing, so the same select may return different results.Repeatable Read (can be reread) This is the default transaction isolation level for MySQL, which ensures that multiple instances of the same transaction will see the same rows of data while concurrently reading the data. In theory, however, this can lead to another tricky problem: Phantom Reading (

Note: This article is reproduced from http://www.hollischuang.com/archives/900Read behavior is a condition that can be encountered when multiple transactions are executing concurrently, while reading data. Understanding them first helps you understand the meaning of each isolation level. These include dirty reads, non-repeatable reads, and Phantom reads.Dirty ReadDirty read also known as invalid data readout, refers to in the database access, transact

operations are rolled back.Non-repeatable read (non-repeatable Reads): One transaction repeats two reads of the same row of data, but it gets different results. For example, in the middle of two reads, there is another transaction that modifies the row's data and submits it.Two update issues (Second lost updates problem): A special case that cannot be read repeatedly. There are two concurrent transactions that read the same row of data at the same time, and one of them modifies the commit, and

, however, this can lead to another tricky problem: Phantom Reading (Phantom read). To put it simply, Phantom reads when a user reads a range of data rows, another transaction inserts a new row within that range, and when the user reads the data row of that range, a new phantom row is found. The InnoDB and Falcon stora

commit the current transaction, is the change to a permanent change rollback: ROLLBACK canTo rollback the current transaction, cancel its change 2, directly using set to change the MySQL auto-commit mode MySQL default is automatically submitted, that is, you submit a query, it is directly executed!We can use set Autocommit=0 to disable autocommit set autocommit=1 turn on auto commit to implement transaction processing. But note that when you use set autocommit=0, all of your later SQL will be t

define the meaning of the four isolation levels in the database theory, but will introduce to you how MySQL/InnoDB defines these four isolation levels. MySQL/InnoDB defines four isolation levels: Read UncommitedUncommitted records can be read. This isolation level is not used and ignored. Read Committed (RC)Snapshot read is ignored.For the current read,RC isolation level ensures locking (record lock) of read Records). Repeatable Read (RR)Snapshot read is ignored.For the current read, the RR

which the transaction deals with the data.In a typical application, multiple transactions run concurrently, often manipulating the same data to complete their work. Concurrency, though required, can cause problems: Dirty Reads (Dirty read)--dirty reads occur when a transaction reads data that has been overwritten by another transaction but has not yet been committed. If these changes are rolled back later, the data read by the first transaction will be invalid. Non-repeatable read

committed, transaction B queries out a uncommitted record. non-repeatable read (Non-repeatable Read): transaction A reads once, at which point the transaction B updates or deletes the data, and transaction a again queries for inconsistent data. Phantom Reading (Phantom Read): Transaction A reads once, at which point the transaction B inserts a data transaction a again query, more records.2. MySQL's underly

data row after the iteration is grouped 4. How does the process of inserting and deleting a data in MySQL database execute at the bottom?MySQL locks, locks are indexed, and row-level locks are index-based. InnoDB uses row-level locks only if the data is retrieved by index criteria, otherwise INNODB will use a table lock! The main points involved in the analysis are: (1) Where conditions are split (2) about the combination of index and transaction isolation level Reference: (1) MySQL transactio

Transactions are different from locks. Transactions have ACID properties:Atomicity: Persistence: Redo log Redo logs to ensure the atomicity and durability of transactions,Consistency: Undo log is used to ensure transactional consistencyIsolation: A transaction sees the results of other transactions, such as Phantom reads, during the operation. Locks are a mechanism for resolving isolation. The isolation level of a transaction is implemented through a

same rows of data while concurrently reading the data. In theory, however, this can lead to another tricky problem: Phantom Reading (Phantom read). To put it simply, Phantom reads when a user reads a range of data rows, another transaction inserts a new row within that range, and when the user reads the data row of that range, a new

or BEGIN statement can start a new transaction commit: Commit can commit the current transaction, is the change to a permanent change rollback: ROLLBACK canTo rollback the current transaction, cancel its change 2, directly using set to change the MySQL auto-commit mode MySQL default is automatically submitted, that is, you submit a query, it is directly executed!We can use set Autocommit=0 to disable autocommit set autocommit=1 turn on auto commit to implement transaction processing. But note t

transaction may have new commits during the instance processing, so the same select may return different results.repeatable Read (can be reread) This is the default transaction isolation level for MySQL, which ensures that multiple instances of the same transaction will see the same rows of data while concurrently reading the data. In theory, however, this can lead to another tricky problem: Phantom Reading (Phan